Method of surface treatment of semiconductor substrates

This invention relates to methods for treatment of semiconductor substrates and in particular a method of etching a trench in a semiconductor substrate in a reactor chamber using alternatively reactive ion etching and depositing a passivation layer by chemical vapour deposition, wherein one or more of the following parameters: gas flow rates, chamber pressure, plasma power, substrate bias, etch rate, deposition rate, cycle time and etching/deposition ratio vary with time.

This invention relates to methods for treatment for semiconductor 
substrates and in particular, but not exclusively, to methods of 
depositing a sidewall passivation layer on etched features and methods of 
etching such features including the passivation method. 
It is known to anisotropically etch trenches or recesses in silicon using 
methods which combine etching and deposition. The intention is to generate 
an anisotropic etch, whilst protecting the sidewalls of the trench or 
recess formed by laying down a passivation layer. 
Such methods are for example shown in U.S. Pat. No. 4,579,623, 
EP-A-0497023, EP-A-0200951, WO-A-94114187 and U.S. Pat. No. 4,985,114. 
These all describe either using a mixture of deposition and etching gases 
or alternate etching and deposition steps. The general perception is that 
mixing the gases is less effective because the two processes tend to be 
self cancelling and indeed the prejudice is towards completely alternate 
steps. 
Other approaches are described in EP-A-0383570, U.S. Pat. No. 4,943,344 and 
U.S. Pat. No. 4,992,136. All of these seek to maintain the substrate at a 
low temperature and at first, somewhat unusually, uses burst of high 
energy ions during etching to remove unwanted deposits from the sidewalls. 
The continuous trend in semiconductor manufacture is for features of ever 
increasing aspect ratio, whence the sidewall profile and the surface 
roughness on the sidewalls, becomes more significant the smaller the width 
of the feature. Current proposals tend to produce a rather bowed or 
reentrant sidewall profile as well as rough sidewalls and/or bases to the 
formations depending on the process being run. 
The manifestation of the various problems depends on the application and 
the respective processing requirements, silicon exposed area (unmasked 
substrate areas), etch depth, aspect ratio, side wall profile and 
substrate topography. 
The method of this invention, in at least some embodiments, addresses or 
reduces these various problems. 
From one aspect the invention consists in a method of etching a trench in a 
semiconductor substrate in a reactor chamber using alternately reactive 
ion etching and depositing a passivation layer by chemical vapour 
deposition, wherein one or more of the following parameters: gas flow 
rates, chamber pressure, plasma power, substrate bias etch rate, 
deposition rate, cycle time and etching/deposition ratio vary with time. 
The variation may be periodic. 
The etching and deposition steps may overlap and etching and deposition 
gases may be mixed. 
The method may include pumping out the chamber between the etching and 
deposition and/or between deposition and etching, in which case the pump 
may continue until 
##EQU1## 
wherein Ppa is the partial pressure of the gas (A) used in the preceding 
step, 
Ppb is the partial pressure of the gas (B) to be used in the subsequent 
step, and 
x is the percentage at which the process rate of the process associated 
with gas (A) drops off from an essentially steady state. 
The etching and deposition gas flows may be continuously or abruptly 
variable. For example the deposition and etching gases may be supplied so 
that their flow rates are sinusoidal and out of phase. The amplitude of 
any of these parameters may be variable within cycles and as between 
cycles. 
It is particularly preferred that the deposition rate is enhanced and/or 
etch is reduced during at least the first cycle and in appropriate 
circumstances in the first few cycles for example in the second to fourth 
cycles. 
The etch rate may be reduced by one or more of the following 
(a) the introduction of a scavenging gas 
(b) a reduction in plasma power 
(c) a reduction in cycle time and 
(d) a reduction in gas flow 
(e) varying the chamber pressure 
The deposition rate may be enhanced by one or more of the following 
(a) an increase in plasma power 
(b) an increase in cycle time 
(c) an increase in gas flow rate 
(d) an increase in deposition species density 
(e) varying the chamber pressure 
Other advantageous features of the method are that the etch and/or 
deposition steps may have periods of less than 7.5 seconds or even 5 
seconds to reduce surface roughness; the etch gas may be CF.sub.x or 
XeF.sub.2 and may include one or more high atomic mass halides to reduce 
spontaneous etch; and the chamber pressure may be reduced and/or the flow 
rate increased during deposition particularly for shallow high aspect 
ratio etching where it may be accompanied by increased self bias (e.g. 
voltage &gt;20 eV or indeed &gt;100 eV). 
The deposition step may use a hydrocarbon deposition gas to deposit a 
carbon or hydrocarbon layer. The gas may include O, N or F elements and 
the deposited layer may be Nitrogen or Flourine doped. 
The substrate may rest freely on a support in the chamber when back cooling 
would be an issue. Alternatively the substrate may be clamped and its 
temperature may be controlled, to lie, for example, in the range 
-100.degree. C. to 100.degree. C. The temperature of the chamber can also 
advantageously be controlled to the same temperature range as the wafer to 
reduce condensation on to the chamber or its furniture to reduce base 
roughness. 
The substrate may be GaAs, GaP, GaN, GaSo, SiGe, Mo, W or Ta and in this 
case the etch gas may particularly preferably be one or a combination of 
Cl.sub.2, BCl.sub.3, SiCl.sub.4, SiCl.sub.2 H.sub.2, CH.sub.x Cl.sub.y, 
C.sub.x Cl.sub.y, or CH.sub.x with or without H or an inert gas. Cl.sub.2 
is particularly preferred. The deposition gas may be one or a combination 
of CH.sub.x, CH.sub.x Cl.sub.y or C.sub.x Cl.sub.y with or without H, or 
an inert gas CH.sub.4 or CH.sub.2 Cl.sub.2 are particularly preferred. 
Although the invention has been defined above it is to be understood that 
it includes any inventive combination of the features set out above or in 
the following description.

FIG. 1 illustrates schematically a prior art reactor chamber 10, which is 
suitable for use both in reactive ion etching and chemical vapour 
deposition. Typically a vacuum chamber 11 incorporates a support electrode 
12 for receiving a semiconductor wafer 13 and a further spaced electrode 
14. The wafer 13 is pressed against the support 12 by a clamp 15 and is 
usually cooled, by backside cooling means (not shown). 
The chamber 11 is surrounded by a coil 15a and fed by a RF source 16 which 
is used to induce a plasma in the chamber 11 between the electrodes 12 and 
14. Alternatively a microwave power supply may be used to create the 
plasma. In both cases there is a need to create a plasma bias, which can 
be either RF or DC and can be connected to the support electrode 12 so as 
to influence the passage of ions from the plasma down on to the wafer 13. 
An example of such an adjustable bias means is indicated at 17. The 
chamber is provided with a gas inlet port 18 through which deposition or 
etched gases can be introduced and an exhaust port 19 for the removal of 
gaseous process products and any excess process gas. The operation of such 
a reactor in either the RIE or CVD modes is well understood in the art. 
When etching trenches, etches, vias or other formations on the surface of a 
semiconductor wafer or substrates, the usual practice is to deposit a 
photo-resist mask with openings revealing portions of the substrate. 
Etched gases are introduced into the chamber and a number of steps are 
then taken to attempt to ensure that the etching process is anisotropic in 
a downward direction so that there is as little etching of the sidewalls 
of the formation as possible. For a variety of reasons it is difficult in 
practice to achieve true anisotropic etching and various attempts are made 
to deposit passivating materials onto the sidewalls so that the material 
can be sacrificially etched. The most successful to date of such systems 
is probably that described in WO-A-94114187 and this system is 
schematically illustrated in FIG. 2. The process described in that 
document uses sequential and discrete etch and deposition steps so that 
after the first etched step the sidewalls are undercut as shown at 20 and 
this undercut is then protected by a deposited passivation layer 21. As 
can be seen from FIG. 2 this arrangement produces a rough sidewall and as 
the etched steps increase, or indeed the aspect ratio increases, there can 
be bowing or re-entrant notching in the profile. The prior art documents 
describe the deposition of CF.sub.x passivation layers. 
The Applicant proposes a series of improvements to such processes to enable 
the formation of more smooth walled formations and particularly better 
quality deep and/or high aspect ratio formations. For convenience the 
description will therefore be divided into sections. 
1. Passivation 
As has been mentioned above previous proposals deposit a passivation layer 
of the form CF.sub.x. The Applicants propose passivating the sidewalls 
with carbon or hydrocarbon layers which will provide significantly higher 
bond energies, particularly if deposited under high self-bias so that the 
graphitic phase is at least partially removed. 
If these films or layers are also desirably deposited at high self biases 
eg. 20 eV upwards and preferably over 100 eV, there is an additional 
significant advantage when it comes to high aspect ratio formations, 
because the high self-bias ensures that the transport of the depositing 
material down to the base of the formation being etched is enhanced to 
prevent re-entrant sidewall etching. This transportation effect can also 
be improved by progressively reducing the chamber pressure and/or 
increasing the gas flow rate, so as to reduce the residence time. In some 
arrangements it may be desirable to drive the deposition to such an extent 
that a positively tapered, or v-shaped formation is achieved. In the 
particular case of shallow (&lt;20 .mu.m) high aspect ratio trenches, the 
feature opening size (or critical dimension) can be in the &lt;0.5 .mu.m 
range. 
The hydrocarbon (H--C) films formed by this passivation have significant 
advantages over the prior art fluorocarbon films. 
The H--C films can for example be readily removed after etching processing 
has been completed by dry ashing (oxygen plasma) treatment. This can be 
particularly important in the formation of MEMS (micro-electro-mechanical 
systems) where wet processing can result in sticking of resonant 
structures which are separated by high aspect trenches. In other 
applications, eg. optical or biomedical devices, it can be essential to 
remove completely the side wall layer. 
The H--C films may be deposited from a wide range of H--C precursors (eg. 
CH.sub.4, C.sub.2 H.sub.4, C.sub.3 H.sub.6, C.sub.4 H.sub.8, C.sub.2 
H.sub.2. etc. including high molecular weight aromatic H--C's). These may 
be mixed with noble gases and/or H.sub.2. An oxygen source gas can also be 
added (eg CO, CO.sub.2, O.sub.2 etc.) can be used to control the phase 
balance of the film during deposition. The oxygen will tend to remove the 
graphitic phase (sp.sup.2) of the carbon leaving the harder (sp.sup.3) 
phase. Thus, the proportion of oxygen present will affect the 
characteristics of the film or layer, which is finally deposited. 
As has been mentioned above H.sub.2 can be mixed in with the H--C 
precursor. H.sub.2 will preferentially etch silicon and if the proportions 
are correctly selected, it is possible to achieve side wall passivation, 
whilst continuing the etching of the base of the hole during passivation 
phase. 
The preferred procedure for this is to mix the selected H--C precursor (eg. 
CH.sub.4) with H.sub.2 and process a mask patterned silicon surface with 
the mixture in the apparatus, which is to be used for the proposed etch 
procedure. The silicon etch rate is plotted as a function of CH.sub.4 
concentration in H.sub.2 and an example of such a plot is shown in FIG. 4. 
It will be noted that the etch rate increases from an initial steady state 
with increasing percentage of CH.sub.4 to a peak before decreasing to 
zero. 
It is believed the graph illustrates the following mechanisms taking place. 
In the initial steady state portion the etch is essentially dominated by 
the action of H.sub.2 to form SiHx reaction products. At around 10% of 
CH.sub.4 in H.sub.2, the CH.sub.4 etching of the substrate becomes 
significant (by forming Si(CHx)y products) and the etch rate increases. 
Deposition of a hydrocarbon layer is taking place throughout although due 
to the etching there is no net deposition on this part of the graph. 
Eventually, the deposition begins to dominate the etching process until at 
around 38% for CH.sub.4, net deposition occurs. 
It has been determined that these varying characteristics can be utilised 
in two different ways. If high self bias or there is high mean ion energy, 
eg. &gt;100 ev, the layer or coating laid down is relatively hard because the 
reduced graphitic phase and the process can be operated in the rising 
portion of the etch rate graph, because the coating is much more resistant 
to etching, than the silicon substrate. It is thus possible to etch the 
silicon throughout the deposition phase. Selectivies exceeding 100:1 to 
mask or resist are readily achieved. It should particularly be noted that, 
whilst there is a significant removal of the graphitic phase due to ion 
bombardment bf the mask 22, the high directionality of the ions means that 
the side wall coating is relatively untouched. 
The process can also be operated at low mean ion energies either with a 
H--C precursor alone or with H.sub.2 dilution. In that latter case it is 
preferred that the process is operated in the descending part of the etch 
graph. ie. for CH.sub.4 at a percentage &gt;18% but &lt;38% when net deposition 
occurs. Typically the range for CH.sub.4 would be 18% to 30%. 
The low values of mean ion energy during the polymer deposition are 
believed to be beneficial in allowing high mask selectivies. Under these 
lower rf bias conditions, the selectivity increase to infinity over a wide 
passivation deposition window. So if high selectivity is required, the low 
mean ion energy approach offers advantages. FIG. 5 illustrates the step 
coverage (side wall deposition measured at 50% of the step height versus 
surface deposition) for H--C films using CH.sub.4 and H.sub.2 under a 
range of conditions including the two embodiments described above. FIG. 5 
shows that high ion energies increase the step coverage, but even with low 
bias conditions, there is sufficient passivation to protect against 
lateral etching. Further, in this latter case the higher deposition rate 
serves further to enhance the mask selectivity. The deposition rate at low 
ion energies is a factor of two greater over the 100 ev case. 
It will thus be appreciated that by using these techniques the user can 
essentially select the combination of etch rate and selectivity, which 
most suits his proposed structure. Further the enhancement of mask 
selectivity can be used to either increase the etch rate and/or reduce the 
notching. 
FIG. 6 illustrates how various parameters of the process may be 
synchronised. 6d shows continuous and unchanging coil power, whilst at 6e 
the coil power is switched to enhance the etch or deposition step and the 
power during etch may be different to that selected for deposition 
depending on the process performance required. 6e, by way of example, 
illustrates a higher coil power during deposition. 
6f to i show similar variations in bias power. 6f has a high bias power 
during etch to allow ease of removal of the passivation film, whilst 6g 
illustrates the use of an initial higher power pulse to enhance this 
removal process, whilst maintaining the mean ion energy lower, with 
resultant selectivity benefits. 6h is a combination of 6f and 6g for when 
the higher ion energies are required during etching (eg. with deep 
trenches). 6i simply shows that bias may be off during deposition. 
In some processes, at least, the acceptable segregation period of the gases 
is determined by the residual partial pressure of gas A (Ppa) which can be 
tolerated in the partial pressure of gas B(Ppb). This minimum value of Ppa 
in Ppb is established from the characteristic process rate (etch or 
deposition) as a function of Ppa/(Ppa+Ppb). 
In FIG. 8, Gas A is 20% CH.sub.4 +H.sub.2, whilst gas B is SF.sub.6. It 
will seen that where Ppa/(Ppa+Ppb)&lt;5%, the process rate is substantially 
steady state. For typical practical conditions a pump out time of less 
than 1.5 seconds will suffice and a plasma can be maintained for over 65% 
of the total cycle time where the process steps are of the order of 2 to 3 
secs and over 80% when the steps are over 5 seconds long. A suitable 
synchronisation arrangement is shown in FIG. 7. It will be noted that the 
etch precedes the pump out as it is desirable to prevent a mixing of the 
deposition and etch step gases. Prior art proposals (eg. U.S. Pat. No. 
4,985,114) propose switching off or reducing deposition gas flow for a 
long period before the plasma is switched on. This can mean that the 
plasma power is on only for a small portion of the total cycle times 
leading to a significant reduction in etch rate. The Applicants propose 
that the chamber should be pumped out between at least some of the gas 
changeovers, but care must be taken to maintain pressure and gas flow 
stabilisation. Preferably high response speed mass flow controllers (rise 
times (100 ms) and automatic pressure controllers (angle change and 
stabilise in (300 ms) are used. 
The Applicants have established (see FIG. 8) that the pump out time period 
necessary to avoid the etch being compromised by the deposition gases. 
However pump out could precede the etch step or both etch and deposition 
step depending on the precise process being run. Pump out also reduces 
micro-loading (which is described in U.S. Pat. No. 4,985,114) and is 
beneficial high aspect ratio etching as described below. 
Many of the parameters, which are varied can be `ramped` as general 
illustrated in FIG. 9(ii). This means that they progressively increase or 
decrease cycle by cycle in amplitude or period, rather than changing 
abruptly between cycles. In the case of the pump out, ramping can be used 
to allow mixing at the start of the process allowing sidewall notching to 
be reduced or eliminated as discussed below. 
Typical process parameters are as follows: 
1. Deposition step 
CH.sub.4 step time: 2-15 seconds; 4-6 seconds preferred 
H.sub.2 step time: 2-15 seconds; 4-6 seconds preferred 
Coil rf power: 600 W-1 kW; 800 W preferred 
Bias rf power: High mean in energy case: 500 W-300 W-100 W preferred Low 
mean in energy case: 0 W-30 W-10 W preferred 
Pressure: 2 mTorr-50 mTorr; 20 mTorr preferred 
2. Etch Step 
SF.sub.6 step time: 2-15 seconds; 4-6 seconds preferred 
Coil rf power: 600 W-1 kW-800 W preferred 
Bias rf power: High mean ion energy case: 50 W-300 W; 150 W preferred Low 
mean ion energy case: 0 W-30 W;15 W preferred 
Pressure: 2 mTorr-50 mTorr; 30 mTorr preferred. 
2. Etch/Deposition Relationship 
The Applicant has determined that the prior art approaches are essentially 
too simplistic, because they neither allow for changing conditions during 
a particular process nor for the different requirements or different types 
of formation. Further the prior art does not address the difficulties of 
deep etching. 
Thus contrary to the teaching of WO-A-94114187 the Applicant believes that 
it will often be beneficial to overlap the etch and passivation or 
deposition steps so that the surface wall roughness indicated in FIG. 2 
can be significantly reduced. The Applicant has also established that 
surprisingly the rigid sequential square wave stepping which has 
previously been used is far from ideal. In many instances, it will be 
desirable to use smooth transitions between the stages, particularly where 
overlap occurs, when reduction of the etch rate is acceptable. Thus one 
preferred arrangement is for the gas flow rates of the etch and deposition 
gases to vary with time in a sinusoidal manner the two "wave forms" being 
out of phase, preferably by close to 90.degree.. As the sidewall roughness 
is essentially a manifestation of the enhanced lateral etch component, it 
can be reduced by limiting this component of the etch. The desired effect 
can be obtained in one of a number of ways: partially mixing the 
passivation and etch steps (overlapping); minimising the etch (and hence 
corresponding passivation) duration; reducing the etch product volatility 
by reducing the wafer temperature; adding passivation component to the 
etch gas e.g. SF.sub.6 with added O, N, C, CF.sub.x, CH.sub.x, or 
replacing the etch gas with one of lower reactive species liberating gas 
such as SF.sub.6 replaced by CF.sub.x etc. 
The Applicant has also appreciated that changes in the levels of etching 
and deposition are desirable at different stages within the process. The 
Applicants propose that the first cycle or the first few cycles should 
have an enhanced deposition by increasing the period of deposition, the 
deposition rate, or any other suitable means. Equally or alternatively the 
etch rate or time can be reduced. 
As has briefly been indicated before, it also can become progressively more 
difficult to deposit material as the formation or trench gets deeper 
and/or the aspect ratio increases. By controlling the amplitude of one or 
more of the gas flow rates, chamber pressure, plasma power, biasing power, 
cycle time, substrate etching/deposition ratio, the system can be tuned in 
an appropriate manner to achieve good anisotropic etching with proper 
sidewall passivation. 
These and related techniques can be utilised to overcome a number of 
problems in the etch profile: 
a. Sidewall Notching 
The `sidewall notching` problem is particularly sensitive to the exposed 
silicon area (worse at low exposed areas &lt;30%) and is also correspondingly 
worse at high silicon mean etch rates. The Applicants believe such 
notching to be caused by a relatively high concentration of etch species, 
during the initial etch/deposition cycles. Therefore the solutions adopted 
by the Applicants are to either enhance the passivation or quench etch 
species during the first cycles. The latter can be achieved either by 
process adjustment (ramping one of more of the parameters) or by placing a 
material within the reactor which will consume (by chemical reaction) the 
etch species, such as Si, Ti, W etc. reacting with the F etchant. Such 
chemical loading has the drawback of reducing the mean etch rate, as the 
quenching is only necessary for the first few etch steps. Thus, process 
adjustment solutions are considered superior. 
It is desirable to reduce/eliminate the sidewall notching without 
compromising or degrading any of the other aspects of the etch, such as 
etch rate, profile control, selectivity etc. Investigations by the 
Applicants have shown the approach of `reducing the etch species 
concentration at the start of etch` is best controlled by beginning with 
process with: 
a. fluorine scavenging gas introduction or 
b. low coil power or 
c. low etch cycle time (step duration) or 
d. low etch gas flow or 
e. an increase of the corresponding parameters a to d above during the 
passivation cycle 
f. a combination of the above. 
followed by increasing the respective parameter(s) to normal pre-optimised 
etch conditions such as are illustrated in FIG. 6. The increase can either 
be abrupt (that is using say a step change in the a to f parameters) or 
ramped. The results of these two approaches are now presented, in 
comparison to the teachings of the prior art. 
The nature of the problem (resulting from directly applying the prior art) 
during a silicon trench etch is shown in FIG. 3 schematically and in the 
SEM's (scanning electron micrographs) shown in FIGS. 10 and 11. These 
Figures show that for a 1.7 .mu.m initial trench opening the CD loss is 
1.2 .mu.m (70%), whilst the notch width is up to 0.37 .mu.m. Such values 
of CD loss are unacceptable for the majority of applications. 
However by using the Applicants method (eg. a.to f.), of varying the 
process parameters during the initial cyclic etch process, the notched 
sidewall can be modified. If abrupt steps are used to vary the process 
parameters, abrupt transitions are produced in the sidewall profiles. The 
SEMs in FIGS. 12 and 13 illustrate this for different process parameters. 
In FIG. 12, the transition in the process parameters is clearly marked as 
an abrupt transition in the sidewall profile at the point of parameter 
change (after 8.5 .mu.m etch depth). (Note that the sidewall notches have 
been eliminated.) FIG. 13 illustrates yet another process parameter 
abrupt/step change. Here the sidewall passivation is high enough to result 
in a positive profile (and no notching) for the first 2 .mu.m. When the 
reduced passivation conditions are applied, it is characterised by the 
transition in sidewall angle and reappearance of the notching. 
By using the `ramped` parameter approach, the notching can be eliminated, 
as well as producing a smooth sidewall profile without any abrupt 
transition, see the SEM in FIG. 14. This shows a 22 .mu.m deep trench 
etch, with a smooth positive profile and no CD loss, whilst maintaining 
etch rate comparable to the non-ramped high underact process. The process 
conditions used in this case are given in FIG. 19a. 
b. Profile control during deep high aspect ratio etching 
The teachings of the prior art are limited where high aspect ratio (&gt;10:1) 
etching is required. Whilst the limitations and solutions are discussed 
here for relatively deep etching (&gt;200 .mu.m), there is equal relevance 
for shallow high aspect ratio etching, even for very low values of CD, 
such as &lt;0.5 .mu.m. 
One of the basic mechanisms, that distinguishes high aspect ratio etching, 
is the diffusion of the etching (and passivation) reactive precursors as 
well as etch products. This species transport phenomenon was investigated 
for the passivation step. The results show clearly that the transport of 
sidewall passivation species to the base of deep trenches is improved at 
low pressure. Increasing platen power also improves this, see FIG. 15. The 
graph illustrates the improved passivation towards the base of the trench 
as the pressure decreases and the rf bias power increases. This data was 
obtained by firstly etching 200 .mu.m deep trenches, then using the 
passivation step only and measuring the variation of sidewall passivation 
with depth using an SEM. This supports the variation of passivation with 
etch depth, and further supports the suggestion that the optimum process 
conditions vary with etch depth. 
The limitations of applying the prior art for such a high aspect ratio 
process is shown by the SEM in FIG. 16. It should be noted that this 
relatively high passivation to etch ratio fixed parameter process still 
does result in initial sidewall notching, but the SEM magnification is not 
sufficiently high to show this for the 10 .mu.m CD, 230 .mu.m deep trench 
etch. From the trends shown in FIG. 15, the profile can be somewhat 
improved by operating at under the desired high bias rf power and low 
pressure conditions. However as a fixed parameter process, the high bias 
and low pressure conditions significantly degrades the mask selectivity 
(from &gt;100:1 to &lt;20:1) as the ion energy is increased. Using an abrupt 
parameter variation results in a corresponding abrupt sidewall change, as 
shown by the SEM in FIG. 17. Ramping the following parameters: increasing 
platen power, decreasing pressure, increasing cycle times and gas flows; 
does produce the desired results whilst maintaining reasonably high 
selectivities &gt;75:1, see FIG. 18. Here the SEM shows a 295 .mu.m deep, 12 
.mu.m CD trench etch (25:1 aspect ratio). The process conditions used in 
this case are given in FIG. 19b. 
FIG. 20 illustrates a synchronisation between deposition and etch gases 
which have been used for the initial cycles to reduce side wall notching. 
Typical operating conditions are given in FIG. 19a and its associated SEM 
in FIG. 14. FIG. 21 illustrates a synchronisation reference to using a 
scavenger gas with method (a) of side wall notch reduction technique. The 
dotted line indicates the alternative of the scavenger gas flow rate being 
decreasingly ramped. 
FIG. 9i shows a synchronisation for achieving a deep high aspect ratio 
anisotropic etch although the ramping technique shown can also be used for 
side wall notch reduction. The conditions of FIG. 19b can be used to 
achieve the results shown in FIG. 18. 
Returning to FIG. 9i: 
1. Shows an average pressure ramp. Note the pressure changes from low to 
high as the cycle changes from deposition to etch respectively. The ramp 
down of pressure then results in the pressure decreasing for both etch and 
passivation cycles. 
2. This shows a rf bias power ramp. Note the bias change from low to high 
as the cycle changes from deposition to etch respectively. This is in 
synchronisation with the pressure change discussed above. The ramp up of 
bias refers to the deposition step only in this case. 
3. This shows another example of a rf bias power ramp. Again the bias 
changes from low to high as the cycle changes from deposition to etch 
respectively, in synchronisation to the pressure. The ramp up of bias 
refers to both the deposition step and etch step in this case. 
In FIG. 9ii general parameter ramping is illustrated. These examples serve 
to illustrate cycle time and step time ramping respectively. 
4. This shows a cycle time ramp, where the magnitude of the parameter (such 
as gas flow rates, pressure, rf powers, etc) is not ramped. In some 
applications this would serve as an alternative to the `magnitude` ramping 
in the above cases. 
5. This shows a cycle time ramp, where the magnitude of the parameter (such 
as gas flow rates, pressure, rf powers, etc.) is ramped in addition. Note, 
the parameter ramp may be increasing or decreasing in magnitude, and the 
decrease may be to either zero or a non-zero value. 
3. Etch Gases 
Whilst any suitable etch gases may be used, the Applicant has found that 
certain gases or mixture can be beneficial. 
Thus, it has been suggested in WO-A-94114187 that it is undesirable to have 
any passivating gas in the etch stage, because it affects the process 
rate. However, the Applicant has determined, that this procedure can 
significantly improve the quality of the sidewall trenches formed and it 
is proposed that the etched gas may have added to it such passivating 
gases as O,N,C, hydrocarbons, hydro-halo carbons and/or halo-carbons. 
Equally, and for the same purpose, it is desirable to reduce the chemical 
reactivity of the etched gas and the Applicant proposes using CF.sub.x for 
example with higher atomic mass halides such as Cl, Br or I. However 
XeF.sub.2 and other etch gases may be used. 
The degree of sidewall roughness can also alternatively be reduced by 
limiting the cycle times. For example it has been discovered that it is 
desirable to limit the etch and deposition periods to less than 7.5 
seconds and preferably less than 5 seconds. 
4.Gallium Arsenide and other materials 
Previous proposals have all related to trench formation in silicon. The 
Applicant has appreciated that by using suitable passivation, anisotropic 
etching of Gallium Arsenide and indeed, other etchable material, can be 
achieved. For example it is proposed that Gallium Arsenide be etched with 
Cl.sub.2 with or without passivating or etch enhancing gases, but in 
general it has been determined that this technique is much more successful 
with the carbon or hydro carbon passivation proposed above. If traditional 
CF.sub.x chemistry is used etch inhibiting compounds can be created which 
increase surface roughness or limit the etch. For Gallium Arsenide lower 
temperatures may be desirable as may be the use of a low pressure, high 
plasma density reactor. Suitable etch chemistries have already been listed 
in the preamble to this specification.