Colorado School of Mines
PHGN/CHGN 435
Modules 1 and 2
Authors:
Garrick Johnson
Trevor Haak
Carl DuBois
Ethan Palay
February 10, 2015
1
Introduction and Background
Integrated circuits are an integral part of almost all modern electronics. Fabrication has become exceedingly
cost effective due to the incredible amount of circuit components that can be constructed per unit operation.
As of 2015, industry can design integrated circuits with on the order of 1010 transistors per chip, and create
on the order of 105 chips per wafer. Photolithography acts as a map for the intricate layout of transistors,
resistors, and metal interconnects within a chip, and allows for features as small as 30nm at the time of
this publication. Photolithography uses UV-light incident on a thin film of photoactive material through
a set of masks in order to pattern the semi-conducting substrate. The patterned photoactive material on
the substrate allows for etching of the substrate, etching of the thermal oxide layer, and deposition of metal
interconnects.
The authors intend to construct a rudimentary integrated circuit from a 3 inch silicon wafer in the
microprocessing laboratory at the Colorado School of Mines. First, various equipment and processes must be
appropriately characterized and optimized. In this publication, thermal oxidation, photoresist application,
and reactive ion etching are characterized in order to optimize SiO2 thickness, photoresist thickness and
uniformity, and SiO2 etch depths and profiles.
Thermal oxides growth involves the introduction of an oxidizing agent (oxygen or water vapor) to a wafer
(silicon) at elevated temperatures ranging between 800-1200 degrees Celsius. Elevated temperatures allow
the oxidizing agent to diffuse from the surface to the bulk of the material and react with the wafer[1]. This
experiment diffuses oxygen through crystalline silicon to produce a silicon dioxide layer according to the
reaction:
Si(solid) + O2 (gas) → SiO2
The thickness of the oxide layer is a function of initial oxide thickness, temperature, and duration.
Photoresist is a photoactive material that is resistant to oxidation. The unique properties of photoresist
give simple methodology for a complicated integrated circuit design. A layer of photoresist is deposited
uniformly on the surface of a silicon wafer using a spin-coater. Photoresist thickness is a function of spin
speed and duration.
Photoresist can then be etched using reactive ion etching (RIE) in order to make patterns on a chip.
In contrast with isotropic, wet chemical etching, RIE is a dry etching technique with high anisotropy.
Anisotropic etching of the photoresist is critical for resolved patterning of the integrated circuit. This is
achieved via vertical ion bombardment in vacuum conditions. In this study, oxygen ions were used to etch
photoresist. It is critical to characterize the etch rate to ensure that all of the desired photoresist is etched
away. Even a small layer of photoresist left can completely protect the underlying layer of silicon. Etch rate
is a function of etch power.
Layer thicknesses were characterized using ellipsometry and surface profilometry. Optimized thermal
oxide growth temperatures, reactive ion etching power, and photoresist spin-coater spin-speed are presented.
2
2.1
Methods
Module 1
The purpose of module 1 was to determine the effect of furnace temperature and ambient on the oxidation
of silicon wafers for semiconductor processing. Clean silicon wafers were placed in two separate ovens that
performed oxidation with dry and wet gas at temperatures ranging from 950◦ C to 1100◦ C in 50◦ C increments.
In order to have silicon substrates of a reasonable size to work with and to use the silicon efficiently, the
silicon wafers were divided into quarters using a diamond scribe.
2.2
RCA Clean
An RCA clean was used to remove impurities from the surface of the silicon so that the oxide can grow
on pure silicon. The RCA clean consists of three steps: an organic clean, oxide strip and ionic clean. The
organic clean was made as a solution containing a 5:1:1 ration of deionized water, hydrogen peroxide and
ammonium hydroxide respectively and held at 75◦ C in a water bath. The silicon substrates were placed in
1
this solution for 10 minutes. This solution removes any organic particles on the surface of the material, but it
leaves a thin oxide layer and attracts ionic metal contaminants to the surface. In order to get rid of the oxide
layer and some of the metal contaminants an oxide strip was performed using a solution of 20:1 deionized
water to hydrogen fluoride. This solution was kept at room temperature, and the samples were submerged
for 15 seconds. The third step of the RCA clean, the ionic clean, removes the remaining ionic contaminants
on the surface of the silicon. This solution consisted of a 5:1:1 ratio of deionized water, hydrogen peroxide,
and hydrochloric acid and was held at 75◦ C in a water bath. The samples were submerged for 10 minutes.
Between each step, the silicon substrates were placed in beakers of deionized water for 1 minute to remove
any traces of the previous cleaning solution. The substrates were then blown dry with nitrogen gas and
transferred to the furnace for oxidation.
2.3
Dry and Wet Oxidization
The Dry and Wet oxidation were performed in a thermal processing furnace. In both furnaces, oxygen gas
was allowed to flow into the chamber. The difference between the two methods is that in the wet oxidation,
deionized water is heated to a boiling temperature on a hot plate, and the water vapor is piped into the
chamber. The water vapor speeds the growing process of the oxide.
The samples were placed in the chambers on a quartz sample holder, and the chamber was set to heat up
to the desired temperature at a rate of 20◦ C per minute. After the chamber reached the desired temperature,
it was kept there for 1 hour before the chamber began cooling down. The oxygen gas and water vapor was
allowed to flow for 30 minutes while the sample was cooling down before being shut off. The samples were
left overnight and removed the next day.
Expected thermal SiO2 thicknesses were calculated using Brigham Young University’s oxide growth
calculator which is based off of the Deal-Grove model.[2] [3] Expected wafer thicknesses based on color were
determined from Wolf’s color chart for thermally grown SiO2 films.[4]
2.4
Ellipsometry
After the photoresist application process, the samples were placed in an ellipsometer to determine the
thickness of the photoresist. An ellipsometer has an angled laser beam that reflects off of the surface of
the sample into a detector which measured the final polarization state. The polarization state is affected
because the light that travelled though the outside material layer and reflected from the second interface will
interact with the light that simply reflected from the outside layer of the sample. Software on the computer
uses these polarization measurements to calculate parameters such as the thickness of individual layers on
the sample and the refractive indices. This process was used to measure the thickness of the photoresist at
four different locations on the silicon substrates.
2.5
Module 2
For module 2, photoresist was spun on to silicon wafers in a spin coater, and some of the photoresist was
removed in a plasma etching process. The purpose of this module is to determine the effect of spin speed on
the uniformity and thickness of the applied photoresist layer and the relationship between the power setting
on the plasma etcher and etch rate.
2.6
Photoresist Application
A standard AMI (Acetone, Methanol, Isopropanol) clean degreasing procedure was used to remove organic
contaminants and drive off moisture to allow for good wafer-photoresist adhesion. Plastic tweezers were used
to hold bare silicon wafer quarters, labeled 2A, 2B, 2C, and 2D, over an empty beaker in a fume hood. A
wash bottle was used to spray the wafer for 30 seconds in order to drive off contaminants. The process was
immediately repeated using methanol and isopropanol respectively. An inert, compressed air, nitrogen gun
was used to remove the isopropanol from the surface of the wafer. The isopropanol evaporates quickly, so it
is important that the nitrogen gun is used immediately after the AMI clean. If streaks or smudges were left
on the wafer, isopropanol was reapplied then subsequently re-dried to leave the surface free of contaminants.
2
A hotplate was used in order to ensure no moisture remained on the surface after the AMI clean. The
wafer was placed on a 115 degree Celsius hotplate for 60 seconds.
The Brewer Science Cee Model 100 Spin Coating System was used for the application of photoresist.
The wafer quarters were placed in the center of the vacuum chuck, and centered. The machine was then
programmed with the following parameters:
Spin Speed 1 (RPM)
Spin Speed 2 (RPM)
500
Varied (2000-5000)
Acceleration 1 (RPM/s)
Acceleration 2 (RPM/s)
100
1000
Time 1 (s)
Time 2 (s)
9
40
Room temperature Shipley 1813 positive photoresist was applied to the surface of the wafer using a
disposable pipette such that approximately 90 percent was covered. The recipe was carried out with the
hatch closed. For a more uniform film, the hole at the top of the hatch was plugged to avoid air currents
from interfering with the photoresist dispersion. The second spin speed was varied between 2000 and 5000
RPM between iterations. This allowed for quantification of expected photoresist film thickness as a function
of spin speed. The authors ran all four wafer quarters at 3000 RPM, but data from other classmates was
included for spin speeds between 2000 and 5000 RPM. Data was compared to expected film thicknesses
published in the manufacturer’s data.
After photoresist application, a soft-bake is necessary to drive off solvents, improve adhesion, and anneal
away stress. This gives the photoresist film better durability. The wafer quarters were placed on a hotplate
at 115 degrees Celsius for 90 seconds.
2.7
Reactive Ion Etch
To remove excess photoresist from the surface of the silicon wafers, a plasma etch (reactive ion etch) was
performed. The silicon samples with photoresist were placed in the plasma etcher, and a glass sheet was
placed over half of the sample so that the amount of photoresist removed could be measured. The power
setting on the etcher was varied from 125W to 200W in 25W increments and the etching time was varied to
determine the etch rate.
In order for the plasma etcher to work, ions must be present in the chamber to bombard the samples
with. For our silicon samples, oxygen gas was used, and the pressure of the chamber was set to be around
400mtorr.
2.8
Profilometry
After the photoresist was etched, the sample was placed in a profilometer. The profilometer drags a stylus
across the surface of the sample to measure relative thicknesses, and when it was dragged across the seperation
between the etched and unetched parts of the sample the amount of photoresist removed was found.
3
Results and Discussion
In order to determine how spun photoresist thickness depends on spin speed, photoresist thickness was
measured using an ellipsometer. Resist was spun at speeds of 2000, 3000, 4000, and 5000rpm. Five samples
were spun and measured at 3000rpm. Additionally, two samples were spun at 4000rpm and one sample was
spun at 2000rpm. Data is shown in Figure 1 with an 85% confidence interval.
3
Figure 1: Photoresist thickness decreases as a function of spin speed. Averaged data points (blue diamonds)
fall on the expected curve (purple dotted line). Thickness from a 2000 rpm spin (red square) was only done
once, so does not have error bars. It’s deviation from the expected curve is called into question due to the
lack of data at this spin speed.
As expected, the resist thickness data generally follows an exponential falloff/decay with increased spin
speed. However, the sample spun at 2000rpm does not follow this trend and deviates slightly from the
manufacturer’s curves. Since only one sample was spun at 2000rpm, it is hard to determine if the average
resist thickness at such a spin speed matches the manufacturer’s curves. Considering the size of the error
for the other spin speeds, it is reasonable to expect the 2000rpm average resist thickness to fall near the
expected thickness of roughly 18500Å. Average thicknesses for all other spin speeds match nicely with the
manufacturer’s curves (for S1813 photoresist thickness as a function of spin speed)(citation). Since a small
number of samples were spun at these other spin speeds of 3000-5000rpm, a large level of error exists in
the data for photoresist thickness at the 85% confidence level. Additionally, the samples did not visibly
appear to have a uniform coating of photoresist, and the color of the sample varied from the center to the
edge. Non-uniformity for photoresist application without coverage of the hole for inserting spin solutions
was verified with profilometer measurements. Because of this, the ellipsometry measurements were taken at
the center of all of the substrates.
Etching/Ashing rate was determined as a function of power. Etch rate is plotted versus power in Figure
2, at a confidence level of 90%. In order to calculate etch rate, a profilometer was used to measure a
step height difference in resist thickness between masked and unmasked areas. Etching was carried out at
different lengths of time ranging from 30-200 seconds at discrete powers of 125, 150, 175, and 200 W. A
discrepancy exists for data collected at 125W. The data collected at this power was reported to have been
actually collected at 175W. However, the RIE was also reported to not be producing any step changes in
resist at 125W from another group. The data (originally reported as 175W) was plotted at 125W in order
to determine if reported powers were mixed up and if there was any trend that could provide evidence of a
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mix-up.
Figure 2: The etch rate of photoresist on silicon substrates for a reactive ion etch process as a function of
the power of the plasma etcher.
Generally, the average etch rate follows a logarithmic relationship, increasing with RIE power. Average
etch rates appear to level off at roughly 58Å/s past 175W. Larger RF power indicates that a larger voltage
was applied between the parallel plates. With a larger voltage, oxygen ions have higher kinetic energy, which
resulted in a faster etch rate, as expected. The logarithmic taper most likely was due a tapering unionized
oxygen number density because all of the oxygen molecules became quickly ionized with high RF power.
However, since error is large for the highest power, 200W, this behavior is not guaranteed. Error in etch
rate increases for each power from 150-200W.
In order to characterize the dry oxidation furnace, dry oxide thickness versus oxidation temperature is
plotted in Figure 3. A published curve for the Deal-Grove model of expected oxide growth (on silicon) is also
plotted in Figure 3. Oxide thickness was measured using an ellipsometer following oxidation. As expected,
the oxide thickness data generally exhibits an exponential dependence on oxidation temperature given by
the following relationship.
◦
thickness(Å) = .994e0.00645T (
◦
C)
(1)
For data at 950, 1050, and 1000 C, measured dry oxide thickness consistently falls above the curve
for expected thickness. Since the thermal oxidation ovens were set with a slow ramp time in an oxygen
environment, the samples most likely oxidized before and after the designated 1 hour of dwell time at the
corresponding temperature. This would lead to a higher oxide thickness than expected, and may explain
such a trend in the data.
5
Figure 3: The dependance of dry oxide growth thickness on the oven temperature.
The dry oxide thickness collected at 1000◦ C does not follow with the rest of the data. A discrepancy
exists over whether the sample was actually run under wet oxidation conditions as opposed to dry oxidation
conditions. Currently, the source of this deviation is unknown. As these samples were both oxidized in the
same time frame, separate from other samples, their data may be ruled as questionable. One reasonable
possibility for this large discrepancy is confusion over different mass flow controllers. The MFCs connected to
the process furnaces display old, incorrect labels for channel number. If the channel numbers were confused,
and oxygen was actually flowing through a different channel the entire time, then oxygen would continue
to flow even after it was thought to be turned off. If the oxygen flowed during the cooling of the sample,
oxide would continue to rapidly grow on the surface. A dry oxidation time of roughly 4.5 hours at a constant
1000◦ C would be required to achieve the same thickness as was measured for the samples reportedly oxidized
for 1 hour. Even with a gradual and natural ramping down of temperature, the cooling process may take
longer than 4.5 hours, and oxidation would still occur.
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Figure 4: The dependance of wet oxide growth thickness on the oven temperature.
In an attempt to characterize the wet oxidation furnace, wet oxide thickness versus oxidation temperature
is plotted in Figure 4. Wet oxide thickness data that was reported as correct is displayed as blue diamonds
at 1050 and 1000◦ C. Red circles at 1100◦ C indicate data that was originally collected at that oxidation
temperature but was later reported to be incorrect. Red circles at 1000◦ C mark the questionable data from
the dry oxide runs at the same temperature. It is plotted here along with wet oxide data in order to determine
whether or not the samples were actually run through a wet oxidation. The ellipsometry-measured oxide
thickness for these samples does not fall close to the expected thickness as predicted by Deal-Grove behavior.
Initially, this plot does not offer anything conclusive about the wet oxidation process. However, when the
colors of the samples were analyzed, the results seem to more closely follow expected Deal-Grove behavior.
The green plus (+) marks indicate what thickness is expected for the reported color of the wet oxidation
samples. Since these markers fall close to the Deal-Grove model, perhaps the thickness of the wet oxide
samples was not correctly measured in the ellipsometry process.
4
Conclusion
Based on the results and data collected following the procedures outlined for Modules 1 and 2, there are
several conclusions and reasons for discussion.
According to the manufacturer’s specifications resist thickness is a function of spin speed. In general
results agreed with the expected film thicknesses for spin speeds between 3000 and 5000 rpm. While the spin
speed plays an important factor in achieving a desired resist thickness, perhaps other factors may also play
a crucial role. These may include the acceleration times used to reach the specified spin speed, which are
not indicated on the manufacturer’s spec sheet, and were set according to previously established guidelines
stated in the procedures for the respective Module. Adjusting the ramp time may play a direct role in resist
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thickness.
Additionally, the manner in which the photo-resist is applied has a direct influence on uniformity. For
example, if the hole in the top of the spin coater is not plugged during the spin process the resist develops a
distinct tie-dyed pattern indicating a non-uniform resist pattern which may give rise to a large degree of a
variation in the measurements, assuming the measurements using ellipsometry are accurate. However, if the
hole is plugged a more uniform resist thickness is achieved. Taking these subtle differences into consideration
is an important part of establishing an optimum procedure for application of photo-resist and achieving a
desired and reproducible thickness. Application of photo-resist tends to be somewhat of an art form, and as
much as it depends on pre-defined parameters it also depends on trial and error.
One of the objectives for Module 2 was to determine the etching rate (ashing) of photo-resist in an oxygen
plasma as a function of power, at a given flow rate and pressure. For each group, etching was performed
at a fixed pressure of 400mtorr, and a different power setting. With larger power came larger uncertainties
in the etch rate. For this reason a lower power of 150 W is suggested to run at a longer time, rather than
a higher power for a shorter amount of time, for a desired etch depth in order to minimize error. When
looking at the results and data for each group individually and as a whole, we can see that there is a general
logarithmic trend, and as can be expected an increase in power tends to increase the etching rate.
For the wet and dry oxidations, one objective of the experiment was to determine oxidation kinetics as a
function of temperature. Each group was to perform a wet and dry oxidation for a specified temperature, for
a pre-established flow-rate, in an oxygen environment. Oxidation rates can be found published elsewhere,
however consideration should be made with respect to the actual oxidation atmosphere, and in particular the
variation in the type of equipment used. When comparing the data and results, to that of expected values for
oxide thicknesses with respect to a given set of parameters we can see that there is substantial deviation. This
may ultimately be due to the size of the oven for a given flow rate, provided that the specified temperatures for
the experiment were maintain throughout the oxidation. In addition, upon further investigation, whether the
equipment was set up correctly seems to be the predominant concern, which would give rise the inconsistent
data for a given set of parameters. The measurements for the oxide thickness and correlation between the
color of the oxide and actual thickness suggests that the measurements were correct, and that perhaps the
actual oxidation parameters where different from those intended. The relationship between oxide thickness
and temperature specific to our equipment and process is given by equation 1. Additionally accurate data
reporting is essential.
In summary, while the processes are outlined in the procedures, having an intimate working knowledge
and experience with the specific equipment used, while fully understanding the parameters that play a
primary role in processes involved are crucial for obtaining accurate and reproducible results.
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2522
2522
3873
3873
5189
5189
6411
6411
Expected Oxide
Thickness (Angstrom)
Expected Oxide
Thickness (Angstrom)
220
220
400
400
663
663
995
995
Appendix A. Oxidation Thickness and Color Analysis
25
25
25
25
25
25
25
25
Duration Initial Oxide Thickness
(hr)
(Angstrom)
1
1
1
1
1
1
1
1
Initial oxide thickness
(Angstrom)
Dry
Temperature
(°C)
Duration
(hr)
25
25
25
25
25
25
25
25
950
950
1000
1000
1050
1050
1100
1100
Temperature
(°C)
1
1
1
1
1
1
1
1
Wet
950
950
1000
1000
1050
1050
1100
1100
Expected Color
Tan
Tan
Tan
Tan
Tan-Brown
Tan-Brown
Dark Violet-Red Violet
Dark Violet-Red Violet
Expected Color
Orange to melon
Orange to melon
Yellow
Yellow
Blue-Green
Blue-Green
Carnation Pink
Carnation Pink
Measured Oxide
Thickness
505
452
1295
1318
824
832
1225
1253
Measured Oxide
Thickness
N/A
N/A
N/A
N/A
1087
540
196
635
Actual Color
Dark Blue
Dark Blue
Dark Blue
Metallic Blue
Brown
Dark Violet
Royal Blue
Royal Blue
Actual Color
N/A
N/A
N/A
N/A
Light Blue
Gold
Yellow-Pink
Yellow-Green
Expected Thickness
Based off Color
1200
1200
1200
1500
700
1000
1200
1200
Expected Thickness
Based off Color
N/A
N/A
N/A
N/A
4900
5700
5800
5600
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References
[1] Campbell, Stephen A.. (2008). Fabrication Engineering at the Micro- and Nanoscale (3rd Edition).
Oxford University Press. Online version available at: https://app.knovel.com
[2] B.E. Deal, A. S. Grove, ”General Relationship for the Thermal Oxidation of Silicon,” J. Appl. Phys.,
36, 3770 (1965)
[3] Oxide Growth Calculator. (1994,
January 1).
http://www.cleanroom.byu.edu/OxideThickCalc.phtml
Retrieved
February
1,
2015,
from
[4] S. Wolf and R.N. Tauber, Silicon Processing for the VLSI Era: Volume 1 - Process Technology, Lattice
Press, 1986.
[5] Shipley Microposit S1800 Series Photo Resists. (1993, January 1). Retrieved February 1, 2015, from
http://inside.mines.edu/ sagarwal/phgn435/shipley1800photoresist.pdf
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