U.S. patent application number 09/254532 was filed with the patent office on 2002-08-08 for reactive ion etching of silica structures.
Invention is credited to BAZYLENKO, MICHAEL, GROSS, MARK.
Application Number | 20020104821 09/254532 |
Document ID | / |
Family ID | 3797129 |
Filed Date | 2002-08-08 |
United States Patent
Application |
20020104821 |
Kind Code |
A1 |
BAZYLENKO, MICHAEL ; et
al. |
August 8, 2002 |
REACTIVE ION ETCHING OF SILICA STRUCTURES
Abstract
The invention relates to a method for etching of silica-based
layers/substrates by reactive ion etching system (10) using an
etching gas mixture of CHF.sub.3/AR through a photoresist mask.
Reactive ion etching is carried out under conditions of
simultaneous isotropic deposition of a carbon-based polymer where
the polymer deposition rate is controlled by adjusting process
control parameters of RF power, sample temperature, O.sub.2 and
CF.sub.4 additions.
Inventors: |
BAZYLENKO, MICHAEL;
(FORESTVILLE NSW, AU) ; GROSS, MARK; (SEAFORTH
NSW, AU) |
Correspondence
Address: |
MERCHANT & GOULD PC
P.O. BOX 2903
MINNEAPOLIS
MN
55402-0903
US
|
Family ID: |
3797129 |
Appl. No.: |
09/254532 |
Filed: |
June 1, 1999 |
PCT Filed: |
October 3, 1997 |
PCT NO: |
PCT/AU97/00663 |
Current U.S.
Class: |
216/24 ;
216/80 |
Current CPC
Class: |
H01J 37/32082 20130101;
G02B 2006/12173 20130101; G02B 2006/12176 20130101; H01J 2237/334
20130101; C23C 16/30 20130101; C03C 15/00 20130101; G02B 6/136
20130101 |
Class at
Publication: |
216/24 ;
216/80 |
International
Class: |
C23F 001/00 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 4, 1996 |
AU |
PO 2818 |
Claims
We claim:
1. A method for etching of silica-based glass layers or substrates
comprising reactive ion etching through a mask executed under
conditions of simultaneous isotropic deposition of a carbon based
polymer.
2. The method of claim 1 wherein the polymer deposition rate or/and
its steady-state thickness on different surfaces of the etched
structure is controlled by adjusting one or several process control
parameters in order to control etched profile, dimension loss,
sidewall and bottom etched surface roughness, and etching
selectivity between the silica-based layer and mask material.
3. The method of any previous claim wherein gas or a mixture of
gases is used, which contain fluorine and carbon atoms.
4. The method of any previous claim wherein a photoresist mask is
used.
5. The method of claims 1 to 3 wherein a non-photoresist mask is
used.
6. The method of claim 5 wherein amorphous silicon is the mask
material.
7. The method of any previous claim wherein the Rf power coupled
into the discharge is the adjusted parameter.
8. The method of any previous claim wherein the substrate
temperature is the adjusted parameter.
9. The method of claim 8 where the temperature is adjusted to
achieve low sidewall roughness and low dimension loss at the same
time.
10. The method of any previous claim wherein a resputtering of any
metal present wihtin or/and in contact with the discharge zone is
prevented.
11. The method of any previous claim wherein reactive ion etching
is performed in a high plasma density hollow cathode etching
system.
12. The method of any previous claim wherein the etching gas
mixture is CH.sub.3F and Argon.
13. A product made by the method of any previous claim.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to creation of silica
structures and in particular to the reactive ion etching of such
structures.
BACKGROUND OF THE INVENTION
[0002] Silica-based channel waveguides, fabricated on silicon or
silica wafer substrates, are potential building blocks of planar
lightwave circuits (PLCs) that are becoming increasingly important
for telecommunications systems. While there are a number of thin
film techniques that have been used to deposit silica waveguiding
layers (flame hydrolysis, chemical vapour deposition and plasma
enhanced chemical vapour deposition (PECVD)), almost all reported
waveguide fabrication schemes have used reactive ion etching (RIE)
to delineate the channel waveguide (core) geometry. RIE is also
commonly used in integrated optics, and in planar waveguide
fabrication in particular, for etching light turning mirrors. It
further finds use in the creation of other Micro Electro Mechanical
Systems (MEMS).
[0003] RIE of silica glass in integrated circuit (IC) manufacture
is a well established and routine process with, for example,
CHF.sub.3 based mixtures being used to obtain high selectivity over
photoresist. Although basically similar, the silica films used in
planar waveguides have several unique differences which influence
the development of suitable RIE processes. Firstly, the thicknesses
of silica in waveguide devices can be as much as 5 to 10 .mu.m, as
opposed to typically less than 1 .mu.m in IC technology. This
places extra demands on mask thickness and/or material
selectivities, as well as on the silica etch rate which should be
high enough to obtain reasonable throughput. Different masking
materials for waveguide etching such as photoresist, amorphous
silicon (a-Si) and chromium have been reported. Generally, the use
of non-photoresist masks allows for larger etching depths and
silica etch rates.
[0004] The roughness of the etched walls of the waveguide
structures or light turning mirrors should ideally be as small as
possible in order to reduce the loss due to light scattering. A
number of works on sidewall roughness reduction for etching with
photoresist masks have been reported. In these cases, however, the
etching depth of a SiO.sub.2 layer was restricted to around 1
.mu.m. Etched profile control is also important and some slope in
the etched profile is sometimes desirable in order to facilitate
filling of the gaps between closely spaced waveguides during
cladding deposition. Profile slope is normally achieved by
controlled photoresist mask erosion. Despite a number of published
works on different aspects of the RIE of silica for waveguide
fabrication, it is unclear as to the effect of all relevant
parameters, such as etch rates, sidewall roughness, profile slope
and the relationship between them
SUMMARY OF THE INVENTION
[0005] It is an object of the present invention to provide for the
development of a high-rate silica RIE process suitable for low
temperature waveguide fabrication.
[0006] In accordance with the first aspect of the present invention
there is provided a method for etching of silica-based glass layers
or substrates comprising reactive ion etching through a mask
executed under conditions of simultaneous isotropic deposition of a
carbon based polymer
[0007] Preferably, the polymer deposition rate or/and its
steady-state thickness on different surfaces of the etched
structure is controlled by adjusting one or several process control
parameters in order to control etched profile, dimension loss,
sidewall and bottom etched surface roughness, and etching
selectivity between the silica-based layer and mask material.
[0008] A gas or a mixture of gases containing fluorine or carbon
atoms is used and a photoresist mask or other form of mask such as
amorphous silicon. Adjustable parameters can include RF power and
substrate temperature. The temperature can be adjusted to achieve
low sidewall roughness and low dimension loss at the same time.
Further, resputtering of any metal present wihtin or/and in contact
with the discharge zone can be prevented.
[0009] The invention is ideally suited wherein reactive ion etching
is performed in a high plasma density hollow cathode etching system
and the etching gas mixture is CH.sub.3F and Argon.
[0010] Various products made utilizing the previous methods are
also disclosed.
[0011] In accordance with a further aspect of the present invention
there is provided a high plasma density hollow cathode etching
system is disclosed which has been shown to provide higher etch
rates than those achievable in previously known standard RIE
systems. Etching was carried out in a CHF.sub.3/Ar mixture with
additions of O.sub.2 and CF.sub.4. The effects of the different
chemistries as well as the use of different masks (photoresist and
amorphous silicon) and the effects the substrate temperature on
etching rates, sidewall roughness and etch profiles have been
investigated. Using a photoresist mask generally results in greater
sidewall roughness compared to an amorphous silicon mask.
Importantly, polymer deposition during the etching process can
exacerbate the development of roughness but is still desirable to a
certain extent in the prevention of the loss of line width during
etching. Two mechanisms of polymer deposition control are
disclosed, namely, the addition of varying amounts of O.sub.2 or
CF.sub.4, and elevating the temperature of the substrate. The
latter was found to give a good compromise between control over the
line width loss and the sidewall roughness. In order to explain the
variety of experimental results obtained, a simple phenomenological
model based on a polymer etching/deposition rate equilibrium on
etched surfaces is proposed and examined.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] Notwithstanding any other forms which may fall within the
scope of the present invention, preferred forms of the invention
will now be described, by way of example only, with reference to
the accompanying drawings in which:
[0013] FIG. 1 is a schematic illustration of the basic layout of
hollow cathode discharge chamber used in the preferred
embodiment.
[0014] FIG. 2a to FIG. 2g illustrate graphs of etch rates and
selectivities over mask material for etching with a-Si (FIG. 2a)
and photoresist (FIG. 2g) masks as a function of RF power. Pressure
is 12 Pa. Gas flow rates: 60 sccm of Ar, 15 sccm of CHF.sub.3.
Sample temperature is 80.degree. C.
[0015] FIG. 3a to FIG. 3d illustrate graphs of etching profile
slope (FIG. 3a) dimension loss (FIG. 3b), sidewall roughness (FIG.
3c) and polymer deposition rate (FIG. 3d) as a function of RF power
for etching with a-Si and photoresist masks. The pressure is 12 Pa.
The gas flow rates: 60 sccm of Ar, 15 sccm of CHF.sub.3. The sample
temperature is 8020 C. The dimension loss was normalized to an
etching depth of 5 .mu.m. The polymer deposition rate was measured
in the area shielded from ion bombardment.
[0016] FIG. 4a to FIG. 4h are Electron microscope images of etching
profiles as a function of RF power for etching with a-Si (FIG. 4a
to FIG. 4d) and photoresist masks (FIG. 4e to FIG. 4h). The RF
power was as follows: FIG. 4a- unetched a-Si mask, FIG. 4b-250 W,
FIG. 4c-500 W, FIG. 4d-650 W, FIG. 4e-unetched photoresist mask,
FIG. 4f-300 W, FIG. 4g-400 W, FIG. 4h 500 W. Pressure is 12 Pa. Gas
flow rates: 60 sccm of Ar, 15 sccm of CHF.sub.3. Sample temperature
80.degree. C.
[0017] FIG. 5a to FIG. 5e illustrate graphs of etch rates and
selectivities with FIG. 5a to 5c illustrating SiO.sub.2 etch rates
and selectivities over an a-Si mask and FIG. 5d to FIG. 5e
illustrating vertical and lateral etch rates of a-Si mask as a
function of sample temperature, for a O.sub.2 flow rate and a
CF.sub.4 flow rate, respectively. RF power is 500 W. Pressure is 12
Pa. Gas flow rates: 60 sccm of Ar, 15 sccm of CHF.sub.3. Sample
temperature 80.degree. C. unless varied.
[0018] FIG. 6a to FIG. 6j illustrate graphs of the etching profile
slope (FIG. 6a to FIG. 6c) sidewall roughness (FIG. 6d to FIG. 6f)
and polymer deposition rate (FIG. 6g to FIG. 6j) as a function of
sample temperature, O.sub.2 flow rate and CF.sub.4 flow rate,
respectively using an a-Si mask. RF power is 500 W. Pressure is 12
Pa. Gas flow rates: 60 sccm of Ar, 15 sccm of CHF.sub.3. Sample
temperature 80.degree. C. unless varied. Polymer deposition rate
was measured in the area shielded from ion bombardment.
[0019] FIG. 7a and FIG. 7b illustrate electron microscope images
for etched sidewalls for sample temperature 80.degree. (FIG. 7a)
and 320.degree. C. (FIG. 7b). The a-Si mask is still in place. RF
power is 500 W. Pressure is 12 Pa. Gas flow rates: 60 sccm of Ar,
15 sccm of CHF.sub.3.
[0020] FIG. 8 illustrates a sectional view of a wafer showing the
four possible etched surfaces with respect to intensity of ion
bombardment and the possibility of polymer film formation: the
surfaces include (i) the top surface of the mask 31, (ii) the
sidewalls of the mask 32, (iii) the sidewalls of SiO.sub.2 33 and,
(iv) the bottom surface of the Sio.sub.2 34. A steady state
thickness of polymer film can be present on (i-iii), whereas (iv)
is assumed polymer free under the etching conditions used in this
study.
[0021] FIG. 9a to FIG. 9c are electron microscope images of time
evolution of the etched profile: with FIG. 9a showing an unetched
a-Si mask, FIG. 9b after 3 minutes etching, FIG. 9c after 6 minutes
etching. RF power is 500 W. Pressure is 12 Pa. Gas flow rates: 60
sccm of Ar, 15 sccm of CHF.sub.3. Sample temperature is 80.degree.
C. Etching selectivity over the a-Si mask is approximately 14:1. A
"negative undercut" is shown to be developed without mask width
reduction. The more vertical profile of the a-Si mask is "buried"
under polymer formed at a steady state angle determined by the
polymer etching and deposition equilibrium.
[0022] FIG. 10 is a schematic illustration of a mechanism of sloped
profile formation. A steady state profile angle 0.degree. is formed
under conditions where Er.sub.polymer=Dr.sub.polymer. For this
angle the steady state thickness of polymer film on the sidewall is
enough to prevent etching. The no-zero polymer etch rate at
0=90.degree. is due to ions scattered in plasma sheath.
[0023] FIG. 11a and 11b illustrate etched sidewalls for etching
with a photoresist mask (FIG. 11a) and a a-Si mask (FIG. 11g).
Pressure is 12 Pa. Gas flow rates: 60 sccm of Ar, 15 sccm of
CHF.sub.3. Sample temperature is 80.degree. C. RF power is 500 W
for etching with the photoresist mask and 650 W for etching with
the a-Si mask.
DESCRIPTION OF PREFERRED EMBODIMENTS
[0024] A first embodiment of the present invention relies upon the
utilisation of plasma enhanced chemical vapour deposition (PECVD)
in a hollow cathode discharge chamber. Turning initially to FIG. 1
there is shown a suitable vacuum chamber assembly 10 including a
top electrode 11 and a bottom electrode 12 connected as shown to RF
source 13 which comprised a 13.56 MHz RF source. In use for the
purposes of PECVD, the chamber 14 is evacuated via pump port 15 and
gases such as CH.sub.4/SF.sub.6 mixtures, CHF.sub.3/Ar mixtures are
introduced via corresponding ports e.g. 16, 17, so as to cause
controlled etching on wafers or substrates 19 located in the RF
field induced plasma located between electrodes 11, 12. This
apparatus 10 is utilised to perform the controlled ion etching
operation as discussed in detail below.
[0025] The high plasma-density hollow cathode discharge etching
system suitable for use is described in C. M. Horwitz, S. Boronkay,
M. Gross and K. E. Davies, J. Vac. Sci. Technology A6, at pages
1837 to 1844 (1988). The two opposing RF powered parallel circular
electrodes 11, 12 are surrounded by a grounded chamber 21. A
conventional diode discharge is produced between each of the
electrodes 11, 12 and the grounded chamber 21 but a high density
plasma is generated between the two RF powered electrodes 11, 12
due to the "electron mirror" effect. Both the upper and lower
electrodes were water-cooled and covered with 100 mm diameter
silicon wafers 18, 19. The latter is to prevent resputtering of the
electrode material (Al) which can result in metal contamination and
subsequent surface roughness and the formation of sloped etching
profiles due to metal-based polymer deposition. Examination of the
polymer deposited in the ion-shielded areas (as described below)
using wavelength dispersive X-ray spectroscopy (WDS) showed that no
traces of Al or other vacuum chamber materials could be detected at
the 0.01% level, thus confirming that metal contamination is not an
issue.
[0026] The silica films used in the etching experiments had a
thickness of 8 .mu.m and were deposited on silicon substrates 19
using the hollow-cathode PECVD technique. Masking layers of 1 .mu.m
of PECVD a-Si or 2 .mu.m of photoresist were then applied to the
wafer 19. The photoresist mask was patterned using conventional
photolithography, while patterning of the a-Si layer was carried
out using conventional photolithography followed by etching in a
CF.sub.4/SF.sub.6 mixture.
[0027] In each run the samples 19 were etched to a depth of around
4-5 .mu.m, with the rates determined by surface profilometry. The
etched profile, sidewall roughness and dimension loss were further
determined by SEM examination. The dimension loss was calculated or
defined as the difference between the line width measured at the
bottom of the mask before etching and the width at the top of the
etched ridge. When determining the sidewall roughness, the SEM
photos were taken from the top side of the ridge, at a glancing
angle to the sidewall. Sidewall roughness figures set out
hereinafter are the average amplitudes of the RIE-induced
corrugations measured over a distance of a few microns. The initial
(unetched) roughness of both the photoresist and a-Si mask edge was
not larger than 0.02 .mu.m.
[0028] The silica films were etched in a 20 % CHF.sub.3 in Ar
mixture with various additions of CF.sub.4 or O.sub.2. The pressure
was kept at 12 Pa in all experiments. All gases had a stated purity
of 99.95% or better. Etching in CHF.sub.3 was accompanied by some
polymer deposition. The polymer deposition was estimated using a
shadowing technique, whereby the polymer deposition is assumed to
be isotropic An overhanging structure consisting of two overlapping
silicon wafers was used and the thickness of polymer deposited
under the overhang (and so shielded from ion bombardment) was
measured using surface profilometry.
[0029] The temperature of the samples was controlled by varying the
thermal contact between the sample 19 and the cooled electrodes eg.
12. For an A-Si mask, three cases were characterized by
thermocouple measurements: (i) no thermal contact between the
sample and the electrode; (ii) partial thermal contact through
several point contacts of vacuum grease; (iii) good thermal contact
through vacuum grease spread on the back of the sample. Good
thermal contact was used in all experiments where the temperature
was held constant and for all photoresist masked samples.
RESULTS
The Effect of RF Power
[0030] Etch rates: The resulting etch rates as a function of RF
power (at 13.56 MHz) coupled into the discharge are shown in FIG.
2a for an a-Si mask and in FIG. 2b for a photoresist mask. It is
seen that the SiO.sub.2 etch rate is slightly higher (around 10%)
in the case of the a-Si mask for similar power levels. With the
a-Si mask the Sio.sub.2 etch rate increases by almost a factor of
three over the investigated power range reaching a value of 0.8
.mu.m/min at the maximum power. The a-Si etch rate increases with
power faster than the Sio.sub.2 etch rate thus causing an overall
decrease in selectivity over a-Si from 20:1 to 12:1. Similarly, the
selectivity over photoreslst also decreases with the power.
[0031] Etched profile and dimension loss: The etched sidewall slope
angle, as a function of power for a-Si and photoresist masks, is
shown in FIG. 3a. The SEM photographs of the corresponding etched
profiles are shown in FIGS. 4a to 4h. For both mask materials the
angle of the profile slope was found to increase with the power,
being greater for an a-Si mask for similar power levels. The
dimension loss (FIG. 3b) (normalised to a depth of 5 .mu.m) for the
a-Si mask was found not to exceed 0.2 .mu.m (although it seemed to
increase slightly with power), at the same time the dimension loss
for the photoresist mask was quite significant (>1 .mu.m) and
clearly increased with power as illustrated in FIG. 3b. This
difference probably points to different mechanisms being
responsible for the sloped profile formation in both cases. It may
be noted from FIGS. 4a to 4h, where the initial mask profiles
before etching are shown, that a facet is developing on the
photoresist mask sidewall, thus possibly contributing to the
observed dimension loss.
[0032] Sidewall roughness: As illustrated in FIG. 3c, the sidewall
roughness appears to be consistently higher for a photoresist mask
than for an a-Si mask. In both cases, however, it was found to
increase with power and, as can be seen in FIG. 3c, the sidewall
roughness for etching with an a-Si mask at the highest power level
is comparable with the sidewall roughness obtained with the
photoresist mask at lower power levels.
[0033] Polymer deposition rate: The polymer deposition rate in the
area shadowed from ion bombardment was found to give results as
indicated in FIG. 3d. It was found to increase by about 30% over
the whole power range. Also, it may be noted that at the minimum
power, the polymer deposition rate was around 3 times smaller than
the SiO.sub.2 etch rate, which means that in ion bombarded areas,
during etching of 1 .mu.m of SiO.sub.2, around 0.35 .mu.m of
polymer is simultaneously removed. As the power increases this
portion of removed polymer is reduced to around 20% or 0.2 .mu.m
for 1 .mu.m of SiO.sub.2.
[0034] The Effect of O.sub.2 and CF.sub.4 Additions, and Sample
Temperature Variation
[0035] As polymer deposition was found to play an important role in
the etching mechanism, different methods of controlling it were
investigated. These include (i) O.sub.2 additions, (ii) CF.sub.4
additions and, (iii) elevated substrate temperature.
[0036] Etch rates: FIGS. 5a to 5f show SiO.sub.2 and a-Si etch
rates and selectivity plotted on the same scales for all three
varied parameters. It is seen that the Sio.sub.2 etch rate
decreases with temperature and O.sub.2 additions, but increases
with CF.sub.4 additions. At the same time the selectivity over a-Si
decreases in all three cases. The a-Si mask etch rate shown in
FIGS. 5a to 5.sub.f has been separated into two components, a
vertical component, which is related to the mask thickness
decrease, and a lateral component, which is related to the mask
width decrease. It can be seen from that under the conditions of no
O.sub.2 or CF.sub.4 and a low sample temperature (80.degree. C.),
the lateral etch rate is essentially zero (<80.ANG./min). At
elevated temperature and with O.sub.2 added it stays initially at
around zero but then rises, ultimately approaching the vertical
etch rate values, which implies isotropic etching of the a-Si mask.
For CF.sub.4 the behaviour is different, the lateral etch rate
increases gradually but the mask etching remains basically
anisotropic. With increases in all three parameters the vertical
a-Si etch rate increases which, together with a decrease in the
SiO.sub.2 etch rate at higher temperatures and O.sub.2 additions,
and a slower rate of increase with CF.sub.4 additions, results in
an overall decrease in selectivity.
[0037] Etched profile: As illustrated in FIG. 6a, the slope of the
etching profile was found to first increase with temperature and
then decrease below the initial value. As shown in FIG. 6b, O.sub.2
additions caused a small initial increase in the slope followed by
a gradual decrease. As illustrated in FIG. 6c, the slope was found
to be essentially independent of the CF.sub.4 flow rate.
[0038] Sidewall roughness. As shown in the sidewall roughness was
found to decrease with both temperature (FIG. 6d) and O.sub.2 flow
(FIG. 6e) but was not effected by the CF.sub.4 (FIG. 6f). It is
seen that the sidewall roughness can be reduced to 0.02 .mu.m
either by elevating the sample temperature or by adding O.sub.2 to
the gas mixture. However, by comparing FIG. 6d with FIG. 5a and
FIG. 5d, it can be seen that, using temperature as a control
parameter, minimum sidewall roughness can be achieved while
maintaining the anisotropy of the a-Si mask etching. O.sub.2 can
also be used to reduce roughness (FIG. 6e), but the same minimum
roughness can only be achieved at the expense of dimension loss,
since, at the required O.sub.2 flow rates, the etching of a-Si
becomes essentially isotropic (FIG. 5e). From a practical point of
view this suggests that the sample temperature is a more useful
control parameter for reducing sidewall roughness compared to the
addition of O.sub.2. The improvement in the sidewall roughness can
be seen in FIGS. 7a and 7b, which shows SEM images of two sidewalls
etched at different temperatures.
[0039] Polymer deposition rate: The polymer deposition rate on a
shadowed surface as a function of sample temperature, O.sub.2 flow
rate and CF; flow rate, is shown in FIG. 6g, FIG. 6h and FIG. 6j,
respectively. It is seen (FIG. 6g) that by increasing the
temperature, the polymer deposition is first reduced and then, with
a further increase in the temperature, is totally suppressed, which
means that there is no polymer deposition above a certain sample
temperature even in absence of ion bombardment. O.sub.2 additions
(FIG. 6h) cause only a small decrease in the polymer deposition
rate, which is noteworthy given the similar effect of temperature
and O.sub.2 on the sidewall roughness. This may indicate a
difference in the mechanisms by which the two parameters reduce the
sidewall roughness. Finally it is seen from FIG. 6j that the
polymer deposition rate increases with C.sub.4 flow by about 25% of
its initial value.
[0040] It is evident from the aforegoing analysis that essentially
isotropic polymer deposition occurs simultaneously with etching.
Furthermore, the formation of polymer films under similar etching
conditions, with thicknesses depending on an equilibrium between
the polymer etch and deposition rates, has been demonstrated in a
number of other investigations.
[0041] Turning to FIG. 8, in the case of the etched structures
described it is possible to specify four surfaces 31-34 on which
such a film may exist. Previous investigations using similar
conditions have shown that SiO.sub.2 surfaces 34 are free of
polymer film for RF bias voltages above 100V (at a pressure of 0.13
Pa), and that the threshold bias voltage between polymer etching
and deposition decreases with pressure. Therefore, using 12 Pa and
400V-600V bias, a polymer free SiO.sub.2 bottom surface 34
generally results. This is supporred by the fact that SiO.sub.2
etch rates do not increase with polymer suppression, either by
O.sub.2 additions, or increasing sample.
[0042] Temperature (FIG. 5a and 5b). Thus, a polymer film can be
present only on surfaces 31, 32 and 33. The polymer on surface 31
determines the etching selectivity, whereas 32 and 33 will effect
the etching profile and sidewall roughness. The presence of a
finite thickness of polymer implies that both etching species and
reaction products must diffuse through the polymer on their way to
or from the etched surface, a mechanism which has previously been
suggested by others. Here, in addition to etching of the polymer
film by normal surface process, it is assumed that etching species
diffusing through the polymer film have a certain probability of
reaction with the polymer, which is proportional to the film
thickness. Porosity in the polymer film can contribute to this
etching mechanism. As the polymer film thickness increases, this
"diffusion" etching component also increases, thus increasing the
total polymer removal rate and preventing continues film growth.
For a constant polymer deposition rate, these effects will give
rise to a certain equilibrium polymer thickness, which will
determine the etch rates of the underlying surfaces.
[0043] While not wishing to be bound by theory, a simplified
phenomenological model describing this mechanism can be written as
follows:
ER.varies.I.sub.a(1-.alpha.d).ltoreq.1 (EQ.1)
ER.sub.polymer.varies.C.sub.1I.sub.a.alpha.d+C.sub.2I.sub.1COS(.phi.)Y(I.s-
ub.a,E.sub.I,.phi.),.alpha.d.ltoreq.1 EQ.2
DR.sub.polymer.varies..gamma.(T)I.sub.p (EQ.3)
[0044] where ER is the etch rate of the surface under the polymer
film, I.sub.a is the flux of active etching species at the polymer
film surface, .alpha. is the probability of polymer etching by
diffusing active species per unit of film thickness, d is the
thickness of the polymer film, ER.sub.polymer is the polymer etch
rate, C.sub.1 and C.sub.2 are empirical constants, I.sub.1 the ion
flux, .phi. is the sidewall slope or effective ion angle of
incidence, Y(I.sub.a,E.sub.1,.phi.) is the reactive sputtering
yield as a function of active species flux I.sub.a, ion energy E
and effective ion angle of incidence .phi.. I.sub.a is the flux of
polymer forming species and .gamma..sup.(T) is the sticking
probability of the polymer forming species as a function of the
surface temperature. The photoresist, a-Si and SiO.sub.2 etch rate
results of FIGS. 2 and 3 can be explained using this model.
[0045] The observed decrease in etching selectivity over both
photoresist and a-Si with power of FIG. 2a and FIG. 2b can be
explained by a decrease in the steady-state polymer thickness on
both mask surfaces 31, 32 (the SiO.sub.2 surface is assumed polymer
free). This occurs in spite of the increasing polymer deposition
rate with power (FIG. 3d). According to Eq. (2) above this means
that the increase in polymer deposition rate is overshadowed by the
increase in both reactive sputter etching and "diffusion" etching
components, thus requiring a smaller polymer thickness to maintain
the polymer etching/deposition equilibrium.
[0046] Increasing temperature, O.sub.2 flow and CF.sub.4 flow all
reduce the etching selectivity over a-Si, mainly through a greater
vertical a-Si etch rate (FIG. 3). Again, since the SiO.sub.2
surface 34 is assumed to be polymer free, this implies a decrease
in the polymer thickness on a-Si. Increasing temperature reduces
the sticking probability of the polymer forming species with causes
a reduction in the polymer deposition rate according to Eq. 3. This
is also confirmed by FIG. 6g where the polymer deposition rate in
the area shielded from ion bombardment is shown to decrease with
temperature. The polymer thickness then decreases (Eq. 2) causing
and increase in a-Si etch rate through Eq. 1. In the case of
O.sub.2 additions is not significant (FIG. 6h). Similarly, CF.sub.4
additions lead to a reduction in the polymer thickness despite a
small increase in the polymer deposition rate (FIG. 6j). In this
case, however, the a-Si etch rate can increase, not only because of
a reduction in polymer thickness, but also due to an increase in
the active species flux, resulting from CF.sub.4 dissociation.
[0047] While the vertical a-Si etch rate increases in response to
all three factors (temperature, O.sub.2 and CF.sub.4), the lateral
a-Si etch rate behaves differently for the temperature and O.sub.2
cases on one side and CF.sub.4 case on the other (FIG. 5d to FIG.
5f). For increasing temperature and O.sub.2 flow the lateral etch
rate increases, approaching the vertical etch rate, thus indicating
isotropic etching of the a-Si mask. In the case of CF.sub.4
additions, however, the lateral a-Si etch rate increase is small
and the anisotropy remains unchanged due to a proportional increase
the vertical etch rate.
[0048] According to Eq. 1, the difference in vertical and lateral
etch rate of the a-Si mask is due to the different steady-state
polymer film thickness on its top surface and sidewalls. The
sidewalls receive less ion bombardment during etching which,
according to Eq. 2, reduces the reactive sputtering component of
polymer etching and causes an increase in its steady-state
thickness to the point where lateral etching of the mask ceases, as
seen in the first few points in FIG. 5d to FIG. 5f. The increasing
lateral etch rate with temperature and O.sub.2 additions is due to
a reduction in polymer thickness on the sidewalls. In the
temperature case, this can be attributed to reduced polymer
deposition FIG. 6g. In the O.sub.2 case, where the polymer
deposition rate data show only a small decrease FIG. 6g, the
similar reduction in sidewall polymer thickness is due to higher
polymer removal rate by active oxygen. The absence of polymer on
the sidewalls for high O.sub.2 flow rates, as opposed to its
presence in the areas shielded from ion bombardment FIG. 6j is
likely to be due to some ion bombardment on the mask sidewalls by
ions scattered in the sheath (characteristic for the operating
pressure around 10 Pa employed which is, although relatively small,
apparently enough to initiate polymer etching by the products of
O.sub.2 dissociation).
[0049] The addition of CF.sub.4, although causing an increase in
the vertical a-Si etch rate in a way similar to increased
temperature and O.sub.2, does not alter the ratio of the vertical
to lateral etch rates. According to the model this implies that
some polymer film remains on both the sidewalls and top surface of
the a-Si mask, with its steady state thickness on both surfaces
reduced proportionally with the CF.sub.4 additions.
[0050] Finally, we note that, despite there being no polymer on the
a-Si mask surface at high temperature or high O.sub.2 flows its
vertical etch rate, although increased, still remains around 5
times smaller than the SiO.sub.2 rate. This suggests that the well
accepted mechanism of Sio.sub.2 etching, where the SiO.sub.2/Si
selectively is due to selective polymer removal by oxygen released
from the SiO.sub.2 during etching, is accompanied by an additional
mechanism in this case. That is, under conditions of intense ion
bombardment, the production of CHF.sub.3 dissociation apparently
etch SiO.sub.2 faster than Si, even when there is no protective
polymer film on the Si surface.
[0051] Sidewall Angle
[0052] A sloped profile in silica, a material with known
intrinsically anisotropic etching characteristics may be produced
in two ways. The first mechanism, mask erosion, produces a sloped
profile through lateral mask etching. This is well documented in
the literature and may be deliberately induced by, for example,
addition of O.sub.2 when using a photoresist mask. The considerable
dimension loss observed when using a photoresist mask (FIG. 3b)
suggests that mask erosion is the cause of the sloped profile in
this case. The reason for high lateral photoresist mask etching, in
the light of similar selectively to SiO.sub.2 as a-Si, is probably
the result of faceting of the photoresist mask edge. This can be
seen in FIG. 4a to FIG. 4h where the photoresist mask is shown
before and after etching. From the round shape before etching (FIG.
4e) the mask sidewall becomes flat and sloped (FIG. 4h) at the
preferential sputtering angle. An estimate of the lateral etching
rate of the photoresist mask due to faceting is around 1000
.ANG./min, which is more than twice the vertical etch rate. The
increase in slope angle with power observed for a photoresist mask
can be attributed to the preferential increase in the SiO.sub.2
sidewall etch rate compared to the increase in the lateral mask
etch rate. Here, the SiO.sub.2 sidewall is not protected by
polymer, since the angle is less than the steady-state value
required for polymer film formation.
[0053] The effect of the second mechanism of sloped profile
formation can be seen in FIG. 9a to FIG. 9c where the time
evolution of an etched silica profile is shown starting with an
unetched a-Si mask (FIG. 9a). Here, it can be seen that the width
of the mask has not changed, but the effective linewidth has
increased, rather than decreased as is the case of mask erosion.
This effect, which has been termed "negative mask undercut" or
"overcut" can occur under conditions of anisotropic etching of the
substrate and mask in the presence of a simultaneous isotropic
deposition process.
[0054] The SiO.sub.2 sidewall etch rate is zero when all the
etching species are consumed in the sidewall polymer film before
reaching the SiO.sub.2 surface, or when .alpha.d=1 (Eq. 1). The
steady-state polymer thickness required to satisfy this condition
is found when ER.sub.polymer=DR.sub.polymer. However, since both
the ion flux to the sidewall and the angular dependence of the
reactive sputtering yield depend on the sidewall slope, the etching
rate of the sidewall polymer will depend on the slope (.phi. in Eq.
2). Thus, assuming isotropic polymer deposition, there is a
sidewall angle .phi..sub.O, at which
ER.sub.polymer(.phi..sub.o)=DR.sub.polymer, and hence zero
SiO.sub.2 sidewall etch rate. the angle were greater than this
equilibrium value then the reactive component of polymer etching
would be lower, DR.sub.polymer >ER.sub.polymer, and net
deposition would occur. Conversely, if the angle were less than the
equilibrium value, then the reactive component of polymer etching
would be higher, DR.sub.polymer<ER.sub.polymer and net etching
would occur. This mechanism is shown graphically FIG. 10. The point
41 where the semicircle 40 representing isotropic polymer
depositior crosses the line 42 representing the angular dependence
of the polymer etch rate corresponds to the angle at which the
polymer etching/deposition balance is achieved. This angle is the
steady-state angle of the SiO.sub.2 sidewall 44. The angular
dependence of the polymer etch rate in FIG. 10 is drawn
schematically and includes both the angular dependence of the ion
flux on the sidewall I.sub.1COS(.phi.) and the angular dependence
of the sputtering yield, Y(I.sub.a,E.sub.1, .phi.) the latter
having a maximum around 60.degree.. As the polymer deposition rate
(the semicircle radius 45) increases, the intersection between it
and the angular dependence curve 42 (the polymer etching/deposition
equilibrium point) shifts upwards thus decreasing the sidewall
angle. Similarly, if the polymer etch rate increases, the
intersection shifts downwards and the angle increases. This
mechanism can be applied to explain the experimental data.
[0055] The etched profile observed using an a-Si mask is seen to be
"overcut" (FIG. 3b and FIG. 9a to FIG. 9c). The effect of higher
power is to increase the reactive sputtering component of polymer
etching due to higher ion energy and density.
[0056] According to the above mechanism, this establishes a new
polymer etching/deposition equilibrium at a higher sidewall angle,
as observed. The increase in polymer deposition rate, which also
occurs with power (FIG. 3d) is apparently less than the increase in
its etch rate.
[0057] One can explain the profile slope dependences on
temperature, O.sub.2 and CF.sub.4 additions in similar terms,
keeping in mind the a-Si mask lateral etch rate tendencies (FIG. 5a
to FIG. 5f). The profile slope initially increases with temperature
and with O.sub.2 flow (although only slightly) and then decreases
in both cases FIG. (6a and FIG. 6b). The initial increase in the
slope profile versus temperature is due to a decrease in polymer
deposition through a reduced sticking probability of polymer
forming species. The new deposition/etching equilibrium occurring
at a higher angle according to FIG. 10. In the case of O.sub.2
flow, the polymer deposition rate decreases only slightly but its
reactive sputtering rate increases due to active oxygen produced in
the discharge thus increasing the equilibrium sidewall angle.
Further increases in temperature and O.sub.2 flow cause total
polymer removal from the sidewalls of the a-Si mask, resulting in
lateral etching of the a-Si and a smaller sidewall angle due to
mask erosion. The sidewall slope is relatively independent of
CF.sub.4 additions, which indicates that the increase in polymer
deposition rate due to CF.sub.4 flow is balanced by the
simultaneous increase in its etching rate, probably due to an
increase in the fluorine flux.
[0058] Sidewall Roughness
[0059] Since the initial mask-edge roughness, determined by SEM
examination of both a-Si and photoresist masks, was significantly
smaller than the post-etch sidewall roughness, this can be
eliminated as a source of the observed sidewall roughness. Thus,
either the mask edge is roughened during etching, or the roughness
is formed on the silica sidewall itself. Both these occurrences can
result from micromasking in the presence of ion bombardment and
polymer deposition. Since the photoresist masked samples were
cooled, photoresist reticulation is not an issue.
[0060] Fig. 11a shows a etched sidewall with a photoresist mask
still in place. It is seen that roughness has been generated in the
photoresist during the process and then transferred to the silica
sidewall where the mask edge has been thinned by the faceting which
is evident. The increase in silica sidewall roughness with power
can be explained by an increase in micromasking as both ion
bombardment and polymer deposition rate increase.
[0061] In the case of an a-Si mask (FIG. 11b), some of the observed
sidewall roughness may be produced by mask roughening, as can be
deduced from FIG. 11b, which shows an etched sidewall with the a-Si
mask still in place. However, while there is some faceting of the
top corner of the mask, the upper part of the sidewall close to the
mask is smoother than the lower part, suggesting that a larger part
of the roughness has not been transferred from the mask edge, but
rather has formed on the sidewall during etching. The reason for
this additional roughness is likely to be the sidewall polymer
which can act as a micromasking material. The sidewall roughness
increase with power can be explained by an increase in
micromasking, in this case in both the mask edge and the sidewall
itself, as both ion bombardment and polymer deposition rate
increase.
[0062] Both increased sample temperature and O.sub.2 additions
reduce roughness (FIG. 5d and FIG. 5e). In both cases this is
likely to be the result of sidewall polymer suppression. In the
temperature case, as a result of a reduction of the polymer
deposition rate to zero (FIG. 6g), and in the O.sub.2 case through
an increase in the polymer etch rate. In the absence of sidewall
polymer, this contribution to micromasking induced roughness is
eliminated. The sidewall roughness does not change with CF.sub.4
additions (FIG. 6f), which is consistent with the fact that the
profile slope also does not change, since the latter implies an
unperturbed balance between polymer etching and deposition and
therefore a constant polymer thickness on the sidewalls.
[0063] Trade Off Between Sidewall Roughness and Dimension Loss
[0064] Using the sample temperature as a control parameter allows
smooth sidewalls to be obtained without dimensional loss, whereas
using O.sub.2 additions does not allow for a process window where
both dimension control and smooth sidewalls can be achieved. In the
O.sub.2 case, active oxygen enhances the polymer etching rate on
both the a-Si mask and SiO.sub.3. sidewalls and therefore, together
with an improvement is sidewall roughness, it brings about
dimension loss due to isotropic etching of the mask. In the case of
increasing sample temperature, the flux of polymer forming species
from the plasma remains unchanged, but their sticking probability
is reduced, thus decreasing the effective polymer deposition rate,
which then results in reduced roughness. However the results
obtained here suggest that with increasing temperature the sticking
probability of polymer forming species to silica is reduced faster
than to the a-Si surface. This can allow for polymer free SiO.sub.2
sidewalls and simultaneously, sufficient polymer remaining on the
a-Si mask sidewalls to prevent dimension loss.
[0065] It can be seen from the forgoing, the application of
reactive ion etching of silica in a high plasma density hollow
cathode etching system to the fabrication of silica based
integrated optic devices can be effectively utilised. This
application imposes specific requirements on the etching depth,
sidewall roughness and profile slope control. Due to the high
plasma density produced in the hollow cathode discharge, high
silica etch rates (over 0.5 .mu.m for a-Si) and significant
dimension losses (>1.mu.m for 5 .mu.m etching depth). The
disadvantages with photoresist are believed to originate from
strong faceting which occurs on the mask sidewall. In a-Si mask the
faceting is negligible. Increasing RF power results in a decrease
in selectivity and an increase in sidewall roughness for both
photoresist and a-Si masks.
[0066] The effects of sample temperature an the addition of
CF.sub.4 and O.sub.2 on the etching characteristics for an a-Si
case have been investigated. The etching selectivity is reduced
with all three parameters. Increasing sample temperature and
O.sub.2 content permit sidewall roughness reduction to 0.02 .mu.m.
However, in the O.sub.2 case, the reduction in sidewall roughness
is accompanies by lateral etching of the a-Si mask causing
dimension loss and a decrease in the profile slope. A similar
effect observed with the sample temperature, however in this case
there appears to be a temperature range where smooth sidewalls can
be obtained, together with almost vertical sidewalls and without
dimension loss.
[0067] Finally based on polymer deposition rate measurements, a
model explaining the variety of experimental data is explained. The
model is based on a balance between isotropic polymer deposition
and etching. A polymer film of a certain steady-state thickness is
formed as a result of this balance on (i) the top surface of the
mask, (ii) the sidewalls of the mask and (iii) the sidewalls of the
SiO.sub.2. The polymer thickness on the top surface determines the
etching selectivity, whereas the polymer thickness on the mask
sidewalls and SiO.sub.2 sidewalls determines the profile slope and
sidewall roughness.
[0068] In conclusion, the silica reactive ion etching process of
the preferred embodiment satisfies all the requirements of planar
waveguide fabrication and can also be-used for other integrated
optics applications or MEMS applications where deep etching of
silica is required along with smooth etched sidewalls and vertical
or sloped etching profiles.
[0069] It would be appreciated by a person skilled in the art that
numerous variations and/or modifications may be made to the present
invention as shown in the specific embodiments without departing
from the spirit or scope of the invention as broadly described. The
present embodiments are, therefore, to be considered in all
respects to be illustrative and not restrictive.
* * * * *