WET CLEANING AS AN IMPROVED FINAL QUALITY CONTROL OF DRIEPRODUCED FEATURES
Meng Guo1, Donald Pfettscher1, Kimberly Pollard1, Richard Peters1, Travis Acra1
Dynaloy LLC, a subsidiary of Eastman Chemical Company
1
Indianapolis, IN,USA
tacra@eastman.com; kpollard@eastman.com
Thierry Lazerand2, Kenneth D. Mackenzie2, Marco Notarianni2
2
Plasma-Therm LLC,
St. Petersburg, FL, USA
thierry.lazerand@plasmatherm.com
ABSTRACT
The deep reactive ion etch (DRIE) process also known as
the “Bosch process” was aimed originally at creating high
aspect ratio features specifically for silicon-based MEMS
devices.
With time and improvements in both dry etch equipment
and process capability, this processing technique has
become a stepping stone for a multitude of devices such as
capacitors, through-silicon vias (TSVs), and also
nanotechnology where deep trench features with high aspect
ratio are required.
The DRIE process consists of a fast alternation of etch and
polymer deposition steps in order to create deep trenches in
the silicon wafer. The polymer deposition step protects the
sidewalls during the next anisotropic etch loop. The final
result produces a sidewall with specific features called
scallops whose size depends upon the process conditions
selected for the polymer deposition and etch steps.
The polymer removal from the sidewalls at the end of the
process is critical in order to assure good adhesion of the
materials that could be deposited in the cavities such as
metals and oxides. Although this can be realized by
controlling the amount of polymer being deposited during
the DRIE process and proceeding with an in-situ plasma
cleaning after completion of the trench features, the cost
sensitivity of any modern device can benefit from a separate
wet cleaning on single wafer or batch systems.
This paper will present the results of a collaborative effort
between Dynaloy and Plasma-Therm LLC related to the
final quality control of DRIE produced trenches on wafers.
Specifically, Dynastrip DL9150 solvent has been employed
in order to remove any residual polymer contaminants and
show a perspective of expanding this application to post
plasma dicing where the wafers are mounted on a tape
frame. In that particular case, one of the main challenges is
to find a suitable solvent for polymer removal and to ensure
the chemistry is compatibility with the dicing tape in order
to avoid any die removal during this operation.
Key words: Wet clean, DRIE, photoresist, removal, residue,
plasma dicing.
INTRODUCTION
The deep reactive ion etch (DRIE) process also known as
the “Bosch process”i was aimed originally at creating high
aspect ratio features specifically for silicon-based MEMS
devices. With time and improvements in both dry etch
equipment and process capability, this processing technique
has become a stepping stone for a multitude of devices.
Today feature dimensions may include thicknesses in the
hundreds of micrometers and aspect ratios on the order of
~20:1.
Nowadays, there are many applications on the market that
implement the DRIE process. All devices where deep trench
features with high aspect ratios are required such as silicon
power devices, actuators for read/write heads, capacitors,
cantilevers, micro mechanical rotors, and through-silicon
vias for 2.5D and 3D advanced packaging, are typical
examples of how this process technique is widely adopted
by the industry. Some of these of these DRIE-created
structures are shown in Figure 1.
The highly directional etch characteristic results in features
with vertical sidewalls. The vertical profile is created
commonly by alternating (1) etch steps using SF6 and (2)
passivation (deposition) ones using C4F8. Residuals of the
polymer that is deposited in the passivation step remain at
the end of the etch process especially on the sidewall of the
trench.ii The deposition steps create a highly fluorinated
polymerized residue that is relatively inert, hydrophobic,
difficult-to-remove and even more difficult-to-dissolve in
standard strip solutions. The polymer on the sidewalls could
be removed with an ash process based on an O2 plasma
treatment at the end of the etching process. This step is not
as efficient as wet strip and offsets valuable productive time
of the plasma etching process chambers. In particular, it is
hard for the O2 plasma to penetrate efficiently into the
trenches in order to remove the polymer especially for high
aspect ratio trenches. For this reason a wet clean solution
may be preferred in order to efficiently remove polymer
from the sidewalls, and as a result increase throughput and
decrease maintenance time of the etching process modules.
All of these factors are key in order to decrease the cost of
ownership (CoO). A robust strip formulation based on bath
life time and loading capacity is required in order to
understand how it could impact the removal of the polymer
content in the context of integration. Further opportunities
exist in studying new materials with a smaller
environmental, health, and safety (EHS) footprint to reduce
risks for operators and the environment due to an accidental
exposure.
Optimization of individual components may
provide for a recyclable solution, where additional value can
be gained for the customer by re-sale of post process
solution. In this case, the waste solution may be further
purified by a 3rd party and re-sold into less demanding
applications. Careful consideration and understanding of all
aspects of formulation development allow tailoring of a
product to meet the specific needs of specific processes.
technique, called Auger electron spectroscopy (AES) is used
to study the detailed level of cleanliness, shown in Figure
4.iii
With the increased use of DRIE in a more diverse
application space, there is the opportunity to diversify the
use of current best methods and processes of record (PORs)
to obtain optimum clean results and improve final quality
control of a wide array of DRIE-produced features. One
such new opportunity is the use of plasma dicing to
singulate chips.v This method uses DRIE in a damage-free
process with very high throughput and with a decreased
requirement for street sizes, thus returning significant
silicon area for additional devices. The purpose of this
paper is to show the results of using a wet clean solution to
remove post-Bosch process fluorinated residue in a plasma
dicing process.
One of the critical aspects of die packaging, especially when
using plasma dicing with fluorine-based gases is to ensure
no fluorine traces on the bond pads and limiting the amount
of polymer left on the die sidewalls.
Figure 2. SEM images of vias (diameter = 5 μm, pitch = 10
μm) before removal of Bosch etch residue: (A0) 3500x
magnification, (A1-A3) 12kx magnification. (B1-B3) EDS
spectra of via bottom, middle and top showing presence of
F-containing residue on via sidewall.
Figure 1. Commercial applications for DRIE: (a) and (b)
micro actuators for a read/write heads; (c) and (d) test
structures for evaluating cantilevers and or capacitance
driven actuators; (e) membrane like structure; (f) microelectro-mechanical rotor structure.
We have previously shown that post-Bosch process residue
can be removed from TSVs for advanced packaging
applications.iii,iv
The definition of clean in a TSV
application includes removal of the polymer-based residue,
including all fluorine and compatibility with the underlying
silicon surface. Commonly, initial studies of the silicon
surface, before and after cleaning (Figures 2 and 3), are
done using (SEM) in conjunction with energy dispersive xray analysis (EDX).iii In the case of TSVs, the detection
limit for fluorine using EDX is too high to ensure absolute
cleaning. As a result, a second more surface sensitive
Figure 3. SEM images of vias (diameter = 5 μm, pitch = 10
μm) after removal of Bosch etch residue using the Dynastrip
DL9150 at 70 ˚C for 5 min: (E0) 3500x magnification, (E1E3) 12kx magnification. (F1-F3) EDS spectra of the via
sidewall at the bottom, middle and top, with no detected
fluorine signal.
A
B
Figure 4. Auger electron spectroscopy of via sidewall
(diameter = 5 μm, pitch = 10 μm) after removal of Bosch
etch residue using Dynastrip DL9150 at 70 ˚C for 10 min.
Fluorine was not detected on the via sidewall through the
depth of the via.
The criticality of fluorine removal on bond pads is related to
the well-known influence of fluorine contamination on
aluminum microchip bond pads.vi Results from these
studies indicate that aluminum bond pads exposed to
fluorine-based plasmas and stored in an ambient
microclimate will continue to oxidize due to co-adsorption
of fluorine-containing compounds during storage. The coadsorption leads to additional build-up of oxyfluoride in the
oxide layer and may reduce reliability for connections made
at that bond pad. The cleaning of bond pads is successfully
done with in-situ plasma cleaning during the plasma dicing
sequence as demonstrated in previous work.VII This paper
focuses on wet cleaning solutions.
One of the motivations of this paper is to look at
compatibility of the TSV sidewall removal solution to an
on-tape process which can help in the overall productivity
and cost of ownership by offloading the cleaning process
from the utilization of the plasma dicing process chamber.
Since plasma dicing is a die singulation technique, the wafer
is adhered to a dicing frame and dicing tape for easy
handling. A wet cleans solution also must be compatible
with the adhesive used to adhere the die after singulation.
In many instances, the materials used as adhesives are of a
similar chemical composition to standard polymers used in
resists. To deal with the chemical similarities between the
passivation and the polymers used for adhesives and create a
robust cleaning process, an understanding of the integrated
process, including the prior and subsequent process steps,
the singulated die size, the tool used, the process needed, the
time/temperature profile, the rinse process, etc. is critical as
all these variables contribute to the success or failure of the
overall process.
Additional challenges related to cleaning using a wet
solution include compatibility with device materials,
including Al, Cu, or alloys of the two, as well as inorganic
and organic encapsulating materials such silicon nitrides,
oxides, oxynitrides, polyimides (PIs), and polybenzoxazoles
(PBOs).
EXPERIMENTAL
Four full thickness 200mm test wafers with 15µm unified
streets and 1mm2 square die were employed in this work.
They all had a photoresist (PR) mask approximately 6µm in
thickness. The wafers were etched with different conditions
in order to understand the effect of the solvent on the
polymer removal. The process employed was always a
typical Bosch process with only variations of the polymer
passivation step time. All the wafers were partially etched to
a depth of ~ 300µm. Post etch, in situ O2 plasma treatment
was performed on some of the wafers in order to remove the
PR mask and to evaluate polymer removal after O2 plasma
clean. Table 1 summarizes the process conditions.
The wafers were cleaved into smaller samples and
immersed in a beaker-type test bath using Dynastrip
DL9150 with low agitation at 70˚C for varying times.
Several characteristics of this solution were measured to
ensure its general use in cleaning tools with a variety of
designs and to understand maximum safe operating
temperatures. A summary of the solvent characteristics are
shown in Table 2.
Wafer
number
#1
Etch Process Conditions
Standard processing conditions; PR mask left
intact on wafer
#2
Enhanced passivation step; PR mask left intact
on wafer
#3
Standard processing conditions; PR mask
stripped by in situ O2 plasma treatment
#4
Enhanced passivation step; PR mask stripped
by in situ O2 plasma treatment
Table 1. Test wafer description
Flash
Specific
Viscosity Viscosity
point
Gravity
@25°C,
@38°C,
(°C)
(g/mL)
(cP)
(cP)
90
1.085
3.4
2.6
Table 2. Characteristics of Dynastrip DL9150
Surface
tension
(mN/m)
38.9
After the DL9150 cleaning step, the samples were rinsed
sequentially using DI water spray and IPA and then dried in
a stream of N2 at room temperature. Inspection of the
samples was completed using SEM and EDX. EDX
analysis was performed at the top, middle, and bottom of the
sidewalls, as shown in Figure 5, to provide some indication
of polymer uniformity before and after solvent cleaning
step. The fluorine content, which is a component of the
passivation layer formed during the DRIE process using
C4F8, was measured by EDX in order to quantify the
efficiency of the solvent for polymer removal. Carbon
levels have not been used as an indicator of polymer residue
removal because of the presence of adventitious carbon in
the SEM vacuum chamber, which cannot be removed and
complicates measurements. As a result, only the fluorine
content has been measured from the EDX spectra in order to
understand if the samples were cleaned or not after
immersion in the Dynastrip DL9150 formulation.
Figure 5. Approximate EDX analysis locations.
In order to understand the baseline fluorine content in the
SEM vacuum chamber, an as-received, unprocessed Si
substrate was analyzed by EDX. A content of 2 wt% was
detected indicating the presence of a fluorine background
signal due to residual contaminants in the chamber (Figure
6). This value was subtracted from the actual signal
obtained from the processed samples in order to effectively
estimate the amount of fluorine content after polymer
removal with the Dynastrip DL9150 solvent.
Figure 6. SEM image of pristine cleaved Si surface used
for collecting background EDX spectrum that exhibits 2
wt.% F.
RESULTS
Figure 7 (a) shows a typical SEM cross-sectional image of
Test Sample #1. Three EDX spectra have been taken as
described before, in order to quantify the fluorine content
before cleaning with Dynastrip DL9150, Figure 7(b). The
fluorine content measured in this case is 40 wt%, 34 wt%,
and 21 wt% at the top, middle and bottom, respectively.
Figure 7. Test Sample #1. (a) cross-sectional SEM image
before clean, (e) EDX data from top, middle and bottom of
sidewall showing initial fluorine concentration, (f) crosssectional SEM image after clean, (g) EDX data from top,
middle and bottom of sidewall indicating fluorine removal
after clean.
After cleaning at 70˚C for 60 min in Dynastrip DL9150 , the
fluorine content was dramatically reduced to the non-detect
level when chamber baseline levels of 2 wt% are factored in
(Figure 7 (c) and (d)). The side wall passivation deposited
during etch was effectively removed by the solvent
formulation in the top, middle and bottom of the trench.
Figure 8 shows a cross-sectional image of Test Sample #2.
The purpose of this test sample was to generate a test
vehicle with large amounts of passivation layer after etch
and study the effect on the cleaning process. Again in this
test case, the photoresist mask was not removed in a
subsequent ash process. As the EDX results summarized in
Table 3 confirm, this sample has the greatest amount of
passivation residue on it after etching, and with the residue
thickly coated across the sidewall. After cleaning, the
passivation deposited during etch was removed. Figure 8(a)
shows the SEM cross-section of an etched sidewall prior to
cleaning and the EDX analysis showing the presence of
fluorine-containing residue at the top (39% F), middle (42%
F) and bottom (37% F) of the sidewall is shown in Figure
8(b).
Figure 8. Test Sample #2. (a) Cross-sectional SEM image
before clean, (b) EDX data from top, middle and bottom of
sidewall showing initial fluorine concentration, (c) crosssectional SEM image after clean, (d) EDX data from top,
middle and bottom of sidewall indicating fluorine removal
after clean.
Figure 9. Test Sample #3. (a) Cross-sectional SEM image
after etch and plasma ash, before clean, (b) EDX data from
top, middle and bottom of sidewall showing initial fluorine
concentration, (c) Cross-sectional SEM image after clean,
(d) EDX data from top, middle and bottom of sidewall
indicating complete fluorine removal after clean.
Analogous analyses were completed after cleaning and are
shown in Figure 8(c) and 8(d). In these images, the
polymeric passivation residue has been completely
removed. The impact of increased passivation residue was
to increase the cleaning process immersion time required to
completely remove the residue.
Figure 9 shows a cross-sectional image of Test Sample #3.
This test sample was generated using a test vehicle with
standard amounts of passivation layer after etch but also
including an additional ash step to remove the photoresist
mask and some of the passivation residue. The effect on the
cleaning process was studied.
As the EDX results
summarized in Table 3 confirm, this sample has lower
concentration of passivation residue on it after etching and
ashing, as measured by the fluorine concentration. After
wet cleaning, all residues were removed. Further detail of
the cleaning can be found in Figure 9. Figure 9(a) shows
the SEM cross-section of an etched sidewall prior to
cleaning and the EDX analysis showing the presence of
fluorine-containing residue at the top, middle and bottom of
the sidewall is shown in Figure 9(b). Analogous analyses
were completed after cleaning and are shown in Figure 9(c)
and 9(d). In these images, the polymeric passivation residue
has been completely removed. This sample had the lowest
initial amount of fluorine (via top, 6%; via middle, 3%; via
bottom, 2%) and required the shortest cleaning process
immersion time (10 minutes) to completely remove the
residue.
Figure 10 shows a cross-sectional image of Test Sample #4.
The purpose of this test sample was to generate a test
vehicle with large amounts of passivation layer after etch
but remove the photoresist mask and some of the residue
using a post-etch ash process. The samples provide an
opportunity to study the effect of higher concentrations of
passivation residue and with a subsequent ash process, on
the wet cleans process. As the EDX results summarized in
Table 3 confirm, this sample has its greatest amount of
passivation residue (22wt%, 10wt%, 3wt%) at the top of the
sidewall. After cleaning, all passivation on the sidewall was
removed. Further detail of the cleaning can be found in
Figure 10. Figure 10(a) shows a cross-sectional SEM image
of an etched sidewall prior to cleaning and the EDX analysis
showing the presence of fluorine-containing residue at the
top, middle and bottom of the sidewall is shown in Figure
10(b). Analogous analyses were completed after cleaning
and are shown in Figure 10(c) and 10(d). In these images,
the polymeric passivation residue has been completely
removed.
This sample had similar initial amount of
fluorine to Test Sample #1 and required commensurate
cleaning process immersion time to completely remove the
residue.
In applications where a photoresist mask is needed to define
the etch feature, the residual photoresist must be removed.
In non-Bosch etch processes, the standard method is to
employ a gas-based resist ashing step, followed by a wet
chemical clean step to remove all post-etch residue, since
photoresists and post etch residues may be chemically
dissimilar and difficult to remove using a single solution.
This study offered an opportunity to determine if the
photoresist mask residue after etch could be removed in a
single step process along with the post-Bosch sidewall
polymer. As described in Table 1, Test Samples #1 and #2
had a PR etch mask intact after etch. Test Samples #3 and
#4 had the PR resist mask removed by ashing and the
surface was observed for defects after wet strip using postBosch process sidewall removal. Figure 11 shows analyses
of the clean sample surface after cleaning in Dynastrip
DL9150 to remove the sidewall polymer. Simultaneous
removal of the photoresist mask and the fluorinated sidewall
residue was observed, opening up the possibility of a onestep resist and etch residue removal process and further
enhancing CoO improvements.
Figure 10. Test Sample #4. (a) Cross-sectional SEM
image after etch and plasma ash, before clean, (d) EDX data
from top, middle and bottom of sidewall showing initial
fluorine concentration, (e) Cross-sectional SEM image after
clean, (f) EDX data from top, middle and bottom of sidewall
indicating complete fluorine removal after clean.
Figure 11. (a) Test Sample #1. Top down SEM image of
wafer surface after cleaning using Dynastrip DL9150 and
EDS analysis showing removal of PR; (b) Test Sample #2.
Top down SEM image of wafer surface after cleaning using
Dynastrip DL9150 and EDS analysis showing removal of
PR; (c) Test Sample #3. Top down image of wafer surface
after cleaning using Dynastrip DL9150 using SEM and EDS
analysis showing removal of PR; (d) Test Sample #4. Top
down SEM image of wafer surface after cleaning using
Dynastrip DL9150 and EDS analysis showing removal of
PR.
Table 3 summarizes the results from these experimental
studies. For the standard plasma dicing process (Test
Sample #1), the Dynastrip DL9150 has a comparable
performance to the O2 post etch plasma clean (Test Sample
#3) indicating that a wet solution is a viable alternative to a
time consuming O2 plasma clean that could increase the cost
of ownership. Increased passivation step times, have shown
an increase in the fluorine content (Test Sample #2) and
Test
Sample
#1
#2
#3
#4
Process (Temp
(°C)/time
(min)
70/60
70/120
70/10
70/60
Sidewall Condition,
Pre-Clean
(F wt%, measured using EDX*)
top
40
39
6
22
middle
34
42
3
10
Sidewall Condition,
Post-Clean
(F wt%, measured
EDX*)
bottom
21
37
2
3
top
ND
ND
ND
ND
middle
ND
ND
ND
ND
Result
using
bottom
ND
ND
ND
ND
Clean
Clean
Clean
Clean
* Fluorine baseline content of 2 wt% removed from these measurements. ND = not detected
Table 3. Summary of Cleaning Results
even after O2 plasma clean (Test Sample #4), the fluorine
content has not been efficiently removed. The Dynastrip
DL9150 has shown also in this case a complete fluorine
removal on both Test Samples #2 and #4. The clear benefit
of this solvent cleaning solution is made apparent in the
applications where significant polymer passivation exists on
the sidewalls.
As expected, when more passivation was present at the end
of the etch process, additional time of the sample immersed
in the solution was needed to remove the residues.
SUMMARY AND CONCLUSIONS
As plasma dicing applications are amalgamated into wafer
processing schemes, cleaning solutions with good
throughput and reasonable CoO are needed. One such
solution is a wet clean process in which Dynastrip DL9150
is used to remove the highly fluorinated passivation left
after etch. Options exist to have the process remove the
photoresist mask as well as the etch residue in one step, or
to match the wet process with a simplified ash process to
generate a clean, fluorine residue-free wafer. This work has
shown good feasibility for wet cleans. Work is on-going.
with the next focus on process optimization and dicing tape
compatibility to yield a robust integratable and high yield
die singulation process.
i
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