Real-time Dose Control for Electron Beam Lithography
UNIVERSITY
OF KENTUCKY
Yugu Yang, Stephen A. Maloney and J. Todd Hastings
Department of Electrical and Computer Engineering, University of Kentucky, KY40506, USA
FPGA Implementation
Fundamental limitation in Electron Beam lithography (EBL): Random Poisson distribution of electron arrival
and resist interaction events [1-4], combined with system imperfections [5], limits the critical dimension (CD)
control, line-edge roughness and throughput of EBL.
Traditional way of determining exposure
dose:
• E-beam current is measured before
exposure and shot exposure time is
calculated to yield desired dose.
• Statistical properties of e-beam are not
taken into consideration.
• Other methods have been implemented
that use intervening apertures [6-7] or
emitter control [8-11].
SEM images of dots exposed with average dose of 0.09 pC.
(a) Dose control was incorporated during exposure to
achieve the desired dose level. The dwell time was set to
600 ms (nominal dose 0.28 pC, variance 0.04 pC) in the
RAITH lithography software to avoid the effect of e-beam
motion on the pattern shape. (b) Feedback control was not
used during exposure. The variance of dose is 0.028 pC.
Virtex-4 User FPGA
( XC4VSX35-10FF668)
clock
BB_RAITH
Dose
Control
Logics
BB_FPGA
Virtex-II Clock
FPGA (XC2V804CS144)
(a)
PCI to User FPGA
Interface Core
User
Interface
Software
Dose
Threshold
data
1mm
1mm
1mm
Std. of Dot Diameter vs. Std. of Exposure Dose without Dose Control
(b)
18
Standard Deviation of Dot Diameter vs. Standard Deviation of Exposure Dose
Feedback
clock
ctrl
14
with dose control
without dose control
12
40MS/s
Spartan-II
Interface FPGA
ADC
Std. of Dot Size (nm)
Sig. from
Hardware:
Scintillator
ctrl
• XtremeDSP Development Kit-IV
Schematic of signal routing among modules on the board and the host computer
motherboard is used for implementation.
• Signal from scintillator is sampled by ADC on the motherboard at a rate of 40 MS/s [12].
• The algorithm is translated into DSP system in System Generator software and then converted into binary file to be
downloaded to Virtex-4 user FPGA. Virtex-II clock FPGA synchronizes ADC with user FPGA [12].
• DIMEScript language provides access to FPGA from host computer such as data transfer (“dose threshold” in the
experiment reported here) and downloading binary design file to hardware [13].
10
8
6
8
6
Dose Var. = ~10 fC
5
10
15
20
25
Std. of Exposure Dose (fC)
12
20
without dose control
with dose control
30
35
40
without dose control
with dose control
14
8
6
6
4
2
8
6
2
2
220
225
230
235
Dot Diameter (nm)
240
245
0
215
220
225
230
Dot Diameter (nm)
235
240
0
200
205
210
215 220 225 230
Dot Diameter (nm)
235
240
245
Exposure was performed with average dose of 0.09 pC. When exposure was performed without dose control,
the dot size spreads out more from the average value as the built-in dose variance increases. In contrast, dots
exposed with dose control present a constant distribution variance.
Setup (a):
sample (a) below mirror
30
18
10
4
4
25
Std. of Dot Diameter vs. Std. of Exposure dose with Dose Control
Std. of Dot Diameter (nm)
10
Number per Bin
Number per Bin
12
8
10
15
20
Std. of Exposure Dose (fC)
16
without dose control
with dose control
10
14
5
Dots exposed at lower or higher average dose
without feedback shows the same trend in std. of
size as shown in the plot on the left when built-in
dose variance increases. Average dot size increases
as the base dose increases.
Dose Var. = ~24 fC
2
0
0
215
Ave. Dot Dia. = 233.8nm
10
12
Dose Control Experiment
Ave. Dot Dia. = 225.8nm
12
2
0
Dose Var. = 0 fC
18
14
Ave. Dot Dia. = 214.5nm
4
4
16
Illustration of feedback system for real-time dose control. The substrate to be
patterned is coated with a scintillating layer that produces an optical signal. The
signal is detected and processed to determine when each pixel has received
sufficient dose so that the control system can stop the exposure.
0.07pC
0.09pC
0.11pC
16
Number per Bin
New approach: Real-time dose control
for every exposed feature
• A scintillating layer in resist stack emits
several photons for each primary electron.
The photons are collected and converted
to electrical signal to estimate the dose
that has been received by each pixel.
• E-beam is blanked once sufficient dose is
achieved.
• Ultimately, individual electrons could be
counted to overcome shot noise limit.
XtremeDSP Development Kit-IV
Host
Computer
PMT-current integration algorithm:
• Start accumulating the electrical
signal sampled by ADC once the
exposure is initiated.
• A new beam-blanking signal from
FPGA is generated to turn off the beam
once the desired dose is achieved, or
keep the beam on if otherwise.
Results
Std. of Dot Diameter (nm)
Introduction
nominal dose 0.22pC
nominal dose 0.25pC
nominal dose 0.28pC
16
14
Ave. Dot Dia. = 226.9nm
Ave. Dot Dia. = 227.6nm
Ave. Dot Dia. = 227.2nm
12
10
8
6
4
2
0
10
20
30
Std. of Exposure Dose(fC)
40
The change in nominal dose and the dose variance
To improve the performance of dose control:
affects neither the average dot size, nor the
standard deviation, when exposure is performed
• Use different accelerating voltage and aperture to achieve higher SNR.
with dose control.
• Improve the quality of the mirror in the LCS to achieve better light yield.
• Implement the algorithm at higher rate. The ADC on XtremeDSP motherboard can sample at speed up to 105 MHz.
Light Detection and Collection
Side (a) of LCS is covered with
copper foil. Type (a) sample is
placed underneath the hole
where E-beam focuses through.
Schematics of elliptical mirror with video
images captured in EBL chamber
Samples
Al (~60nm, E-beam evaporation)
Scintillator (~450nm)
Silicon Substrate
Scintillator (~450nm)
Glass Substrate
Type (a)
Type (b)
Scintillator emits light in an upwards direction or
through the edges of the film. LCS sits above the sample.
Light emitted from scintillator is reflected downwards by Al film. Glass
substrate allows the light to reach to the LCS underneath the sample.
Scintillator:
• Composed of primary scintillator (p-terphenyl), secondary scintillator (POPOP), and polyvinyl
toluene (PVT) polymer. Spin-coated from chlorobenzene solvent (C6H5Cl).
• 420-nm emission wavelength
Acknowledgements
This work is supported by the National Science Foundation under Grant No. 0601351. Facilities and technical assistance for this work were
provided by the University of Kentucky Center for Nanoscale Science and Engineering (CeNSE). We acknowledge Brian Wajdyk and Chuck
May (CeNSE) for their valuable technical assistance. The FPGA design tools were donated by the Xilinx University Program.
0.02
0.03
Time (s)
PMT Signal vs. Time
0.04
0.05
0.4
0.3
0.2
0.1
0
0.01
0.02
0.03
Time (s)
(a)
0.04
0.02
0.03
Time (s)
PMT Signal vs. Time
0.04
0.05
0.01
0.02
0.03
Time (s)
(b)
0.04
0.05
0.4
0.3
0.2
0.1
0
-0.1
0
0.05
0.01
Comparison of EBL system beam blanker signal (upper traces, “BB_RAITH” in the schematics above) with
scintillator signal from the PMT (lower traces). A voltage of 0 indicates that the beam would be on in the absence
of feedback control. (a) The dwell time was set to 10 ms (nominal dose 2095 fC) and the feedback system achieved
the required dose (629 fC) by terminating the exposure early. (b) The dwell time was set to 1.5 ms (nominal dose
314 fC) and the feedback system extended the dwell times to achieve the desired dose.
PMMA (~60nm)
SiO2 (~40nm, sputtering)
0.01
3
2.5
2
1.5
1
0.5
0
-0.5
0
Dot arrays in RAITH software
• 7x7 dots in each field are
separated by 0.9 mm to avoid
damaging the scintillator under
neighboring dots.
• Same base dose is applied to
all the fields in the same row
and different dose variations are
introduced intentionally to each
column.
• Base-dose factor increases
vertically (bottom to top) from
50% to 200%.
• Dose variance increases
horizontally (left to right) from
0 to 29% of base dose.
Determining the “Dose Threshold”
• Signal from PMT is captured by
oscilloscope when scanning the sample
near the focus of the elliptical mirror
before performing exposure.
• Calculate the average voltage VPMT for
the duration of desired dwell time (dt).
• Dose threshold = (# of samples sampled
by ADC in dt) *VPMT
Standard Deviation of Electrons/Pixel (%) vs.Seconds Per Sample
24
No Control
Control 1 (Pulse Counting)
Control 2 (Pulse Analysis)
24
22
20
18
16
14
12
10
20
18
16
14
12
10
8
8
6
No Control
Control 1 (Pulse Counting)
Control 2 (Pulse Analysis)
22
2
3
4
5
6
7
Photons Per Electron
(a)
8
9
10
6
0
0.5
1
1.5
2
2.5
Seconds Per Sample
(b)
3
3.5
4
Signal from Scintillator on the Incidence of Electrons vs. Time
0.8
0.7
Simulation Results
• Both algorithms offer improvement in
standard deviation when the ratio of photons
to electrons is larger than 5 for pulse analysis, and 3 for pulse counting respectively.
• Pulse counting algorithm depends highly on sampling speed, and provides significant improvement on the
performance at sampling speed of 100 MS/s and higher.
Conclusions
The work reported here serves as a proof of concept for the novel approach of real-time feedback control of dose
during electron-beam lithography without any modification of the patterning tool. The experimental results
demonstrate that single-pixel exposures with intentionally varied dose are well controlled to yield constant variance
and average value of feature size. Implementation on FPGA provides the flexibility to switch from one algorithm to
the other, or combine both for future investigation.
VPMT  0.58V for dt
0.6
0.5
References
0.4
0.3
0.2
0.1
dt
0
-0.1
-100
0
100
200
300
Time (us)
400
500
-8
x 10
Investigation of the effects of system parameters on the performance of the control
algorithms. (a) Normalized standard deviation of electrons required to expose one pixel as
a function of photons produced by scintillator on the incidence of electron. (b)
Normalized standard deviation of electrons required to expose one pixel as a function of
sampling speed. Photons produced per electron is set to 5.
Beam Blank Signal from RAITH vs. Time
Beam Blank Signal from RAITH vs. Time
3
2.5
2
1.5
1
0.5
0
-0.5
0
-0.1
0
PMMA (~60nm)
• Setup (a) was used to test the hardware functionality
• Accelerating voltage: 10 keV; Working distance: ~24mm
Voltage (V)
• LCS and power supply on
sample holder with metal shield.
• Side (b) of LCS is covered
with copper foil. Type (b)
sample sits on top of the mirror.
• Power supply is shielded with
a sheet of Al.
Hardware testing
Voltage (V)
Pro/E light collection system model
Standard Deviation of Electrons/Pixel (%) vs. Photons Per Electron
26
Standard Deviation of Electrons/Pixel (%)
Side (b)
Voltage (V)
Side (a)
Schematic of the experimental setup. The signal from PMT is routed through a transimpedance
amplifier and ADC and finally processed by a Virtex-4 FPGA. The beam-blanking control signal
from the EBL system (original connection shown with “X”) now provides the control signal for
the logic, and a new signal from the FPGA controls the beam blanker. Transimpedance amplifier
converts the negative current from PMT to 0-1V positive voltage.
Control algorithms
1) Pulse counting: Count current pulse
associated with each electron arrival until
proper exposure achieved, then stop
writing the pixel.
2) Pulse analysis: Divide each current pulse
by the average signal value produced by a
single electron arriving. Once appropriate
number of electrons have arrived, stop
exposure.
Standard Deviation of Electrons/Pixel (%)
Setup (b):
sample (b) above mirror
Voltage (V)
Light Collection System (LCS)
• Material: UV-curable photopolymer resin
• Ellipsoidal hole formed in rectangular solid.
• Light source (scintillator) is located at one focus and
light sensor at the other.
• Aluminum (Al) is thermally evaporated on the inside
surface of cavity to increase the reflectivity.
• LCS increases the light captured by PMT by 12 times.
• Shielded with copper foil and aluminum to prevent
charging
Simulation of Future Algorithms
Voltage (V)
Light Sensing
• Hamamatsu R4700U photomultiplier tube (PMT)
offers gain up to 3 10 6 with peak sensitivity through
most of the visible spectrum.
• Hamamatsu C4900 high voltage power supply
provides 0 to -1250 V to power the PMT.
• Low voltage electrical feed-through to EBL vacuum
chamber
600
Exposure settings
• Setup (b) was used to write the patterns
• Accelerating voltage: 10 keV; Working distance: ~5mm
• Dot settling time was set to 1 ms to eliminate the modulated dwell time affecting the
exposure of the next dot.
[1]
[2]
[3]
[4]
[5]
[6]
G. P Patsis, et al., Microelectronic Engineering 87, 1575-1578, 2010.
P. Kruit, et al., Journal of Vacuum Science & Technology B 24, 2931-2935, 2006.
N. Rau, et al., Journal of Vacuum Science & Technology B 16, 3784-3788, 1998.
H. I. Smith, Journal of Vacuum Science & Technology B 4, 148-153, 1986.
S. H. Lee et al., Photomask and NGL Mask Technology XVII, (SPIE, 2010), pp. 77480J.
M. A. McCord and A. D. Brodie, US Patent 7091486 (2006); M. Mankos, et al., US Patent 6555830 (2003); A. Yamada and Y. Oae, US Patent 5449915
(1995); Y. Fujikura, US Patent 4937458 (1990).
[7] J. S. Kim, et al., Journal of Nuclear Science and Technology, 515-517, Jun 2008.
[8] T. Rahman, et al., Journal of Vacuum Science & Technology B 25, 655-660, Mar-Apr 2007.
[9] K. Wilder, et al., Journal of Vacuum Science & Technology B 17, 3256-3261, Nov-Dec 1999.
[10] L. R. Baylor, et al., Journal of Vacuum Science & Technology B 20, 2646-2650, Nov-Dec 2002.
[11] C. S. A. Durisety, et al., Analog Integrated Circuits and Signal Processing 48, 143-150, 2006.
[12] Xilinx XtremeDSP Development Kit-IV Reference Guide, NT 107-0272 – Issue 3.
[13] Nallatech DIMEscript User Guide, NT 107-0113 – Issue 3.