Hot -Wire Sensing of Micro-Jet Flow
By
Yanxia Sun
B.S., Precision Instruments and Mechnology
Tsinghua University, China, 1999
Submitted to the Department of Mechanical Engineering
in Partial Fulfillment of the Requirement for the Degree of
Master of Science
at the
Massachusetts Institute of Technology
February 2002
©Massachusetts Institute of Technology 2001. All rights reserved.
Signature of A uthor....................................................................................
Department of Mechanical Engineering
June 12,2001
Certified by...............................................
..... ......
Taiqing Qiu
Associate I
essor of Mechanical Engineering
Thesis Supervisor
A ccepted by....................................................A
Ain A. Sonin
Professor of Mechanical Engineering
Chairman, Department Committee on Graduate Students
MASSACHUSETTS INTITUTE
OF TECHNOLOGY
MAR 2 5 2002
LIBRARIES
BARKER
1
Hot -Wire Sensing of Micro-Jet Flow
By
Yanxia Sun
Submitted to the Department of Mechanical Engineering
On January 16, 2001, in partial fulfillment of the
requirements for the Degree of
Master of Science
ABSTRACT
Uniformity of chemical vapor deposition (CVD) depends greatly on gas delivery systems.
A typical CVD machine uses metering tubes with a linear array of micro-orifices to
deliver gas-phase reactants. This work developed a measurement technique to
characterize sub-millimeter scale jet flow produced by a metering tube. It utilizes a hotwire probe as the sensing element, and the probe is scanned across a flow field narrower
than the probe width.
A general heat transfer model was established to describe the probe response to a highly
non-uniform jet flow. Both hot-film and hot-wire probes were tested for jet sensing
experimentally. Hot-film sensors were found to be more robust and easier to handle. The
effect of different sensor operation modes was studied experimentally. Results show that
a probe operated under constant current provides a higher sensitivity of detecting small
variation of jet velocity than that under constant temperature mode. On the other hand,
2
constant-temperature probes are more suitable for detecting flow directions. Scanning
profiles of micro-jet reveal non-uniform flow distribution along a typical metering tube.
Optical imaging of the tube shows that manufacturing defects of the orifices contribute
significantly to the non-uniformity of jet flow. This work lays a solid foundation for
building flow-testing systems in the semiconductor industry.
Thesis supervisor: Taiqing Qiu
Title: Associate Professor of Mechanical Engineering
3
TABLE OF CONTENTS
ABSTRACT ........................................................................................................................
2
TABLE OF CONTENTS .................................................................................................
4
LIST OF FIGURES.......................................................................................................
6
LIST OF TABLES ............................................................................................................
10
CHAPTER 1 INTROD UCTIO N ......................................................................................
11
CHAPTER 2 NUMERICAL SIMULATION OF JET FLOW ......................................
14
2.1 Exit jet velocity ...................................................................................................
15
2.2 Lam inar jet flow ...................................................................................................
2.3 Incom pressible turbulent jet flow ........................................................................
17
21
CHAPTER 3 HOT-WIRE SENSING OF MICOR-JET ...............................................
25
3.1 Introduction .............................................................................................................
3.2 Structure of hot-wire probes...............................................................................
3.3 Heat transfer model............................................................................................
3.4 Results and discussion........................................................................................
3.4.1 Hot-wire sensing in still air ..........................................................................
3.4.2 Hot-wire sensing in uniform flow ....................................................................
3.4.3 Hot-wire sensing profiles for micro-jet flow ...............................................
25
26
28
34
34
37
40
Chapter 4 EXPERIM ETNAL SETUP ..........................................................................
46
4.1 Air supply ................................................................................................................
46
4-2 Hot-wire probes and signal conditioning .............................................................
4.3 Probe scanning system ........................................................................................
47
50
Chapter 5 HOT-WIRE SCANNING OF MICRO-JETS ..............................................
5.1
5.2
5.3
5.4
Effect of sensor types sensing results.................................................................
Sensitivity to axial position.................................................................................
Sensitivity to backpressure.................................................................................
Reliability of hot-wire sensing for m icro-jet flow ..............................................
5.5 Jet direction .............................................................................................................
53
53
55
57
59
61
4
5.6 Sensing profiles at different backpressures........................................................
5.7 Uniformity of micro-jets .....................................................................................
64
70
Chapter 6 CONCLUSION AND FURTHER WORK .................................................
85
RE FE CE N C E S ..................................................................................................................
87
5
LIST OF FIGURES
Figure 1-1. Schematic diagram of CVD deposition......................................................
12
Figure 1-2. Schematic diagram of flow metering tube...............................................
12
Figure 2-1. Schematic diagram of gas flow in a metering tube ..................
14
Figure 2-2. Enlarged view of flow near an orifice ........................................................
15
Figure 2-3. Exit jet velocity........................................................................................
17
Figure 2-4. Jet flow from an orifice ..............................................................................
18
Figure 2-5. Axial velocity profile for laminar flow (z=3mm) .....................................
20
Figure 2-6. Radial velocity profile for laminar flow (z=3mm).....................................
20
Figure 2-7. Axial velocity profile for turbulent flow (z=1.0mm) .................................
23
Figure 2-8. Radial velocity profile for turbulent flow (z=1.0mm)...............................
23
Figure 2-9. Axial velocity profile for turbulent flow (z=2.5mm) .................................
24
Figure 2-10. Radial velocity profile for turbulent flow (z=2.5mm)...............................
24
Figure 3-1. H ot-w ire operation......................................................................................
25
Figure 3-2. Optical images of hot-wire and hot-film probes....................
26
Figure 3-3. Structure of hot-wire and hot-film probes .................................................
27
Figure 3-4. Schematic model of hot-wire and hot-film probes ......................................
29
Figure 3-5. The heat transfer model for wire support ...................................................
32
Figure 3-6. The temperature distribution along the wire (TSI1210-60, CC, 1=100 mA) .35
Figure 3-7. Electrical resistance of hot-film probe in still air ........................................
36
Figure 3-8. Average wire temperature in still air..........................................................
37
Figure 3-9. Effect of convection on wire temperature (I=100mA)...............................
39
6
Figure 3-10. Correlation of probe resistance (I=lOOmA) ............................................
40
Figure 3-11. Horizontal wire in mixed convection and conduction...............................
41
Figure 3-12. Temperature distribution along the wire when HFA sits at the jet's center. 42
Figure 3-13. Probe temperature during scanning (I=lOOmA)......................................
43
Figure 3-14. Effect of jet velocity on scanning profiles...............................................
44
Figure 3-15. Effect of axial position on scanning profiles.............................................
45
Figure 4-1. Schematic diagram of air supply system...................................................
46
Figure 4-2. Calibration of pressure transducer...............................................................
47
Figure 4-3. Schematic diagram of TSI1210 general purpose probe (-T1.5, -20, -60) ...... 48
Figure 4-4. Constant-current circuit ...............................................................................
49
Figure 4-5. Constant-temperature circuit .....................................................................
50
Figure 4-6. Experim ental setup ...................................................................................
51
Figure 5-1. Hot-wire probe sensing of micro-jet flow .................................................
54
Figure 5-2. Hot-film probe sensing of micro-jet flow.......................................................
55
Figure 5-3. Scanning along jet centerline......................................................................
56
Figure 5-4. Hot-film sensing of micro-jet flow near orifice ....................
57
Figure 5-5. Hot-film scanning of orifice #23 ....................................................................
58
Figure 5-6. Hot-film scanning of orifice #24 ....................................................................
59
Figure 5-7. Optical im age of orifice #35........................................................................
60
Figure 5-8. Repeatability of hot-wire scanning.............................................................60
Figure 5-9. Hot-film profiling of jet at relatively low velocity ..................
62
Figure 5-10. Hot-film profiling of jet at relatively high velocity.................63
Figure 5-11. Hot-film profiling of jet at relatively high velocity (CC mode).........64
7
Figure 5-12 . Three working backpressures when four orifices open............................65
Figure 5-13 . Scanning profiles of jet #22 (CC mode).................................................
66
Figure 5-14 Scanning profiles of jet #23 (CC mode)....................................................
66
Figure 5-15 Scanning profiles of jet #24 (CC mode)....................................................
67
Figure 5-16 Scanning profiles of jet #25 (CC mode)....................................................
67
Figure 5-17. Scanning profiles of jet #22 (CT mode)...................................................68
Figure 5-18. Scanning profiles of jet #23 (CT mode)...................................................69
Figure 5-19. Scanning profiles of jet #24 (CT mode)...................................................
69
Figure 5-20. Scanning profiles of jet #25 (CT mode)...................................................70
Figure 5-21. Scanning profiles of four orifices ............................................................
71
Figure 5-22. Optical images for four neighboring orifices ..........................................
72
Figure 5-23. Scanning profiles at low backpressure (CC mode) ...................................
73
Figure 5-24. Scanning profiles at medium backpressure (CC mode) ............................
73
Figure 5-25. Scanning profiles at high backpressure (CC mode).................................74
Figure 5-26. Scanning profiles at low backpressure (CT mode)...................................74
Figure 5-27. Scanning profiles at medium backpressure (CT mode) ............................
75
Figure 5-28. Scanning profiles at high backpressure (CT mode) .................................
75
Figure 5-29. Scanning profiles of jet #22......................................................................
76
Figure 5-30. Scanning profiles of jet #23......................................................................
77
Figure 5-31. Scanning profiles of jet #24......................................................................
78
Figure 5-32. Scanning profiles of jet #25......................................................................
79
Figure5-33. Scanning profiles along the metering tube .................................................
81
Figure5-34. Scanning profiles of jets near metering tube inlet ......................................
82
8
Figure5-35. Scanning profiles of jets near the dead end of metering tube....................83
Figure5-36. Optical images of orifice #3 and #35 .......................................................
84
9
LIST OF TABLES
Table 3-1. Dimensions of hot-wire probes...................................................................
28
Table 3-2. Physical properties used in simulations .......................................................
33
Table 4-1. Characteristics of TSI hot-wire probes .......................................................
48
10
CHAPTER 1 INTRODUCTION
Chemical vapor deposition (CVD) of thin films is widely used in the semiconductor
industry to produce integrated circuits, liquid-crystal displayers and solar cells. Figure 11 shows a schematic diagram of a typical CVD deposition process (Dobkin et al., 1995).
Gas-phase reactants are first introduced into an injector head through separate metering
tubes and then directed into a straight channel. As the reactants leave the channel, they
form a sheet of one-dimensional jet, impinging on a heated silicon wafer. As the
chemicals approach the wafer, the gas temperature increases, initiating chemical reactions
and thin-film deposition. The depleted reactants are pumped away from the wafer
through the exhaust channels of the injector.
The uniformity of CVD deposited films depends greatly on the uniformity of flow along
the long and narrow sheet of jet exiting the injector head. As the wafer diameter increases
from 200mm to 300 mm, the requirement for flow uniformity is significantly increased.
Metering tubes play an important role in controlling flow uniformity. A typical metering
tube consists of a lineal array of 30 to 50 sub-millimeter orifices. Figure 1-2 is a
schematic diagram of a metering tube. There exit two fundamental factors affecting flow
uniformity: manufacturing defects and fluid mechanics. The fine orifices on a metering
tube are typically produced through electrical-discharge machining (EDM). During EDM
the fine discharge wire wears and deforms, resulting in variations in orifice diameters and
11
pointing directions. Furthermore, the molten metal may not be removed completely
during EDM, forming solid debris around the orifices.
Injector head
Exhausj
Metering Tubes
sass
I
Gasga
N2
A-
J1L~
11-
"
wafer
Heated Hloor
Figure 1-1. Schematic diagram of CVD deposition
Metering Tube
Inlet
On*fices
Figure 1-2. Schematic diagram of flow metering tube
12
The second factor causing flow non-uniformity is due to two competing physical
processes. As chemicals flow within a metering tube, friction at the wall results in a
gradual decrease of the gas pressure. On the other hand, as the gases exit the tube through
the orifices, the mean velocity of the gases drops along the tube, which in turn leads to an
increase of gas pressure. Numerical simulations show that these two competing processes
produce a non-uniform pressure distribution and thus a non-uniform jet velocity
distribution along the tube (Mokhtari et al., 1997). They found that this intrinsic flow
distribution depends significantly on the orifice diameter and total inflow rate.
The objective of this work is to develop a sensing technique capable of measuring both
the magnitude and direction of sub-millimeter jet flows. It will provide a better
understanding of jet flows from a linear metering tube. It will also be used in the
semiconductor industry for quantitative evaluation of metering tubes.
A hot-wire probe is used in this work as the sensing element. Chapter 2 simulates
velocity distributions of ideal laminar and turbulent jets. Since the characteristic jet width
is much smaller than the sensing length of a typical hot-wire probe, chapter 3 develops a
heat transfer model to describe responses of hot-wire probes in a highly non-uniform jet
flow. It establishes the physical foundation for this unique hot-wire sensing technique.
Chapter 4 describes details of the experimental system, and chapter 5 presents the
experimental data and discusses the results. This work is summarized in chapter 6.
13
CHAPTER 2 NUMERICAL SIMULATION OF JET
FLOW
Figure 2-1 shows a schematic diagram of gas flow in a metering tube. For typical CVD
applications, the flow is laminar, single-phase and non-reacting within the tube. The tube
temperature is about 20 0 C higher than the inlet gas temperature, which is around 22 0C.
The inlet gauge pressure, P, controls the total flow rate through the tube. The objective of
this chapter is to estimate the exit jet velocity and velocity distribution of the jets. The
numerical simulations are conducted under the following assumptions.
1. Both the pressure variation and slight heating along the metering tube are
neglected.
2. The orifice diameter is 0.2 mm, and the orifices are circular and smooth. The
inside diameter of the tube is 5.10 mm and the wall thickness is 0.28 mm.
V1
P1
V2
P2
Vj
Pj
Figure 2-1. Schematic diagram of gas flow in a metering tube
14
2.1 Exit jet velocity
This section estimates the exit velocity of jets. Applying the Bernolli's equation along a
streamline (figure 2-2), the exit jet velocity, Ve, is related to the inlet gas pressure, P, as
P + IpV
2
12
2
pVe2 + APO + AP + AP2+
(2.1)
A,
(2)
d
L
P,V 1
(1)
Figure 2-2. Enlarged view of flow near an orifice
where V1 is the inlet gas velocity. The pressure change along the tube, AP, is
APO = I pKOVe 2
2
(2.2)
where the pressure coefficient, Ko can be positive or negative (Mokhtari, et al., 19997).
The pressure loss near the orifice entrance, AP, is
AP = I pKIVe 2(2.3)
2
15
where the loss coefficient due to sudden contraction is K1=0.42 (White, 1994). Since the
wall thickness of the metering tube, L, is larger than the orifice diameter, d, pressure loss
due to friction at the orifice wall, AP 2 , is
AP
I
2
L
2
d
1
(2.4)
2
2
At a typical Reynolds number of Re=1000, the friction coefficient, f, is 0.064. The
pressure loss at the orifice exit, AP3, is
AP = 1PK
2
3
(2.5)
V,2
The value of K3 =1 for sudden expansion is used here. Since the exit jet velocity, Ve, is
much larger than the inlet gas velocity VI, jet velocity can be found from equation 2.1 as
V
Ve
--
I
2
p(1+ K + K +K+K
(2.6)
pK
Figure 2-3 presents jet velocity as a function of the inlet pressure at different value of
total pressure loss coefficient, K.
16
I
10C
I
I
I
I
I
I
I
I
I
I
90
K=3
8C
K=4
-'O
70
I
I
)
60
0
50
7a)
40
x
uJ 30
20
10
I I I I I I I I I I I I I I
C
1
2
3
4
5
6 7 8 9 10 11 12 13
Inlet pressure, P (kPa)
14 15
Figure 2-3. Exit jet velocity
2.2 Laminar jet flow
Figure 2-4 illustrates the essential features of a jet issued from an orifice into still air. The
velocity profile is assumed to be uniform at the orifice exit here. There exists a region
called the potential core of the jet (0 : z < Z, ). Within the potential core, the jet velocity
is not affected. Outside this region, significant fluid mixing exists, resulting a monotonic
decrease of the jet velocity.
17
The potential region is generally within 5-7 orifice diameters. Beyond the potential core
(Z>Zc), the effects of viscous shear are active across the whole width of the jet, and the
jet flow becomes self-similar.
A
Jet Edge
Z=Z2
Z=Zc
-
-.
-
-=-
-
-
-
-
I-
-
-
Potential core
.
r
Z=L
Figure 2-4. Jet flow from an orifice
Jet flow can be characterized by the Reynolds number based on the exit velocity, Ve,
and the orifice diameter, d,
Re =
dV
e
V)
(2.7)
At a small Reynolds number, e.g., Re<30, the jet flow is laminar. Far away from the
orifice, the axial velocity, Vz and radial velocity Vr, have been reported in the
literature (Schlichting, 1979; Turns, 2000),
18
V
(2.8)
[1+9 ]2
8Vc q (1-_72)
(2.9)
Re (1+)r2)2
where Vc is the axial velocity at the jet center,
3
d
Re-Ve
32
z
Vc =
the similarity parameter,
(2.10)
ij,
is
- 6 Re 16
z
(2.11)
and z and r are the axial and radial positions measured from the orifice, respectively.
The radial position at which the axial velocity is half of the center velocity is
ro. =-
1.49
xz
Re
(2.12)
the jet width at half center velocity is
dO.5=
2.98
--- X z
Re
(2.13)
It increases with the axial position and decreases with the increase of the Reynolds
number.
Figures 2-5 and 2-6 present simulated velocity profiles at z=3mm at different exit jet
velocities. The results show that the radial jet velocity is significantly smaller than the
axial velocity.
19
5
4 .5
Ve=5m/s
4
E 3 .5
3
2
0
75
2
76
Ve=3m/s
1 .5
0 .5
-.
A
-500 -400 -300
100 200
0
-200 -100
Radial position, r (um)
300
400
500
Figure 2-5. Axial velocity profile for laminar flow (z=3mm)
0.04
Ve=5m/s
0.035
CD
E
0.03
0.025
Ve=3m/s
-.
0
-
0.02
4'
~
%
%
I
4'
j
I
I
I
/
I
II
,~
/
1
0.01
I
I
j
j
'
1
I
0.005
II
I
I
I
II
I~
~I
0
-4 00
I
-300
-200
/
-100
0
100
200
Radial position, r (um)
I
300
I
400
Figure 2-6. Radial velocity profile for laminar flow (z=3mm)
20
2.3 Incompressible turbulentjet flow
At relatively high Reynolds number, jet becomes turbulent. Turbulence greatly enhances
fluid mixing, which in turn increases internal friction of fluid. This effect can be modeled
by replacing the kinetics viscosity v in laminar flow case by eddy viscosity
C
(Schlichting, 1979),
e=0.0143Re v
(2.14)
Far away from the orifice and assuming that the jet momentum is conserved, the
similarity solution of jet velocity are given by Schlichting (1979),
V"
-
=
8Vc 17(l1-17 2
[3 Re (1+172)2
V= 6.6dVe
z
(2.15)
(2.16)
(2.17)
where the similarity parameter, 11, is
7/ = 7.59 r(2.18)
z
Figures 2-7 to 2-10 present velocity profiles at locations z=lmm and z=2.5mm. The
radial velocity is found to be less than 2% of the axial velocity.
The location at which the axial velocity is half of the center velocity is
21
70 .5 =0.64
or
ro
= 0.08 4 z.
(2.19)
The full width of the jet at half center velocity is
dO5 = 0.17z
(2.20)
The jet width is independent of the Reynolds number. The center jet velocity decreases
with z. It drops to half of the exit jet velocity at location zO,
z05 =13.2d
(2.21)
For the orifice used in this work, d=0.2mm, z0 sis 2.6mm. The width of the jet at this
location is about 0.5mm. The results suggest that sensing of the jet flow should be
conducted within a few millimeters of the orifice, and the jet velocity is highly nonuniform within a radial range of 0.5mm.
22
90
-
80
--- Vexit=40m/s
Vexit=60m/s
-
-
_
Vexit=80m/s
--
70
E
N
-
-
60
50
0
40
I /
I
-
-
30
20
' '
'
'
1
U
-400
-300
-200
0
-100
100
200
Radial position, r (lam)
300
400
Figure 2-7. Axial velocity profile for turbulent flow (z=1.Omm)
I2
-. Vexit=40m/s
Vexit=60m/s
-- Vexit=80m/s
1.8CO)
E
-
1.6
1.4
0
0i
1
II
/
Xi 0.8-
I
/
/Ii
,
I
\
-\
0.4
0.
I. 'I
-200
-150
-100
-50
0
50
100
Radial position, r (gm)
150
200
Figure 2-8. Radial velocity profile for turbulent flow (z=1.Omm)
23
4-
Vexit=40m/s
Vexit=60m/s
Vexit=80m/s
-
-
40
- -
-35
E
-
-
N 30
I It
.25
0
0
/
0 20
\
I%
~15
-
-.-
10
5
200 400
- 000 -800 -600 -400 -200 0
Radial position, r (Gm)
600
800 1000
Figure 2-9. Axial velocity profile for turbulent flow (z=2.5mm)
0.9
- Vexit=40m/s
- - - Vexit=60m/s
Vexit=80m/s
0.8
_
E 0.1
-
44
C.)
0
(D
0.4
40
Pe
0.
Cz
-
Im
44/
/
0.2
''
0.1
U'
-200
-150
-100
-50
50
0
Radial position, r (um)
100
150
200
Figure 2-10. Radial velocity profile for turbulent flow (z=2.5mm)
24
CHAPTER 3 HOT-WIRE SENSING OF MICOR-JET
3.1 Introduction
A hot-wire probe is a thermal transducer based on the convective heat transfer from a
heated wire to fluid flow. Consequently any change in the flow, which affects heat
transfer from the heated element, can be detected with hot-wire probe system.
Figure3-1 shows a schematic diagram of hot-wire operation. An electric current is passed
through a fine wire, heating the wire to about 2000 C. As the flow velocity varies, heat
transfer from the wire varies. This in turn causes a variation in the heat balance of the
wire and wire temperature. The wire is made from a material possessing a relative large
temperature coefficient of resistance, e.g., tungsten and platinum. The wire temperature is
monitored by measuring the wire resistance. A hot-wire probe can be operated either at
constant temperature or constant current mode.
Flow velocity U, Temperature Tf
Hot Wire/Film Sensor
Average probe
resistance, R
Average
temperature, T
Current: Im
Voltage: Vw
Figure 3-1. Hot-wire operation
25
Conventional applications of hot-wire anemometry require that the flow velocity along
the wire is uniform. In this work the jet flow is confined in a sub-millimeter width.
Existing models for hot-wire operation are not valid. The objective of this chapter is to
develop a general heat transfer model to describe hot-wire responses to highly nonuniform flow.
3.2 Structure of hot-wire probes
Two types of probes are used in this work. Figure 3-2 shows the optical images of the
hot-wire probe and hot-film probe. The hot-wire probe uses a fine tungsten wire as the
sensing element. The sensing element for the hot-film probe is a quartz rod coated with a
thin layer of platinum. The ends of the rod are coated with gold and welded to the
supporting structure. The hot-film probe has a much thicker sensing element than the hotwire probe.
Figure 3-2. Optical images of hot-wire and hot-film probes
26
Figure 3-3 presents dimensional parameters used in this work to characterize hot-wire
and hot-film probes. Table 3-1 lists dimensions of the probes used in this work.
L
ds
dl
d2
Ls
(a)Hot-wire probe
L
-- - --
d2
Ls
I
(b) Hot-film probe
Figure 3-3. Structure of hot-wire and hot-film probes
27
Table 3-1. Dimensions of hot-wire probes
Probe Type
Hot-Wire
Hot-Film
Hot-Film
Model
1210-T1.5
1210-20
1210-60
Sensing Diameter d,
3.8um
50.8um
152.4um
Sensing length, Ls
1.27mm
1.02mm
2.03mm
Total Length, L
1.52mm
1.65mm
3.05mm
Diameter of Wire Support, di
40um
-
Diameter of Supporting Rod, d2
100um
200um
200um
50
30
30
--
3.3 Heat transfer model
Operation of a hot-wire probe involves Joule heating in the sensing element, convective
heat loss from the probe to surrounding fluid, and conductive heat loss form the sensing
element to sensor supports. Compared to convection, radiation heat loss by the wire is
negligible.
The energy equation for a hot-wire probe is
pA(x)C,
aT
a
at
ax
7 =-
(A(x)K(x)
aT
ax
) + P(x)h(x)(To - T(x)) + I 2 p, (x)
(3.1)
28
where p, C, and K are the density, heat capacity and thermal conductivity of the wire
respectively, A is the cross sectional area, P is the wire perimeter, To is the ambient
temperature, I is the current, and p, is the electrical resistance per unit length. Equation
3.1 is a general equation applicable to both hot-wire and hot-film probes. A hot-film
probe has a uniform cross sectional area (Figure 3-4). On the other hand, a hot-wire probe
has a much smaller diameter in the sensing zone (region 2 in Fig. 3-4). The electrical
resistance in regions 1 and 3 is much smaller than that in region 2.
I-I
Region1
x=-0.5L
Region3
Region2
x=-0.5Ls
x=0.5Ls
x=O
x=0.5L
(a) Hot-film sensor
Region1
II
x=-0.5L
Region3
Region2
II
-
x=-O.5Ls
x=O
I
I
x=0.5Ls
I
x=0.5L
(b) Hot-wire sensor
Figure 3-4. Schematic model of hot-wire and hot-film probes
29
The convective heat transfer coefficient, h, can be calculated from the Nusselt number,
Nu,
h(x) = Nu(x)Ka
d,(x)
(3.2)
where Ka is the thermal conductivity of air, and d, is the wire diameter.
For natural convection, the Nusselt number is correlated to the Rayleigh number
(Incropera and Dewitt, 1996),
NuN = (0.6+0.321Ra 1 / 6 ) 2
Ra =
g(T -T0 )d, 3 #
ay
(3.3)
(3.4)
where a is the thermal diffusivity of air, and 8 = 1 is the expansion coefficient.
For forced convection, the Nusselt number is correlated to the Reynolds number
(Incropera and Dewitt, 1996),
NuF = 0.3+0.48
Re
Re,
Vd_
L
(3.5)
(3.6)
where V is the velocity component normal to the wire. Contribution to convective heat
transfer by flow component parallel to the wire is neglected here.
30
For mixed convection, the effective Nusselt number is
3
3
Nue = (NuF + NuN ) 1/3
(3.7)
From equation 3.1, the thermal time constant of the sensing element is
Ir =
pC A
(3.8)
Ph
As steady state, equation 3.1 becomes
d
-(A(x)K(x)
dx
dT
) + P(x)h(x)(To -T(x))
dx
+I
2
p(x) = 0
(3.9)
The boundary conditions for the sensing wire are
dT
dx
(3.10)
x=0, -=0
x
=
L
-,2T = T,
2
- KA-
dT
=Q
dx
(3.11)
The supporting rods for the sensing wire are modeled as infinitely long fins of uniform
cross sectional area (Figure 3-5). The rods conduct heat away from the sensing wire.
Since the rods are outside micro-jet flow, they lose heat mainly through natural
convection,
Qb =7w1d2
2
(3.12)
NUKaK(Tb TO)
where d2 is the rod diameter, Kr is the thermal conductivity of the rods. The Nusselt
number is assumed to be 0.6 here.
Combining equations 3.11 and 3.12 yields the boundary condition at x =
L
2
31
L
x=+ -KA
dT
2
dx
1
7Td
2
-
NuKK,(T -T,,)
(3.13)
Heat flux from
the wire: Qb
Base tem perature,
Tb
Wire su pPort
Ln
0%
d2
V
Figure 3-5. The heat transfer model for wire support
The electrical resistivity of sensing wire is
_po(I+ao(T - To))
A
(3.14)
where po is the electrical resistivity at 00 C, and ao is the temperature coefficient of
resistivity. The total sensor resistance is
L/2
R = fpw(x)dx =
-L/2
L/2
jdx
-L/2
(3.15)
A
32
where the electrical resistance outside the sensing region is neglected.
The steady-state energy equation is solved numerically using the finite different method.
The sensing wire is divided into 100 small elements. Table 3-2 lists properties used in the
simulations. The fluid properties are evaluated at a film temperature of 110 0C.
Table 3-2. Physical properties used in simulations
] Thermal conductivity of air, Ka
0.034W/m-K
Thermal diffusivity of air, a
.83* 10- m2/s
Viscosity of air, v
2.64*10 rm2/s
Thermal conductivity of tungsten, K
159W/m-K
Thermal conductivity of quartz, K
7.6W/m-K
Heat capacity of tungsten, pCp
2.6*106 J/m 3-K
Heat capacity of tungsten, pCp
2.3*10 6 J/n 3 -K
Thermal conductivity of support, Kr
16W/m-K
The total heating power is
QiI= I2R
(3.16)
Heat loss by convection is
LQ2
=cn
Jh(x)P(x)(T - To)dx
(3.17)
-L/2
33
Heat loss by conduction to wire supports is
Qcond = 2 Qb
(3.18)
The numerical solutions are found to satisfy the energy balance within 0.01%,
Qin
- Qcond -
conv
<; 0.01%
(3.19)
Qin
3.4 Results and discussion
3.4.1 Hot-wire sensing in still air
Experiments were conducted to validate the heat transfer model for hot-wire probes. A
TSI1210-60 hot-film probe was heated by a constant current in still air. Figure3-6 shows
the simulated temperature distribution along the probe at a constant current of lOOmA.
Heat is generated mainly within the 2mm long sensing region at the center. Conductive
heat loss through the wire supports is significant, leading to a highly non-uniform
temperature distribution. The total electrical resistance of the probe can be calculated
through equation 3.15. Figure3-7 compares the predicted probe resistance with
experimental results. Model prediction agrees with the experimental results.
Figure3-7 compares the average probe temperature. The probe temperature is deduced
from measured probe resistance,
34
-
R-R
T=
ROa
(3.20)
0
where RO is the probe resistance at 00 C. The predicted average sensing temperature is the
average wire temperature in the sensing zone,
-
L /2
T =- L
T(x)dx
(3.21)
L/2
The results show that the center wire temperature can be 40 0 C higher than the average
wire temperature at lOOmA.
250
200
150
0Z
L.
a 100
E
50
-
.......
0
0
0.5
1
1.5
Wire Temperature
Averaged Temperature
2
2.5
3
Position along the wire y(mm)
Figure 3-6. The temperature distribution along the wire (TSJ1210-60, CC, 1=100 mA)
35
6.2-
.
Experimental data
Curve fits
cc
57------
Simulation data
--
0
C
S5.2/
4.7
TS11210-60
4.2-
0
20
40
60
80
100
Input current,l (mA)
Figure 3-7. Electrical resistance of hot-film probe in still air
36
m
Experimental data
Curve fits
250---------
Simulation results
200-
00150
E
.
100-
Cz
50 -
5.0-
.
0
TS11210-60
I
I
20
40
.
I
60
80
100
Current input, I (mA)
Figure 3-8. Average wire temperature in still air
3.4.2 Hot-wire sensing in uniform flow
Figure3-9 compares the predicted average probe temperature with that deduced from
measured probe resistance in a uniform flow. The flow velocity was determined using a
pitot tube. The TSI1210-60 probe was operated at a constant current of lOOmA. By
neglecting conductive heat loss by probe supports, Joule heating of the sensing wire is
balanced by convective heat loss,
37
I 2 R = rds LS h(T -TO)
(3.22)
The convective heat transfer coefficient can be expressed as
h = C + DS
(3.23)
where C and D are probe constant (Bruun, 1995). The probe resistance is related to the
probe resistance is related to the probe temperature,
R = R (1+ aT)
Ra
(3.24)
= R0 (1+ aTO)
(3.25)
where Ra is the probe resistance at the ambient temperature, To. Combing the above
equations lead to
R
R~
R- R
+ DV)
7rsLs(C+DJ= A + B-
_7dL(C
ROaO 12
at constant sensing current.
(3.26)
A and B are constants. Figure3-10 presents the ratio, R/(R-Ra), as a function of flow
velocity. Above 5m/s, the experimental data follow a straight line, which agrees with the
theory prediction. The results suggest that the experimental data are valid. At low
velocities, conductive heat loss to the probe supports becomes more and more important.
As a result, the experimental data fall below the straight line.
38
Sensor Temperature in Air Flow (1=100 mA)
100
-
Simulation results
Experimental results
90
C)
80
40~
9
E
U)
70
.9
'4
60
U)
L.D
U)
50
-4
401L
5
10
15
20
25
30
35
40
Flow exit velocity (rn's)
45
50
55
60
Figure 3-9. Effect of convection on wire temperature (I=lOOmA)
39
.
7.5-
experimental data
Linear curve fit result:
7.0-
R
=3.62 + 0.64V
No
R-Ra
6.56.0C55
CF
5.5-
5.04.5-*
TS11210-60
4.01
2
3
4
Vi_ (
5
6
mis)
Figure 3-10. Correlation of probe resistance (I=lOOmA)
3.4.3 Hot-wire sensing profiles for micro-jet flow
By combined the heat transfer model of hot-wire probes with jet velocity fields presented
in chapter 2, local wire temperature can be determined. Figure3-11 presents a schematic
distribution of the wire temperature. The typical width of micro-jets is smaller than the
length of a sensing wire. A micro-jet produces intense cooling in a small region of the
wrie.
40
micro-Jet flow
Large-scale uniform flow
Temperature profile(micro-jet)
Temperature profile
(large scale unifrom flow)
Wire elemen\
Tm Tp
d.
4T
1
0
U2
Prong
x
I
I
dx
L
Figure 3-11. Horizontal wire in mixed convection and conduction
Figure 3-12 shows the predicted temperature distribution profile along a TSI 1210-60
probe when the probe is at the center of a jet. The probe is operated at a constant current
of 100mA and is located at an axial position of z=lmm.
41
When the probe is placed across the center of the jet, convective cooling by the jet is the
strongest, resulting in the lowest average wire temperature. If the probe is located near
the edge of the jet, the sensing wire is only slightly cooled, leading to a higher average
wire temperature. Figure 3-13 simulates the average probe temperature as the probe scans
across the jet.
15C
14C
13C
12C
j-
11C
10C
9C
0
8C
c*
7C
E
%
6C
5C
4C
3C
.5
0
2
2.5
1
1.5
0.5
Position along the wire, y (mm)
3
3.5
Figure 3-12. Temperature distribution along the wire when HFA sits at the jet's center
42
24(
Ve:=40m/s, z=1 mm,
TS11210-60
22C
20C
U>
3 18C
U)
E 16C
14C
)
cD
12C
10
.5 -0.4 -0.3 -0.2 -0.1
0
0.1
0.2
0.3
0.4
0.5
Radial position, r (mm)
Figure 3-13. Probe temperature during scanning (I=lOOmA)
When the exit jet velocity increases, whatever the jet is laminar or turbulent, the cooling
effect becomes stronger and the average probe temperature decreases (Figure 3-14).
According to the simulation results in chapter 2, when the axis position increases, the jet
expands, and the centerline jet velocity decreases. Figure3-15 shows that average probe
wire temperature decreases with the axial position and the scanning.
43
ST11210-60 HFA, axis position z=1mm
24C
-
-
Vexit= 4 m/s
Vexit=6 )m/s
Vexit=8 m/s
Cz
20C
/4.f
C
CD
1-
12C
a
.44f
CO
> 1 4C
'4e
1 2C
.000
10C
-C.5
-0.4
-0.3
-0.2
-0.1
0
0.1
0.2
0.3
0.4
0.5
Radial scanning position, x (mm)
Figure 3-14. Effect of jet velocity on scanning profiles
44
t
240
-
22C
--- Z3=1.0mm
Z2=1.5mm
Z3=2.Omm -
/ -TI
20C
-F-.
I
18C
I.
-V
16C
14
I
-1-
>14C
.\
I.
120
100
1
-0.8
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
0.8
1
Radial position, x (mm)
Figure 3-15. Effect of axial position on scanning profiles
45
Chapter 4 EXPERIMETNAL SETUP
An experimental system was established to characterize micro-jet flow with hot-wire
probes. It consists of supply of regulated compress air, hot-wire probe and signal
conditioning circuits, probe scanning system and data acquisition system.
4.1 Air supply
Figure 4-1 shows a schematic diagram of the air supply system. A dry diaphragm air
compressor is used to avoid oil contamination of the metering tube. A large gas filter is
used to remove particles larger than 1 pim. The gas flow rate is monitored by a rotary
flow meter. A pressure transducer monitors the transient line pressure. A digital pressure
gauge with ± 1% accuracy in serial is used to calibrate the pressure transducer. The
pressure transducer has a linear response,
(4.1)
P=25.OV(V)-26.2 (kPa)
Filter/Reservoir Flow Meter
Metering Tube
IT
Compressor
Pressure
Transducer
12 VDC Power Supply
- Presure
Gauge
MicroJetrFgaw
Signal to
Computer Pv
Figure 4-1. Schematic diagram of air supply system
46
Experimental data
Linear Fit
P=25.OVp(v)-26.2 (kPa)
0
100-
80-
7
60-
40-
~20-
0
I
1.0
I
*
I
*
I
*
I
"
I
"
I
1.5
2.0
2.5
3.0
3.5
4.0
Voltage output of pressure transducer, Vp (v)
"
I
*
4.5
5.0
Figure 4-2. Calibration of pressure transducer
4-2 Hot-wire probes and signal conditioning
Commercially available TSI hot-wire probes are used for micro-jet sensing. Figure 4-3
illustrates typical dimensions of the probes. Table 4-1 lists electrical characteristics of the
probes. The probes were operated at a nominal temperature of 2000C in this work.
47
12.7mm Hot Film
9.5mm Hot Wire
38mm
-i
1-
I
-0
t==
T
t-4.6mm
3.2mm
Figure 4-3. Schematic diagram of TSI1210 general purpose probe (-T1.5, -20, -60)
Table 4-1. Characteristics of TSI hot-wire probes
Probe type
Hot-Wire
Hot-Film
Hot-Film
Model
1210-T1.5
1210-20
1210-60
Serial number
225479
#27
996002
Working Temperature
2000 C
200 0 C
2000 C
Total probe resistance at 00 C, Ro
5.90Q
5.94 2
4.21 2
Total probe resistance at 200 0C
6.7492
6.4292
4.692
Lead resistance
0.19Q
0.652
0.202
Working current
34mA
65mA
lOOmA
Temperature coefficient
0.0042 / 0 C
0.0024 / 0 C
0.0024 / 0 C
The probes can be operated either under the constant-current mode or constanttemperature mode. Figure 4-4 shows the circuit used for the constant-current mode.
48
"V1
Reference resistor
R1
Current
source:1
Hot wire/film
sensor, R
Vw
Figure 4-4. Constant-current circuit
For constant-temperature operations, a commercially available control module from TSI
is used. It contains a Wheatstone bridge circuit with the sensor as one arm of the bridge.
A differential feedback amplifier senses the bridge unbalance and adds current to hold the
sensor temperature constant. Before the probe is placed in operation, the control resistor
is set to a value so that the bridge will be balanced at the working temperature of the
probe. An increase in fluid velocity cools the sensor and unbalances the bridge. This
causes the feedback amplifier to increase the sensor heating current and to bring the
bridge back into balance. Since the feedback loop responses rapidly, the sensor
temperature remains constant. The voltage drop across the probe is related to the fluid
velocity. Figure 4-5 shows a typical constant-temperature circuit.
49
Bridge voltage
R2
R1
D.C Diferential
Amplifier
|
Sensor
Control resistance,
Rc
Flow
Figure 4-5. Constant-temperature circuit
4.3 Probe scanning system
Figure 4-6 shows a schematic diagram of the probe scanning system. A hot-wire probe is
mounted on a three-dimensional transnational stage. The probe position in the y and z
directions is adjusted manually with micrometers. During measurements, the stage is
driven along the x direction at a constant speed by a DC motor. An encoder provides the
stage position with ±1 gm accuracy.
50
Gas inlet
M et ering
Tube
Orifice
0
Jet
X
3-D linear moving station
Gas metering
Figure 4-6. Experimental setup
A typical experiment involves (1) determination of probe position, (2) probe alignment,
(3) alignment of the scanning stage, and (4) probe scanning. A dummy probe is used to
determine the probe position. It has the same structure and size as the working probes
except there is no wire attached between wire supports. As the probe is moved slowly
along the z direction approaching the metering tube, the electrical resistance between
them is monitored. When the probe contacts the tube, the resistance drops sharply and the
probe position in the z direction is set to z=O. The probe is then moved into a jet and
adjusted along the y direction until it is centered. To align the scanning stage parallel to
the tube, the distance between the probe and metering tube is determined at the two travel
51
extremes of the stage along the x direction using the above electrical resistance method.
Over a travel range of 25mm, the stage is aligned parallel to the metering tube within
20um. During measurements, the probe is driven at a constant speed of 60um/s when the
probe is near or within a jet and at about 200um/s when the probe is traveling between
jets.
52
Chapter 5 HOT-WIRE SCANNING OF MICRO-JETS
This chapter investigates effects of different operation modes and different probes on
micro-jet sensing sensitivity, orifice-to-orifice flow uniformity, and relationship between
jet flow and orifice defects.
5.1 Effect of sensor types sensing results
Hot-film probes have a much larger diameter and thermal time constant than hot-wire
probes. Figure 5-1 and 5-2 present typical results from a TSI 1210-T1.5 hot-wire probe
and TSI1210-20 hot-film probe under the constant-current condition. The results show
that the two probes provide similar sensitivity and signal-to-noise ratio. Since hot-wire
probes are significantly shorter than hot-film probes, they are difficult to align to the
center of orifices. They are also very fragile. Hot-film probes are used in this work.
53
0.36
Hot-wire sensor TSI1210-T1.5, CC mode
0.35
0.34
CD)
0
0.33
0.32
.C
40
CO)
Cl)
0
0.31
0.3
0.29
0
0.28
0.27
0
200
400
600
800
1000
1200
1400
1600
1800
2000
Scanning position, x (gm)
Figure 5-1. Hot-wire probe sensing of micro-jet flow
54
0.46-
Hot-film sensor, TS11210-20, CC mode
U)
0
>
0.45
.0
0- 0.44
CD
CO,
CD,
0
o 0.430
0.42-
0.41
0
200
400
'
600
'
800
'
1000
1200
1400
Scanning position, x (pim)
Figure 5-2. Hot-film probe sensing of micro-jet flow
5.2 Sensitivity to axial position
The cooling effect to a hot-wire probe is the strongest when probe is at the centerline of
the jet. Figure 5-3 shows the average probe temperature versus probe axial position at a
constant-current of 100mA. The nominal wire temperature in still air at this current is
200C. As the probe moves away from an orifice, the flow velocity decreases. The
overall convective cooling of the probe decreases, leading to an increase of the averaged
probe temperature. Also, as fluid particles move away from the orifice, turbulent eddies
develop bigger and bigger, resulting a larger fluctuation of the wire temperature. The
probe temperature decreases with the increase of the exit jet velocity. The probe sensing
55
noise decreases with the jet velocity, suggesting that hot-film sensing technology has
good sensitivity for micro-jet flow when the probe is close to the orifice.
16C
k
TSI1210-60, I=100mA
15C
P1=0.8KPa
0
V
14C
..............................................................
.............
.....
..................................
13C
E 12C
Ci)
11C
.....................................................................
..............
..........
P3=3 6KPa
-
10C
...........................................
.........................
..................................................................
-
P4=1 4.3KPa
90
80
70
1.5
2
2.5
3
Axis position, z (mm)
3.5
Figure 5-3. Scanning along jet centerline
When a probe is very close to the orifice, the probe response is not sensitive to the probe
position (figure 5-4), indicating that the probe is within the potential core of the jet.
56
0.47 r-
TSI1210-20, CC mode
0.46
Z=0.5mm
0.45-
0.440)
.0
0
0.42-
0.41
0
'
200
400
600
800
1000
1200
1400
Scanning position, x (gm)
Figure 5-4. Hot-film sensing of micro-jet flow near orifice
5.3 Sensitivity to backpressure
Figures 5-5 and 5-6 show the scanning profiles at different pressures for two neighboring
offices, #23 and #24. The orifices are indexed from the tube inlet. At the nominal gauge
pressure of 7.5kPa, the exit jet velocity is estimated between 50m/s and 70m/s. Although
the drop of the backpressure is only 0.7%, the hot-wire probe can clearly distinguish this
slight variation.
The variation of the exit jet velocity is related to the backpressure as
57
AV = 0.5 AP
(5.1)
P,
Using the nominal pressure of P, =7.5kPa and nominal velocity of V" =70m/s, the hotfilm sensing system can detect jet velocity changes as small as 0.3m/s.
0. 44
'
I
Orifice #23, z=O.5mm
TSI 1210-20,CC mode
CI)
4.
43
7E- 0. 42
41
0
a) 0.
0 40
2
P2=7.5 kPa
39
0
38
----
o.
P i=7.55Kpa
n
-0.2
-0.1
0
0.1
0.2
0.3
Scanning position, x (mm)
Figure 5-5. Hot-film scanning of orifice #23
58
0.44
0.43
0,
0
0.42
Orifice #24, 2=0.5mm
T111210-20, CC mode
0.41
0.40
P2=7.5
0
C,,
0)
L..
kPa
0.39
0.38
0
0.37
0.36
-C .2
r P 1=7.55 K p a
0.1
0
-0.1
Scanning position, x (mm)
0.2
0.3
Figure 5-6. Hot-film scanning of orifice #24
5.4 Reliability of hot-wire sensing for micro-jet flow
Figure 5-7 shows an optical image of a typical orifice, #35. From the image, small
irregular defects can be observed in the left part of the orifice. Multiple scans of the jet
show identical profiles (Figure 5-8). The scanning profiles overlap perfectly in the
smooth parts (i.e., x=800-1200gm). In the left side of the jet, i.e., x=500-800 gm, the
profiles are slightly not identical and include irregular features, indicating the impact of
orifice defects on the jet flow. The results show that hot-wire sensing is reliable.
59
Figure 5-7. Optical image of orifice #35
0.47
Orifice 35, z=0.5mm,
TS11210-20, CC mode
0.46
Test 1
Test 2
0.45
C,,
E 0.44
0.43
2 0.42
0.41
0.40
0
1
200
400
1
1
600
800
1000
1200
Scanning position, x ([tm)
1
1400
1600
Figure 5-8. Repeatability of hot-wire scanning
60
5.5 Jet direction
When the sensing probe is scanned perpendicular to the jet, the ideal scanning profiles
should be symmetric. The asymmetric profiles indicate that the orifice is non-circular and
contains defects. Furthermore, the direction of jet flow can be deduced by tracing the
center point of the symmetric scanning profiles at different axial positions.
Assuming aht the jet center points are 01 (x1, z1) and 02 (x2, z2), the angle between the
jet and the z axis, 6, is
x1)
6 = tan- 1 (x2
( z2 - zl)
(5.2)
Figure 5-9 presents scanning profiles for a typical orifice under the constant-temperature
mode. The axial position of the probe is 1 mm, 1.5mm and 2mm, respectively, and the
backpressure is about 1kPa. At a relatively high jet velocity, the scanning profiles
become asymmetric (figure5-10), and the jet center cannot be determined accurately.
Similar results for hot-film probe working in the constant-current mode are found (figure
5-11).
61
2.5 TS11210-20
2.4
Z-2.0nmr
Z=1. 5mm
P=_1kPa
2.3
'2.2
62.1
7E- 2
1.9
a..
1.8
1.7
1.6 ,
0
200
400
600
800
1000
1200
1400
Scanning position, x (pm)
Figure 5-9. Hot-film profiling of jet at relatively low velocity
62
3.2
TSI1210-20
a
Z=0.5mm Z=0.5mm
Z=1 .0mm
Z=1.Omm
CT mode
P=7.5kPa
A
1.5m
0)
(U
~0 2.4
Z=2.0m
-0
0
2
1.8
200
400
600
800
1000
Probe scanning position x (jim)
Figure 5-10. Hot-film profiling of jet at relatively high velocity
63
0.44
I
P = I .5I
I
TSI 1210-20, CC mode, P=7.5kPa
0.43
0.42
0.4
Z=2.Omm
> 0.41
)
1
0.
:= 0.40
0
Z=1.5mm
2 0.39
0.
0.38
Z=1.0mm
0.37,
0
200
400
600
Scanning position, x (pin)
Figure 5-11. Hot-film profiling of jet at relatively high velocity (CC mode)
5.6 Sensing profiles at different backpressures
This section investigates the effect of backpressure on hot-wire scanning profiles. Figure
5-12 shows the pressure variation during typical hot-wire scanning. The three inlet
pressures used in testing are P1=0.70kPa, P2=2.05kP and P3=7.30kPa. The pressures are
stable. Only four orifices were open in this experiment, and the rest were blocked.
64
P3=7.30tpa
7.5
0O
5.0
CD
CL
a)
P2=2.05]Ya
2.5
P1=0.70- a
0
-300
-200
-100
0
100
200
300
400
Scanning position, r (pm)
Figure 5-12. Three working backpressures when four orifices open
Figures 5-13 to 5-16 show the scanning profiles of jets #22-25 using a TSI 1210-20 probe
under constant-current mode.
65
0.44
Orifice #22, z=0.5mm, l=54.3mA
0.43
0.42
0
,> 0.41
CD
cz 0.40
...
...
...
...
.-...
...
..
..
...
...
...
...
...
...
...
...
...
...
... ......
0
c,
-0
0
0.39
P2
P3
0.37
0.36'
-300
-200
-100
100
0
Scanning position, x (jim)
300
200
Figure 5-13. Scanning profiles of jet #22 (CC mode)
0.44
Orifi e #23, Z=0.5mm
1=54.3rr A
0.43
0,
0.42
0)
0.41
0.40
0
Pi
0.39
P2
0.38
0.37
0.36
P3
-300
-200
200
100
0
-100
Scanning position, x (jim)
300
Figure 5-14. Scanning profiles of jet #23 (CC mode)
66
0.44
Orfice #24, -=0.5mm, I=54.3mA
0.43
0.42
0.41
V,
\J
0.40
>
0
0
1
P
0.39
__P_2
0.38
0.37
I
0.36
-300
__
-200
_
_
_
_P3
100
0
-100
Scanning position, x (prm)
200
300
Figure 5-15. Scanning profiles of jet #24 (CC mode)
0.45
0.44
-
_Orifice
#25, Z=0.5mm, I=54.3mA
0.43
0.42
(D
CU
0
.0
0.41
0.40
0.39
0.38
0.37
0.36
-300
P3
-200
100
0
-100
Scanning position, x (rm)
200
300
Figure 5-16. Scanning profiles of jet #25 (CC mode)
67
The scanning profiles are symmetric and smooth. When the inlet pressure increases, CC
mode curves are symmetric and smooth. When the pressure as well as jet exit velocity
increases, the average wire temperature decreases. Since the scanning plane is only 2.5orifice diameter away from the orifices, the probe is within the potential core of the jets.
In this region, the jet velocity increases with the backpressure. The jet width, however, is
not strongly affected by the pressure.
Figures 5-17 to 5-20 show scanning profiles under the constant-temperature mode. The
results are similar to those under the constant-current mode.
3.2
P3
3
I-,
0
Orifice #22: CT mode
z=0.5mm
2.8
2.6
0)
Ca
2.4
0
P1
CD
n02.2
0
a.
'
2
/
1.8
1.6
600
-400
-200
0
200
400
600
800
Scanning position, x (gm)
Figure 5-17. Scanning profiles of jet #22 (CT mode)
68
3.2
P3
3-
Orifice #23, CT mode,
z=0.5mm
2.-
5'
Ca -2.84-P
--
8P1
a~2.2-
24
0.
1.8
-600
-400
-200
0
200
400
600
800
Scanning position, x (p m)
Figure 5-18. Scanning profiles of jet #23 (CT mode)
3.2
P 3
3
Orifice #24, CT mode,
z=0.5mnm
2.8
2.6
P 1
z
2.4
-
2.2
-2
1.8
1.6
-600
-400
200
0
-200
Scanning position, x (pim)
400
600
800
Figure 5-19. Scanning profiles of jet #24 (CT mode)
69
3.2
P3
3
P2
I-,
0)
Orifice #25, CT mode,
z0.5mm
2.8
7E)
2.6
P1
CO
2.4
.0
..
..
..
.. ...
..
...
..
..
..
..
..
..
...
..
..
..
..
..
...
2.2
..
..
..
....
..
..
...
..
..
..
..
..
...
..
..
..
..
..
..
..
.
2
1.8
1 .6
-400
-200
0
200
400
600
800
Scanning position, x (pm)
Figure 5-20. Scanning profiles of jet #25 (CT mode)
5.7 Uniformity of micro-jets
Figure 5-21 presents hot-wire scanning profiles when only four orifices, #22 - #25, are
open. The sensing probe, TSI1210-20, was operated under constant-current mode. The
axial position of the probe is 0.46mm, and the inlet pressure is 7.3kPa. The spacing
between jets #22, #23, #24 and #25 are found to be 5.1mm, 4.7mm and 4.5mm,
respectively. Jet #25 has the highest velocity.
70
0.44
0.44
3
022
0.43
0.43
0.42
0.42
0.41
0.41
0.4
0.4
0.39
0.39
0.38
0.38
037
0.37
.
0.4
0.5
0.6
0.7
0.8
20
0-
1
1.1
1.2
0.36
5. 7
nA--
0.44
0.43-
0.43
0.42
-240-4
0.42
0.41
-
0.41
0.4
-
0.39
0.38
6.2
6.1
6
5.9
5.8
Scanning position, x (mm)
044
0
0.9
_23
.
025
0.4
0.39
0.38
-
0.37
0.361
10.4
0.37
10.5
10.6
10.7
10.8
10.9
11
-
0.36
15
15.1
15.2
15.3
15.4
15.5
Scanning position, x (mm)
Figure 5-21. Scanning profiles of four orifices
Figure 5-22 shows optical images of the solid debris can be seen on the inner edges of
orifices #23 and #25. Orifice #25 is also not circular.
71
15.6
Orifice#22
Orifice#24
Orifice#23
Orifice#25
Figure 5-22. Optical images for four neighboring orifices
Figures 5-23 to 5-30 compare the sensing profiles of the four jets under different
operation modes, and at different exit velocities. The axial position of the probe is
0.5mm. The results show that the jet velocity increases consistently form jet #22 to jet
#25. Scanning profiles are smooth and symmetric at inlet pressures less than 2kPa. At the
inlet pressure of 7kPa, irregular features are present near the center of the scanning
profiles. The constant-current mode provides better velocity resolution than the constanttemperature mode.
72
7.36
CD
/
7.27 k......
-..........
Cn
U)
ft
P1=0.7OkPa
/.
I/I
7.18
0
1*...
)
22
I...
)23
7.09
7.00
..
........
4
015
2
-
-300
-200
-100
0
100
200
300
Radial position, r (gm)
Figure 5-23. Scanning profiles at low backpressure (CC mode)
7.36
P2=2.05kPa
1-0
U)
7.18
-0
0
I...
7.09
22
7.00
024
025
6.91
-500 -400
-300 -200 -100
100
0
Radial position, r (im)
200
300
400
500
Figure 5-24. Scanning profiles at medium backpressure (CC mode)
73
7.09
P3=7.3OkPa
7.00
4)
0
6.91
4)
4)
023
0
4)
6.82
4)
025A
0024
6.73
-500 -400
-300 -200
0
100 200
-100
Radial position, r (Elm)
300
500
400
Figure 5-25. Scanning profiles at high backpressure (CC mode)
025
2.7
024
2.6
P1=0.7kPa
ai)
2.5
00
22.3
-1--
2.2
2.1
2
1.9
1.8
1 .7
-800
-600
-400
-200
0
200
400
600
800
Radial position r (urn)
Figure 5-26. Scanning profiles at low backpressure (CT mode)
74
3.0
02C
025
2.8
P2=2.05kPa
*-a
~0 2.4
023
22
2.2
2.0
1.8
1.6
- 600
-400
-200
0
200
400
600
800
Radial position r (urn)
Figure 5-27. Scanning profiles at medium backpressure (CT mode)
3.2
3
P3=7.3OkPa
24
02
2.8
0
023
2.6
0
I
2.4
02
j
_____________U
2.2
2
al.
1.8
-600
-400
-200
0
200
400
600
800
Radial position r (urn)
Figure 5-28. Scanning profiles at high backpressure (CT mode)
75
Figures 5-29 to 5-32 present scanning profiles of jets #22 to #25 when all the 49 orifices
in the center region were open. The TSI1210-20 probe was operated under the constantcurrent mode and at an axial position of 1mm. The results show a continuous increase of
the jet velocity for jets #22 and #24. The jet velocity drops slightly for jet #25 compared
to jet #24.
Qifice #2, z=1.OM CC mode
V1.
C1
*~8
~0
01
?D7.6
P1=05kPa
7.4
P2=6.CkPa
79
-300
-200
-100
0
Scaning position, x (pm)
100
200
300
Figure 5-29. Scanning profiles of jet #22
76
8.6
Orifice #23, z=1.Omm
8.4
8.2
C.)
8
0
C,,
7.8
7.6
P1=0.5kPa
7.4
P2=Z.OkPa
72
-300
-200
-100
0
Scanning position, x (pm)
100
200
300
Figure 5-30. Scanning profiles of jet #23
77
Oifice #24, z=1.Orm
CO
E
0
4-1
UD
8
L..
C)
.0
0I...
0L
C)
7.8
a
7.6
P1=O5kPa
7.4
P2=6,UPa
72
I.
-300
-200
-100
0
100
200
300
Soaning position, x (pnj
Figure 5-3 1. Scanning profiles of jet #24
78
a6
Qifie#25 z-1.Omi
8.4
C,,
a)
T78
0
L-
CD
P1:=.5kPa
7.4
F2-6.0kPa
72
-300
-200
-100
0
100
200
300
Smanrirg posifion, x (pm
Figure 5-32. Scanning profiles of jet #25
79
Figure5-33 shows scanning profiles of every eight orifices along the metering tube. The
center positions of the jets are arbitrarily shifted to make the results more distinguishable.
The jet velocity has a relatively uniform distribution in the center region. At both ends of
the tube, jet velocity is highly non-uniform (figure5-34 and figure 5-35). Optical images
show significant defects at the inner surface of orifice #3 (figure5-36). The axial position
of the TSI1210-20 probe is 0.5mm± 0.1mm along the entire tube. As this sensing
position, the scanning profiles are not sensitive to a small change of the axial position.
The inlet pressure is 1.0kPa.
80
0. 0.sl
I
-
-
0.47
0.46
0.45
D0.44
Cr
0
-0.43
0
0-
0.42
-03
c#43
0.41
0.4
-500
-
0
9
0#
500
To measure the velocity fields
for every 8 orifices along the
tube.
0#35
1000
-
1500
2000
2500
Scaming position. x* (pm)
Figure5-33. Scanning profiles along the metering tube
81
0.49
I
I
P=1.OkPa, z=0.5mm
0.48
Orifice35 scanned for comrparison.
0.47
0.46
0
0.45
0.44
-16
0.43
0.42
I0#
0.41
0.4
-500
0
500
1000
1500
2000
2500
Scanning position, x* (pm)
Figure5-34. Scanning profiles of jets near metering tube inlet
82
0.47
03 is scanned for comparison
0.46
0.451
a)
0
.0
0-
0.441
0.43F
I...
0.421
CV47
CM46
0.411
=
=3
P=1 .0kPa, z=O.5rrr
0.41
-5CxJ
0
500
1000
1500
2000
Scanning position, x*(pm)
Figure5-35. Scanning profiles of jets near the dead end of metering tube
83
Figure5-36. Optical images of orifice #3 and #35
84
Chapter 6 CONCLUSION AND FURTHER WORK
This work investigated applications of hot-wire probes for flow characterization of
micro-jets, both theoretically and experimentally. A general heat transfer model was
developed to describe the probe response to highly non-uniform jet flows. The
experimental study established general features of hot-wire scanning of micro-jets.
First, the sensitivity of hot-film probes are similar to hot-wire probes. Since hot-film
probes are more reliable and easier to align, they are recommended. Second, both the
constant-temperature and constant-current models can be used to detect jet direction and
velocity. The constant-current mode provides a slightly better resolution for resolving jet
velocity, while the constant-temperature mode provides a slightly better delectability of
the jet center. Third, within five-orifice diameters, the scanning profiles of a jet are not
sensitive to the axial position of the probe. The profiles are smooth and symmetric at
relatively low backpressures, e.g., 2kPa. At relatively high pressures, e.g., 7kPa, irregular
features develop in the profiles. Fourth, the output signal from a hot-wire probe becomes
increasing noisy as it moves away form an orifice. Fifth, large jet-to-jet velocity
variations are found to be caused by orifice defects. In conclusion, this work
demonstrated that a combination of hot-wire scanning and optical imaging techniques can
be sued for rapid identification of manufacturing defects and qualification of gas delivery
system of CVD machines.
85
In further studies, two or more wires can be integrated into one probe together. And then
the jet flow can be scanned by wires from different directions simultaneously. Moreover,
different work modes (CC/CT) can be applied to different wires for best sensing results.
It will greatly reduce the sensing time and collect more information about the jet velocity
filed together.
In addition to wires integrations, a high-resolution imitating system can also be integrated
along with the sensor and capture the orifice sidewall defects simultaneously. The
relationship between the distortion of the sensing curve and the orifice shape defects can
be studied in details in the future.
86
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No.3, 3-9, 1980.
Bruun, H.H., Hot-wire Anemometry: Principles and Signal Analysis, Oxford University press,
Oxford, 1995.
Dobkin D.M. Mokhtari S., Schmidt M., Anil P. and Robinson L., "Mechanisms of Deposition of
SiO2 from TEOS and related organosilicon compounds and Ozone," J. Electrochem. Soc., Vol. 142,
No. 7, 2332-2339, 1995
Fingerson L.M. and Freymuch, P., "Thermal anemometers', in Fluid Mechanics Measurements," (ed.
R.J. Goldstein), pp. 99-154, Hemisphere, Washington, 1983.
Freymuch P. and Fingerson L.M., "Electronic testing of frequency response for thermal
anemometers", TSI Quart., 3, No.4,5-12, 1977a.
Freymuch, P. "A comparative study of the signal-to-noise for hot0film and hot-wire anemometers," J.
Phys. E.: Sci. Instru., 10, 705-710, 1978
Freymuth, P. "Frequency response and electronic testing for constant temperature hot-wire
anemometers," J. Phys. E.Sci. Instruments, 10, 705-710, 1977b.
Incropera F.P. and DeWitt D.P., Fundamentals of Heat and Mass Transfer, fourth edition, John
Wiley &Sons, Inc., 1996
Lomas, C.G., Fundamentals of Hot Wire Anemometry, Cambridge University Press, Cambridge,
1986
Mayer B., "Small signal analysis of source vapor control requirements for APCVD," IEEE
Transactions on semiconductormanufacturing,Vol. 9, No.3, 344-365, 1996
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Mokhtari S., Kudriavtsev V.V., and Danna M., "Flow uniformity and pressure variation in multioutlet flow distribution pipes," ASME, PVP.Vol. 355, Advances in Analytical, Experimental and
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Oh H.J., Rhee S.W, Kang I.S., "Simulation of CVD Process by Boundary Integral Technique," J.
Electrochem. Soc., Vol. 139, No. 6, 1714-1720, 1992
Perry, A.E., and Morrison, G.L., "A study of the constant-temperature hot-wire anemometer," J.
Fluid Mech., 47, 577-599, 1971
Perry, A.E., Hot-wire Anemometry, Claredon Press, Oxford, 1982
Schlichting, H., Boundary-LayerTheory, McGRAW-HILL BOOK COMPANY, 1979
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White, F.M., Fluid mechanics, third edition, McGRAW-HILL, INC., 1994.
88