Journal of The Electrochemical
Society
You may also like
An Overview of Dry Etching Damage and
Contamination Effects
To cite this article: Stephen J. Fonash 1990 J. Electrochem. Soc. 137 3885
View the article online for updates and enhancements.
- Black silicon method X: a review on high
speed and selective plasma etching of
silicon with profile control: an in-depth
comparison between Bosch and cryostat
DRIE processes as a roadmap to next
generation equipment
H V Jansen, M J de Boer, S Unnikrishnan
et al.
- Foundations of low-temperature plasma
enhanced materials synthesis and etching
Gottlieb S Oehrlein and Satoshi
Hamaguchi
- High quality shadow masks for top contact
organic field effect transistors using deep
reactive ion etching
Muhsen Aljada, Karyn Mutkins, George
Vamvounis et al.
This content was downloaded from IP address 128.227.1.50 on 21/04/2023 at 16:51
An Overview of Dry Etching Damage and Contamination Effects
Stephen J. Fonash*
Center for Electronic Materials and Processing, The Pennsylvania State University,
University Park, Pennsylvania 16802
ABSTRACT
Plasma or dry etching techniques, such as reactive ion etching, magnetron reactive ion etching, electron cyclotron resonance etching, ion beam etching, and plasma etching, can result in damage and contamination of the materials used in
device structures and interconnects. The damage that can take place occurs because of ion b o m b a r d m e n t , radiationinduced b o n d i n g changes, and charge buildup. If not removed, damage can have effects which vary from gettering to
creating traps in insulators and gap states in semiconductors. The contamination that can take place can have two forms:
residue layers and permeation. The former are ultrathin layers of reaction products that can coat surfaces whereas the latter is the e m b e d d i n g or implantation, and perhaps subsequent diffusion, of impurities. These impurities can range from
metals whose source is the processing chamber to hydrogen which is often found in the etch recipe or may even be found
due to the presence of water in the processing chamber. If not removed, residues can interfere with s u b s e q u e n t processing
such as oxidation and can cause series resistance in contacts. Impurities, if not removed, can have a range of effects from
modifying carrier lifetimes to interfering with subsequent processing. Hydrogen, in particular, can affect materials in a
n u m b e r of ways including modifying doping activation.
Dry etching techniques such as reactive ion etching
(RIE), magnetron reactive ion etching (MRIE), plasma
etching (PE), ion beam etching (IBE), electron cyclotron
resonance (ECR) etching, and reactive ion beam etching
(RIBE) can cause damage and contamination effects in exposed materials (1-14). I n fact, damage is often inherent in
these processes due to the presence of ion bombardment,
which can create b o n d i n g damage in semiconductors and
insulators (1-9, 11, 13), as well as due to the presence of UV
radiation, which can create b o n d i n g damage in insulators
(10). Contamination is also often inherent in these processes due to the presence of residue layers made up of reactant species and reaction products and due to the presence of impurities which may permeate the etched
material during the dry etching exposure (3,4, 11-22).
These damage and contamination effects that can be inherent in dry etching are shown schematically in the
model of Fig. 1 together with an indication of their depth
of influence for the case of silicon. As may be seen, these
inherent damage and contamination effects basically can
produce three different kinds of layers in a material: residue layers (surface films), permeated layers (impurity permeation), and damaged layers (bonding damage) (4). The
boundaries between these layers are not distinct; often
they meld into one another. Damage can also occur due to
charge b u i l d u p and discharge resulting from etching (5),
and, in addition, surface roughening can take place due to
partial masking of an etching surface by residues and impurities (23, 24).
If not controlled, these various damage and contamination effects present in dry etching, if not controlled can
quite adversely influence material properties, subsequent
processing, and device performance. Dry etching damage
and contamination effects manifest themselves in a number of ways in etched dielectrics from shifts in neutral trap
densities and positive charge (10) to increased leakage currents (25). Dry etching effects manifest themselves in
etched silicon in changes in surface properties (4), in
changes in generation lifetimes (12), in changes in doping
activation (20-23), a n d can even result in changes in diffusion lengths (26). S u b s e q u e n t processing steps on a silicon
surface, from silicidation (27) to oxidation (19), can be impacted if the reacting surface has been exposed to prior
dry etching. I n fact, gate oxides grown on dry etched silicon have been demonstrated to have properties that
strongly depend on the etching and the subsequent
cleaning processing (19). Capacitor structures fabricated
on dry-etched Si surfaces show that the oxides display increased interface state densities which are strongly dep e n d e n t on the etching ion species and energy (28) as well
as breakdown voltages that also depend on etching history
(5, 19, 25). All of these various effects of dry etching on material properties and s u b s e q u e n t processing propagate
* Electrochemical Society Active Member.
into devices in a n u m b e r of ways: from shifts in the turnon voltage in FETs to extraneous series resistance in contacts.
Given the crucial importance of dry etching to the fabrication of micron and submicron features, it becomes necessary to be fully aware of all of the damage and contamination aspects of dry etching and of their implications to
material properties, s u b s e q u e n t processing, and device
performance. This overview examines the causes of damage and contamination, the methods of detection of damage and contamination, and approaches to damage and
contamination control.
Damage and Contamination
Residues--their effect, detection, and repercussions.-The residue layer is a surface film stemming from the etching process. Whether or not such a layer is present depends on the etching chemistry. When present, it is composed, to varying degrees, of reactant species and reaction
products; hence, its composition depends on the etching
gas and the exposed materials. It may also contain impurities. The selectivity and even anisotropy, in some cases, of
m a n y dry etching recipes is based on the formation of a
residue in the form of a reaction-blocking coating layer on
the slowly (or effectively not) etched material (17, 29, 31).
Surface
Region I
Residue Layer
(Tens of Angstroms)
Region 2
Bonding
Damaged Layer *
(Up to hundreds
of Angstroms)
Region 3
Impurity
Permeation Layer
(Up to ~10 microns)
p~.SSIBI F IMPIJRITIE5
IN REGION 3
[~
Oxygen
Metaliic~
Hydrogen
Fig. 1. Schematic representation of the various damage and contamination layers produced by dry etching. Depths indicated are intended to
give a representation of the possible extent in silicon.
J. Electrochem. Soc., Vol. 137, No. 12, December 1990 9 The Electrochemical Society, Inc.
3885
J. Electrochem. Soc., Vol. 137, No. 12, December 1990 9 The Electrochemical Society, Inc.
3886
Hence, this type of contamination is inherent in many selective dry etching approaches.
I n the case of fluorocarbon-based RIE chemistries, the
presence of reaction-blocking, polymer-like residue layers
on Si for recipes designed to etch SiO2 over Si has been
confirmed by several groups (17, 32, 33), and the effect of
the ion b o m b a r d m e n t on the polymeric nature of these
RIE-produced films has been explored (30). A recent review of RIE etching (31) has examined reaction-blocking
residue layer formation as a function of substrate (photoresist, Si, WSi2, TiSi2, and Ti) and etching chemistry. That recent study also explores residue layers on the RIE etching
surface. That is, it examines RIE etching situations where
the residue layers are not part of a selectivity mechanism
but are simply layers of reactants and reaction products on
the surface of the material that is undergoing etching. In
particular, that work showed that CF4/Oz RIE of Si produced a SiFxO~ layer whose composition and thickness
control the etch rate of the silicon (31).
The presence of residue layers as well as the nature of
their b o n d i n g has also recently been established for CC12F2
MRIE of silicon (11), as well as for CF4ECR etching of silicon (13, 14). The latter work also compared the residues
present in CF4ECR etching of Si with those present if RIE
is used or if a hybrid RF-biased ECR (hybrid ECR/RIE) is
used. The results showed that ECR etching exhibited a
t h i n n e r residue layer than the RIE or the hybrid ECR/RIE.
Interestingly, the ECR etching has the lowest self-bias
voltage suggesting that, for this chemistry, residue formation was enhanced by ion bombardment.
The principal techniques that have been used to establish the presence and chemical makeup of these different
kinds of residue layers are x-ray photoelectron spectroscopy (XPS), secondary ion mass spectroscopy (SIMS), and
Auger electron spectroscopy (AES) (14, 16, 29, 31); in addition, electrical measurements have been used to infer the
presence of residue layers since these layers, when sufficiently thick, can cause series resistance in Schottky barrier and ohmic contacts (4). Ellipsometry has also proven
to be useful in determining the presence of residue layers
(34-36). Recently it has been used for in situ process monitoring of residue effects (23, 24).
An obvious repercussion of the presence of residue layers is that these layers can cause series resistance as seen
in Fig. 2. These data show the current-voltage (I-V) characteristics for Au Schottky barrier contacts made on n-type
Si (35). The control I-V is for devices made on wet etched
Si surfaces; the other I-V is for Au contacts made on
CC1F~-Iz RIE exposed Si surfaces. As can be seen, the forward characteristic of the device made on the RIE exposed
Si surface shows the presence of what can be phenomenologically described as a series resistance. Further charac1
IO - 4
I'
/
I
/
--
I
1
I
I
/Fcrword
i
/
~
/
/
I~
/ /
/
~
Reverse
///Sy -- ......
eve, e
I
0
,
r
O.l
I
0.2.
f
O.3
t
r
[
0.4
0.5
0.6
BIAS (VOLTS}
[
0.7
l
(3.8
I
0.9
.LO
Fig. 2. I-V characteristics of Au/.-Si. the CCIE3 + H2 RIE was 50%
overetch ef blanket Si02 en Si.
terization in this case substantiated that this resistance
was indeed coming from a residue layer on the Si surface
(35). Another repercussion of the presence of surface layers containing reaction products, and perhaps impurities,
can be surface roughening. This can occur if the residue
layer is discontinuous or if there are islands of impurities
on the surface. The resulting micromasking can result in
roughening of the etching surface (23, 24). Residue layers
can also affect further processing. As noted earlier, their
presence can affect, for example, the interface quality
(3, 10), growth (3, 10), and breakdown characteristics (19) of
oxides grown on RIE- and PE-exposed Si surfaces.
Residues--controL--Determining if a residue layer is
present and establishing its composition are obvious, necessary steps in reconditioning an exposed surface after dry
etching. For example, rapid thermal annealing (RTA) for
the removal of the dry etching-induced b o n d i n g damage in
silicon, discussed later, is not successful in cases where
polymer-like surface films are present unless these residue
layers are removed prior to RTA (37, 38). A n u m b e r of detailed studies have been u n d e r t a k e n to examine residue
layer removal (19, 31, 37-39). I n a study of silicon overetching using the highly SiO2 selective etch CC1FJH2 RIE, it
was found that both 02 ashing and 02 RIE, followed by an
HF dip, were effective in removing a polymer-like residue
on the Si surface (35). It was also found that, i r a Cr~O3 etch,
followed by an H F dip, was used after O2 ashing, the surface further improved due to additional Si removal (35). An
HF dip is required after these aforementioned residue removing procedures since they all produce a thin silicon
oxide layer (35). Rather than using oxygen-based plasma
techniques (Oz ashing, O3 RIE) to volatilize residue layers
or all wet-chemistry techniques (35), alternative approaches such as the use of hydrogen-based plasma (39) or
UV/O~ exposures for volatilization (40) have recently been
explored.
Impurity contamination and permeation--the effect, its
detection, and repercussions.--Impurities, particularly metallic impurities such as Fe, Ni, A1, Na, Cr, K, and Zn, have
been detected by a n u m b e r of groups (2, 3, 13, 14, 19, 28) on
the surface, and in the near-surface region, of materials exposed to dry etching techniques as varied as PE, RIE,
ECR, and MRIE. These impurities are believed to arise
from dry etching processing chambers and their fixturing.
In a recent study it has been found, for example, that when
"Lexan" (a high-temperature polymer) is used to cover
wafer trays in an RIE reactor in place of the tray coating
"Ardel," the a m o u n t of metallic impurities such as Ca, Na,
K, and Z n present on the surface, and in the near-surface
region, of an etch sample can be greatly diminished (19). In
the case of ECR and hybrid ECR/RIE etching, it has been
shown that the high plasma densities involved in these
techniques result in significant contamination from the
ECR chamber walls (13). This was also solved by covering
the involved surface. I n this situation the chamber walls
were covered with an anodized A1 liner (13).
The presence of metallic impurities can be detected in a
n u m b e r of ways. Since it is well established that oxidation
stacking faults (OSF) can be caused by metallic impurities
(41), OSF can be used as a detection mechanism. For example, high-temperature oxidation of RIE-exposed Si, followed by oxide stripping and a SECCO etch, has been
used to demonstrate the presence of OSF on an RIEexposed Si surface with an optical microscope (19). Since
this was done on Si surfaces where the residue had been
removed prior to oxidation, the presence of these stacking
faults was attributed to metallic impurities such as Cr and
Fe (19). This was substantiated using SIMS. Since residue
cleaning procedures, including RCA cleans, were not able
to remove this metallic contamination, it was suggested
that the metallic contamination must have permeated
(-100/~) into the silicon during the RIE (19). It is interesting to note that the same study did not find these stacking
faults for PE-exposed material. In general, PE involves
lower plasma potentials and should, therefore, cause less
"scrubbing" of the chamber and fixturing. The lower
plasma potentials should also result in less effective "ira-
J. Electrochem. Soc., Vol. 137, NO. 12, December 1990 9 The Electrochemical Society, Inc.
planting" of impurities into the etching surface. A similar
stacking fault study was also u n d e r t a k e n to compare
CF4RIE, ECR, and hybrid ECR/RIE etching of Si. The results of this study, for the case when a chamber lining was
used, indicated that high stacking fault densities were only
found for the RIE exposure (13).
This metallic contamination and permeation which can be
present due to dry etching can also be detected by SIMS
in m a n y cases (19, 28). Of course, the utility of SIMS will
depend on the detection limit for a given species. It has
also been detected in some studies by electrical techniques such as deep level transient spectroscopy (DLTS)
(41-43) and minority-carrier generation lifetime (3, 12, 13).
I n general, the presence of metallic impurities from dry
etching can affect s u b s e q u e n t processing and device performance. A n example is the poor-quality oxides (poor interface state properties, low breakdown voltage) that can
result if the metallic impurities, resulting from the dry
etching of the silicon, are not successfully removed prior
to oxidation (2, 3, 19, 44). The use of such oxides in gate/
SiOJSi structures leads to severely degraded device performance (2, 3, 19, 44). The repercussions of the metallic
impurity contamination and permeation that can result
from dry etching can also be significant for p-n junctions.
For example, in cases where RIE was done prior to the
j u n c t i o n formation, it has been found that high leakage
currents are present, if care is not taken to remove metallic
impurities. These high leakage currents are believed to be
due to stacking faults, which are nucleated and made electrically active by metallic contaminants from the etching
chamber (45).
Impurity contamination and permeation--control.Clearly the best way to control impurities is to avoid their
presence using techniques such as the coatings discussed
in the previous section. However, when impurities are
present, just as in the case of residue layers, care must be
exercised in their removal. Currently, wet or mixed dry/
wet cleaning approaches are being utilized; however, there
has also been exploration of all-dry cleaning procedures
for removing metallic impurities (46). These approaches
usually rely on actually removing surface layers of the contaminated and permeated materials themselves. I n some
cases these cleaning techniques have been combined with
various types of gettering procedures (42-45).
Some of the dry/wet and wet cleaning procedures that
are currently being used to remove the residue layer and
metallic impurity contamination and permeation present
after dry etching of Si are listed in Table I, We note that
cleaning procedures that remove more of the Si at the dryetched Si surface at low temperatures are more effective in
restoring the material. This is believed to be due to the fact
that the use of low temperatures retards deeper movement
of the metallic impurities while the consuming of the Si
near-surface region allows the capture of these impurities
(45). This experimental observation provides further evidence for the various kinds of permeation conceptualized
in Fig. 1.
Hydrogen permeation--the effect, its detection, and repercussions.--Hydrogen is present in m a n y dry etching
recipes. However, even w h e n it is not purposefully part of
the etching, hydrogen is present as an impurity since it is
relatively abundant, in the form of H20, at the pressures of
current technological interest in dry etching. Regardless of
its source, hydrogen is readily converted in a dry etching
processing chamber into species such as atomic hydrogen
and H § which can then easily move through SiO2 and Si
(47, 48). The first demonstration that this hydrogen permeation effect is present in dry etching showed that hydrogen
at the atomic percent level permeated 400A beneath a
CFgH2-etched Si surface (17). S u b s e q u e n t studies of hydrogen permeation during dry etching have showed the
surprising result that hydrogen can actually permeate microns below an etched surface at concentrations sufficient
to cause significant dopant deactivation (18, 20, 22, 49).
Very recent work has established that this hydrogen permeation can take place through both n-type and p-type Si
(50). This result is important because it sheds light on the
3887
Table I. Some post dry etching surface cleaning approaches for silicon
Name
02 ash/acid clean
[Ref. (35)]
02 ash/acid clean/
dilute HF dip [Ref.
(35)]
O3ash/acid clean/Si
wet etch [Ref. (35)]
02 RIE/acid clean
[Ref. (35)]
Si wet etch
[Ref. (35)]
H2 plasma clean
[Ref. (39)]
O2 ash/acid clean/
RCA clean
[Ref. (45)]
O2 RIE/acid clean/
RCA clean
[Ref. (45)]
02 ash/chromic acid
etch/acid clean/
RCA clean
[Ref. (45)]
Procedure~
02 ashing followed by hot H2SO4I-INO3
clean
02 ashing followed by hot H2SO4/I-INO3
clean and then a 5% HF dip.
02 ashing followed by hot H2SO4/HNO3
clean/BHF (40:1) dip. Si then subjected to silicon etch using chromic
(Cr203) acid solution with a follow-on
HC1/H202 clean.
02 RIE followed by hot H2SOJHNO3
clean.
Silicon etch using chromic (Cr203) acid
solution.
H2 plasma with low RF power, high
pressure, and high H2 gas flow.
02 ashing followed by buffered HF dip,
followed by H2SO4/HNO3clean, followed by an RCA clean.~
O2 RIE, followed by buffered HF dip,
followed by H2SOjHNO3 clean, followed by an RCA clean,b
02 ashing, followed by buffered HF
dip followed by a CrO3-H20-HFetch
followed by H2SO4/HNQ clean, followed by an RCA clean,b
a Deionized water rinses are used as appropriate.
b RCA clean is a wash in NH4OH-H202 solution followed by a
wash in HC1 + H202 solution.
question of whether the hydrogen permeates during dry
etching exposures as a neutral or charged, mobile species
(47). Other very recent work further suggests that hydrogen is amphoteric in silicon and can move as a positively
charged species in p-type Si and as a negatively charged
species in n-type Si (51).
Whatever the actual permeating hydrogen species is, its
infusion as a result of dry etching can be detected using a
variety of techniques. These range from the 1H + 15N nuclear reaction technique (17, 35) to SIMS and to spreading
resistance profiling. The I~N nuclear reaction technique
determines the total hydrogen population in a sample; i.e.,
it detects H2, H, H-, and H § species. However, it can only
detect a fraction of an atomic percent at best. SIMS also
determines the total hydrogen population, and by using
deuterium in place of hydrogen in etch recipes, the hydrogen background problem (hydrogen coming from the
SIMS chamber itself) can be avoided. If deuterium is used,
total hydrogen concentrations of permeated hydrogen
>-1015 cm -3 can be detected (52).
The spreading resistance profiling (SRP) approach to
monitoring the permeation of hydrogen caused by dry
etching makes use of the fact that the presence of hydrogen in silicon results in doping deactivation and, hence, results in a change in carrier concentration (i.e., resistance).
Every lost carrier is attributed to effective doping deactivation by a hydrogen species (18). This doping deactivation effect occurs to some extent in n-type silicon but it
is very p r o n o u n c e d in p-type Si (47). It is noted that SRP
need not detect all the hydrogen that is present in a
sample; it only detects hydrogen that is involved in doping
deactivation. Using SRP, hydrogen injected during dry
etching has been detected at concentrations as low as 1014
cm -3 (52).
From this discussion of spreading resistance profiling it
is obvious that one repercussion of hydrogen permeation
during dry etching is this p h e n o m e n o n of doping deactivation in p- and n-type materials. Figure 3 shows doping
deactivation in p-type Si samples caused by hydrogen permeation during CHF3/C2F6 PE or CHFJO2 RIE. The PE parameters were 60 sccm: 60 sccm CHF3:C2F6 at 700W and
500 mtorr. The RIE heavy damage parameters were 75
sccm: 9 sccm CHF3:O2 at 1300W and 20 mtorr whereas the
RIE light damage used 1000W and 60 mtorr. Fortunately,
doping deactivation anneals out; for example, in p-type
material it is found to anneal out at temperatures in the
200~176 range (22). We note that Fig. 3 also displays another p h e n o m e n o n associated with hydrogen permeation
J. Electrochem. Soc., Vol. 137, No. 12, December 1990 9 The Electrochemical Society, Inc.
3888
I
1
I
f
S U L K ACCEFTOR CONCEZq'TRATIt3N
9
4~4k 4~ 9
gee
"/~e ee e 9
~o
LJ
~
.~ o o
9,~, o
0
PE HEAVY D A M A G E
,a~. oo
='~' o o
Do
o
~. REGION I
9 REGION II
oo
RIE HEAVY D A M A G E
o REGIDN |
a
9 REGIDN l!
~z
1
t
I
I
2
3
t
t
4
5
OEm'HO~)
Fig. 3. Active dopant concentration profiles calculated from spreading resistance data for bulk p-type silicon etched under the RIE and PE
heavy damage parameters. These ore defined in the text. Note the delineation between region I and II.
in p-type Si during dry etching; namely, a two-region behavior. I n general it is found that there is a near-surface
region in which hydrogen-related species causing deactivation move more slowly and in which doping deactivation anneals out differently compared to the second
deeper, but adjacent layer (22, 49).
A repercussion of this hydrogen permeation in terms of
device structures is seen in the SRP data of Fig. 4. Here a
p-n j u n c t i o n has been formed by 100 keV implant of B §
(dosage of 1El3) into n-type silicon. The resulting junction
after an activation anneal is seen in the figure. As may also
be seen, a subsequent dry etching exposure after the activation anneal causes the j u n c t i o n to shift -2000/1, due to
preferentially more effective deactivation of the boron by
the hydrogen which permeated during the dry etching
(53). This j u n c t i o n shift caused by the dry etching necessitates an additional anneal (at -20O~ ) to reactivate all the
dopant and restore the junction position. Hydrogen permeation that occurs during dry etching may also have repercussions, other than that seen in Fig. 4, on subsequent
processing and device structures. I n particular, the presence of hydrogen injection raises the question of whether
hydrogen present in materials due to dry etching can collect at SiOJSi interfaces and at metal/silicon interfaces
during subsequent processing. In the latter regard, it has
been suggested that the presence of hydrogen from dry
etching can affect contact resistance (54). We also note that
hydrogen can affect processing and device behavior
through the b o n d i n g damage and b o n d i n g passivation
19
I
....... IEI3
-----IEI3
E
u18
ACTIVATED
AFTER DEUTERATION
Z
O
m
I.-.
<(
Z
hi
Z
Q
.................. . / / :
n"
tlJ
ill:
0
-J
14
I
.25
I
I
.I
.50
.75
I.O
D E P T H (t, ffcrons}
l
1.25
I
L5
Fig. 4. Carrier concentration profiles for n-type epitaxial silicon implanted with HB+ at 100 keV to a dose of IE|3 cm -z after activation
and subsequent deuterafion.
changes it can cause. These are discussed further in the
section which deals with b o n d i n g damage.
Hydrogen permeation--controL--Hydrogen permeation
is very much a part of dry etching as it is now practiced.
The use of new dry etching approaches such as magnetically enhanced RIE or ECR etching will not remove
this hydrogen permeation problem. I n fact, present evidence suggests that these dry etching approaches may actually exacerbate the problem due to the a b u n d a n t presence of H and H § (54). Removing the source of the
hydrogen, present in m a n y etch recipes and present ultimately through the base pressures currently used for dry
etching appears to be the only solution to fully preventing
hydrogen permeation during dry etching. However, it is
currently not clear how serious a problem this hydrogen
permeation really presents. It does obviously cause dopant
deactivation but, at least in the case of p-type silicon,
where the effect is significant, it is easily annealed out. It
does also cause extended structural defects, which we
treat in the next section as part of dry etching bonding
damage; these have been reported to exist as far as 0.5 ~m
below etched Si surfaces (55, 56). However, its most insidious role may lie in its effects at interfaces and in dielectrics. These remain to be explored and understood.
Bonding damage--the effect, its detection, and repercussions.--The b o n d i n g damage that results from dry etching
exposure has its roots in three distinctly different causes.
One is the ion b o m b a r d m e n t inherent in at least all current
anisotropic dry etching, another is UV radiation, and the
third is hydrogen. These causes can obviously be present
together. The ion b o m b a r d m e n t inherent in current anisotropic dry etching techniques is needed to create that
anisotropic etching by doing one or more of the following:
(i) it produces physical sputtering of chemically weakened
bonds, (ii) it can result in damage-enhanced chemical reactivity, (iii) it can provide energy to drive chemical reactions, and (iv) it can remove residue layers exposing material to etching attack (4).
This same b o m b a r d m e n t that gives anisotropy also produces undesired b o n d i n g damage. This damage often increases with ion energy (4, 7, 11, 57); however, recent work
comparing RIE, ECR, and hybrid ECR/RIE etching underscores the fact that this may not always be the case (13).
That study points out that the rate and extent of ion bomb a r d m e n t damage plays off against the rate of damage removal by etching. The result is that there can be less residual ion damage with increased ion energy, if ion damage
removal by etching dominates with the increasing of ion
energy (13).
The effects of dry etching ion b o m b a r d m e n t damage are
easily seen with the very simple yet very sensitive monitor
provided by current-voltage characteristics of metal/
semiconductor contacts made on etched surfaces (6). The
data of Fig. 2 give one example for n-type Si. As noted in
the first section, the presence of a residue layer can be seen
in these data. However, the figure also shows that dry etching exposure (CCIF~rI2RIE) has resulted in a lower barrier
height for contacts made on the RIE-treated n-type Si surface. This is seen in the increased reverse current of the
contact made to the RIE surface. Figure 5 shows the same
barrier lowering p h e n o m e n o n for n-type Si subjected to
CCl4 RIE, a chemistry for which residue effects are relatively insignificant (58). This etching was done at a power
level of 300W and the pressure, as seen in the figure, was
varied between 20 and 100 mtorr. The damage is seen to increase as the pressure during this RIE etching is reduced.
Figure 6 shows the use of the I-V characteristics of
metal/Si contacts to p-type Si to monitor dry etching damage. Here the pressure was held constant at 100 mtorr of
CClgI-Ie while the power was varied from 100 to 600W. I n
the case of p-type material the barrier is seen to increase
with RIE exposure. I n particular, it increases more with
higher power. I n general, it is found that dry etching damage causes metal/silicon contact barrier heights to decrease on n-type Si and increase on p-type Si. It is well
documented for PE and RIE that this damage-caused barrier height shift at etched Si surfaces is enhanced with in-
J. Electrochem. Soc., Vol. 137, No. 12, D e c e m b e r 1990 9 The Electrochemical Society, Inc.
I
I
3889
100 --, 600 Watts
I
50~
Without RIE
I00
4O
[
3O
20
V
600
500
400
I0
I L~IA) 0,
-I0
-20
300
-30
-40
-60
200 tO0 Watts
Fig. 6. Effect of power during RIE on linear I-V curves for Au/p-Si
contacts. Au contact was made after RIE treatment. Gas used was
CCI4 + He and x-axis is 0.2V/div. and y-axis is 5 mA/dlv.
I
-3
1
I
]
I
I
-Z
-1
I
2
3
v (~ttsl
Fig. 5. Effect of pressure for CCI4 RIE (300W, i-1/2 rain) as seen for
linear I-V curves for Au/n-Si contacts. The Au/n-Si contacts were made
after RIE.
creased power or decreased pressure (57). Increasing
power and decreasing pressure in RIE or PE are both well
k n o w n to increase the energy of b o m b a r d i n g ions (55, 59).
This shift in barrier height (Fermi level position) at a dryetched silicon surface is found to be ubiquitous. It is well
k n o w n to occur for ion beam-etched Si (7, 59, 60), plasmaetched silicon (4, 19), reactive-ion-etched silicon (4, 8), and
magnetron-reactive-ion-etched silicon (i1). Interestingly,
it is also found to occur for argon ion-implanted silicon
(61). Because the effect is i n d e p e n d e n t of etching tool and
etching chemistry and is present in inert gas ion implantation, where impurity implantation and permeation should
be very minimal, this barrier height change effect is attributed to b o n d i n g damage caused by ion b o m b a r d m e n t (4).
The threshold energy for this ion bombardment-caused
b o n d i n g damage is found to be 30-50 eV in silicon (8).
One can see this b o n d i n g damage implied by Fig. 2, 5,
and 6 directly in Fig. 7. This figure shows the reflected
high-energy electron diffraction (RHEED) signature of an
unetched control Si wafer and of a Si wafer given a blanket
CC14 etch. This is the same etch used for the data of Fig. 5
and, as noted earlier, this RIE etching chemistry leaves a
relatively insignificant residue (58). Hence, the haze of
amorphized material and the rings indicative of a polycrystalline Si structure seen in the RHEED pattern of this RIEexposed Si surface can be interpreted as direct evidence of
Si b o n d i n g damage (37). Other direct measures of b o n d i n g
damage such as electron spin resonance (ESR) (37) and
Rutherford backscattering (RBS) (17) have been used to
directly ascertain that the b o m b a r d m e n t of dry etching
does cause b o n d i n g damage.
Techniques, which offer more potential for routine assessment of dry etching damage, have also been examined. These include ellipsometry (23, 24, 36), which has
been employed in situ, spectroscopic ellipsometry (34, 62),
and the therma-wave technique (52, 63). Some thermawave (TW) data are shown in Fig. 8 as a function of anneal
temperature. Here the following designations have been
used: RIE heavy--CHFJO2 at 1300W and 20 mtorr, RIE
light--CHF3/O2 at 1000W and 600 mtorr, and P E - - C H F J
C2F6 at 700W and 550 mtorr (52). As may be seen from these
data, reduction of the TW signal (at about 200~176
anneals) does not m e a n the dry etching damage is removed.
Indeed, the figure shows the TW signal rises above control
at the higher annealing temperatures and finally falls to
the control at temperatures -<800~ The effects of anneal-
ing on b o n d i n g damage are discussed more fully in the following section.
The b o n d i n g damage caused by b o m b a r d m e n t is seen in
both silicon and insulators, whereas UV b o n d i n g damage
is only seen in insulators. Bonding damage from UV radiation manifests itself in the creation of neutral traps and
positive charge in the case of SiO~ (10). Hydrogen-caused
b o n d i n g damage has been reported so far only in the case
of silicon where it has been observed, using cross-sectional
transmission electron microscopy, that hydrogen can permeate Si and produce fissures and "bubbles" as far as
0.5 ~m beneath the surface (55, 56). Of course, hydrogen
b o m b a r d m e n t also produces surface b o n d i n g damage and
it can cause the same b o n d i n g damage effects seen in
Fig. 5 and 6. From the point of view of RHEED, the surface
actually appears more damaged when hydrogen is purposefully present in an RIE recipe (18). However, such a
surface may appear electrically (through I-V characteristics) to be less damaged when hydrogen is present due to
the well-known ability of hydrogen to passivate bonding
damage including b o n d i n g damage caused by the hydrogen itself (18, 64).
Bonding damage---controL--One obvious solution to the
problem of b o n d i n g damage is to eliminate the presence of
its causes during dry etching. However, anisotropic etching may require particle energies that are high enough to
cause b o n d i n g damage for any dry etching technique. Another solution is to have the etching rate sufficiently high
that ion-bombardment-damaged layers are effectively removed by the etching itself. This will necessitate very careful control of physical and chemical effects. If bonding
damage does occur, the only recourse is to remove it or to
anneal it out. I n the case of Si m a n y of the silicon cleaning
procedures listed in Table I are designed to remove at least
some of the damaged silicon layer shown schematically in
Fig. 1.
If annealing is chosen to remove dry etching bonding
damage, one has to be careful that the high temperatures
involved do not drive in impurities. Consequently, any impurities and residue layers present must be removed before attempts are made to anneal out b o n d i n g damage.
Rapid thermal annealing of b o n d i n g damage has been
shown to be effective, for example, if it is utilized after residue and impurity removal (38).
I n general, relatively high temperatures are required for
dry etching damage removal. Using Schottky barrier
structures as a monitor it has been found that furnace anneals as high as 800~ or more (for 30 min) can be required
to return a dry-etched surface to its original electrical
properties (26). This can be seen in the data of Fig. 9 which
give the reverse currents (at 1.5V) for Schottky diodes fabricated on Ar ion beam-etched p-type silicon. The control
line is for Schottky diodes made on wet-etched Si subjected to the same anneals; the Ar-etched data are for
J. Electrochem. Sac.,
3890
Vol. 137, No. 12, December
1990 9 The Electrochemical
S o c i e t y , Inc.
Fig. 7. RH EED pattern for < 100> pSi (a, left) offer only wet chemical etching, and (b, right) after CCI4 reactive ion etching. As may be seen, the
pattern of (b) is far different from the streaks and Kikuchi lines of the clean, smooth (100) Si surface pattern of (a).
Schottky diodes made on Ar dry-etched Si after the various (30 min) anneals indicated. Similar results are seen for
PE- and RIE-exposed Si (50, 52). I n particular, the thermawave data of Fig. 8 are for PE- and RIE-etched p-Si and it
can be seen that they also show that anneals at -800~ are
needed for full recovery. It is interesting to note that the
bonding damage created by the b o m b a r d m e n t of dry etching can have advantageous effects, if understood and utilized (26). As ar~ example, Fig. 10 shows the diffusion length
measured in the p-Si samples of Fig. 9 by the surface photovoltage technique. These SPV diffusion length
data are presented as a function of the anneal temperature
used after the Ar dry etching. The diffusion length enh a n c e m e n t seen in the Ar-bombarded samples relative to
the control (wet-etched) for anneals in the 600~176 range
is believed to be due to gettering by the dry etching damage (26). On the other hand, this gettering by dry etching
damage can also be disadvantageous. For example, deleterious gettering caused by dry etching b o n d i n g damage
has been reported in cases of trench etching (54).
Charge buildup damage--the effect, its detection, and repercussions.--There is an additional type of damage,
charge buildup damage, that can occur in dry etching. Its
origins lie in the fact that charge buildup can occur at dielectric/conductor interfaces as a result of exposure to dry
etching. The presence of this charge can lead to insulator
breakdown or to increased leakage current (5, 65). These
damage effects are believed to occur when this charge at a
~
dielectric/conductor interface becomes isolated from potentially neutralizing charge in the plasma, due to turningoff of the etching plasma, or from potentially neutralizing
charge in the substrate, due to isolation of the conductor
by etching, and a transient discharge occurs across the insulator (5, 65). Charge buildup damage is readily apparent
in changes in breakdown in dielectrics and in changes in
leakage currents.
Charge buildup damage--control.--Attempts at controlling charge buildup damage focus on avoiding transients
and on trying to provide charge that can readily reach and
neutralize the stored charge at dielectric/conductor interfaces (5, 65). An example can be found in recent work
which has shown that charge buildup damage effects may
be less severe in hybrid ECR/RIE etching than in RIE (65).
This is proposed to occur basically because the self-biasing voltages and, therefore, the stored charges, involved in
ECR plasmas themselves are relatively small. Hence, if the
higher voltage RF substrate biasing (using to obtain the
ion energies needed for anisotropic etching in hybrid ECR/
RIE etching) is turned off before t u r n i n g off the ECR
plasma, the result can be substantially reduced charge
buildup damage. This reduced level of damage is believed
to occur because the ECR plasma provides the charge
needed to neutralize the stored charge built-up by the voltages of the RF bias used during etching while the ECR
plasma itself results in little charge buildup (65). In addition, it was demonstrated that this reduced level of damage that is found to be present in this approach can be annealed out with a 450~ anneal.
RJE HE.~VY EPI
10-31
508 "
PE EPI
' 9
RIE LIGHT EP[
400 9
10 -4.
E
control
line
300 "
,.
200
10-5.
t~
g
100
10.6"
.-I
2;0
,;o
,;o
---"-EP---
8;~
10 -7
0
TEMPERATURE
Fig. 8. Therma-wave signal as a function of annealing temperature
for high temperature (30 rain) isochronal anneals of epitaxial silicon
subjected to the RIE heavy damage case, RIE light damage case, and PE
heavydamage case etches.
400eV
J 9 9 ~ 9
200
400
Ar bombardment
9 i
600
9 =
800
,'
1000
Temperature (~
Fig. 9. Leakage current of Schottky barriers mode on Ar IBE p-type
and on wet-etched controls as a function of anneal prior to metalli-
zation.
J. Electrochem. Soc., Vol. 137, No. 12, December 1990 9 The Electrochemical Society, Inc.
300
200
,5
o:
'~
1 O0
-----0--9
0
400ev Argon Bombardment
9
0
Controls
i
200
~
400
9
~
S00
-
9
800
1000
Temperelure (~
Fig. 10. Diffusion length as measured by SPV for the materials in
Fig. 9. Diffusion length was measured after the anneals indicated.
Summary
Dry etching is an e x t r e m e l y important technique for pattern transfer in t h e fabrication of microelectronic circuits.
It can, however, affect the materials being etched as well
as subsequent processing and ultimately, device performance. The detrimental effects of dry etching include residue layers, i m p u r i t y and hydrogen permeation layers, and
b o n d i n g d a m a g e layers, as well as surface roughening effects and charge b u i l d u p damage.
Acknowledgments
The research reported in this article that was u n d e r t a k e n
at the Center for Electronic Materials and Processing at
P e n n State was s u p p o r t e d b y the National Science Foundation, IBM, and Intel Corporation. The author wishes to
thank Dr. X.-C. Mu of Intel for his efforts in proofreading
and Ms. F a w n Houtz for her efforts in preparing the manuscript.
Manuscript received Dec. 8, 1989; revised manuscript received J u l y 10, 1990. This was P a p e r 243 presented at the
Hollywood, FL, Meeting of the Society, Oct. 15-20, 1989.
The Pennsylvania State University assisted in meeting
the publication costs of this article.
REFERENCES
1. L. M. Ephrath, IEEE Trans. Electron Devices, ED-28,
1315 (1981).
2. L. M. E p h r a t h and R. S. Bennett, This Journal, 129,
1822 (1982).
3. S. Pang, Solid State Technol., 27, 249 (1984).
4. S. J. Fonash, ibid., 28, 201 (1985).
5. T. Wantanabe and Y. Yoshida, ibid., 27, 263 (1984).
6. S. Ashok, S. J. Fonash, and R. Singh, Appl. Phys. Lett,,
39, 423 (1981).
7. R. Singh, S. J. Fonash, S. Ashok, P. J. Caplan, J. Shappirio, W. Hage-Ali, and J. Ponpon, J. Vac. Sci.
Technol., A1, 334 (1983).
8. S. Ashok, P. Chow, and B. J. Baliga, Appl. Phys. Lett.,
42, 687 (1983).
9. F. Ohira, This Journal, 130, 1201 (1983).
i0. L.M. Ephrath and D. J. DiMaria, Solid State Technol.,
24, 183 (1981).
Ii. T. Kuroda and H. Iwakuro, Jpn. J. Appl. Phys., 29, 923
(1990).
12. J. P. Gambino, C. C. Parks, G. S. Oehrlein, and M. A.
Jaso, Abstract 205, p. 304,The Electrochemical Society Extended Abstracts, Vol. 89-1, Los Angeles, CA,
May 7-12, 1989.
13. A. S. Yapsir, G. Fortufio-Wiltshire, J.P. Gambino,
R. H. Kastl, and C. C. Parks, J. Vac. Sci. Technol., A8,
2939 (1990).
14. A. S. Yapsir, G. S. Oehrlein, G. Fortufio-Wiltshire, and
J. C. Tsang, Appl. Phys. Lett., In press.
15. J.W. Coburn, " P l a s m a Etching and Reactive Ion Etching," American V a c u u m Society Monograph Series,
New York (1982).
16. J. L. Vossen, J. H. Thomas III, J. S. Maa, O. R. Mesker,
and G.O. Fowler, J. Vac. Sci. Technol., A1, 1452
(1983).
3891
17. G. S. Oehrlein, R. M. Tromp, Y. H. Lee, and E. J. Petrillo, Appl. Phys. Lett., 45, 420 (1984).
18. X.-C. Mu, S. J. Fonash, A. Rohatgi, and J. Rieger, ibid.,
48, 28 (1986).
19. F. K. Moghadam and X.-C. Mu, IEEE Trans. Electron
Dev., 36, 1602(1989).
20. X.-C.Mu, S. J. Fonash, and R. Singh, Appl. Phys. Lett.,
49, 67 (1986).
21. S. J. Jeng and G. S. Oehrlein, ibid., 50, 1912(1987).
22. J. M. Heddleson, M. W. Horn, and S. J. Fonash, J. Vac.
Sci. Technol., B6, 280 (1988).
23. D. J. Thomas, P. Southworth, M. C. Flowers, and R.
Greef, ibid., BT, 1325(1989).
24. D. J. Thomas, P. Southworth, M. C. Flowers, and R.
Greef, ibid., BS, 516 (1990).
25. F. J. Montll]o, R. G. Frieser, and W. K. Chu, Abstract
279, p. 681, The Electrochemical Society Extended
Abstracts, Vol. 81-2, Denver, CO, Oct. 11-16, 1981.
26. R. Herlocher Ill and S. J. Fonash, in "Plasma Processing," PV 90-14, G. S. Mathad and D.W. Hess, Editors, p. 304, The Electrochemical Society Softbound
Proceedings Series, Pennington, NJ (1990).
27. A. Climent and S. J. Fonash, J. Appl. Phys., 56, 1063
(1984).
28. S. W. Pang, D. D. Rathman, D. J. Silversmith, R.W.
Mountain, and P. D. DeGraff, Appl. Phys. Lett., 54,
3272 (1983).
29. J. W. Coburn and H. F. Winters, J. Vac. Sci. Technol.,
16, 391 (1979).
30. J. H. Thomas III, X,-C. Mu, and S. J. Fonash, This Journal, 134, 3122 (1987).
31. G. S. Oehrlein, S. W. Robey, J. L. Lindstrom, K . K .
Chan, M . A . Jaso, and G . J . Scflla, ibid., 136, 2050
(1989).
32. C. G. Tuppen, R. Heckingbotton, M. Gill, C. Heslop,
and G. J. Davies, Surf. Int. Anal., 6, 267 (1984).
33. K. Suzuki, K. Ninomiya, and S. Nishimatsu, Vacuum,
34, 953 (1984).
34. X.-C. Mu, S. J. Fonash, B. Y. Yang, K. Vedam, A. Rohatgi, and J. Reiger, J. Appl. Phys., 58, 4282 (1985).
35. X.-C. Mu, S. J. Fonash, G.S. Oehrlein, S.N. Chakravarti, C. Parks, and J. Keller, ibid., 59, 2958 (1986).
36. J. L. Buckner, D. J. Vitkavage, and E. A. Irene, ibid.,
63, 5288(1988).
37. S. J. Fonash, R. Singh, A. Rohatgi, P. Rai-Choudhury,
P . J . Caplan, and E . H . Poindexter, ibid., 58, 862
(1985).
38. S. J. Fonash, Y~-C. Mu, S. Chakravarti, and L. C. Rathbun, This Journal, 135, 1037 (1988).
39. J. P. Simko, G. S. Oehrlein, and T. M. Mayer, ibid., To
be published.
40. J. Ruzyllo, G. T. Duranko, J. T. Kennedy, and C. G.
Pantano, in " U L S I Science and Technology/1987,"
PV 87-11, S. Broydo and C.M. Osburn, Editors,
p. 281, The Electrochemical Society Softbound Proceedings Series, Pennington, NJ (1987).
41. T. Hosoya, Y. Ozaki, and K. Kirata, This Journal, 132,
2436 (1985).
42. M. O. Watanabe, M. Taguchi, K. Kanzaki, and Y: Zohta,
Jpn. J. Appl. Phys., 22, 281 (1983).
43. M. Matsumoto and T. Sugano, This Journal, 129, 2823
(1982).
44. J. P. Gambino, G. S. Oehrlein, C. C. Parks, and J. F.
Shepard, in "Microcontamination Conference Proceedings," p. 51, Canon Publications, Inc., Santa
Monica, CA (1987).
45. J. P. Gambino, M. D. Monkowski, J. F. Shepard, and
C. C. Parks, This Journal, 137,976 (1990).
46. D. F r y s t a k and J. R. Ruzyllo, in " P l a s m a Processing,"
PV 88-22, G. S. Mathad, G.C. Schwartz, and D.W.
Hess, Editors, p. 125, The Electrochemical Society
Softbound Proceedings Series, Pennington, NJ
(1988).
47. S. T. Pantelides, Materials Science Forum, 11-12, 573
(1986).
48. S. J. Pearton, J. W. Corbett, and T. S. Shi, Appl. Phys.,
A43, 153 (1987).
49. M. W. Horn, J. M. Heddleson, and S. J. Fonash, Appl.
Phys. Lett., 51, 490 (1987).
50. J. M. Heddleson, M. W. Horn, and S. J. Fonash, This
Journal, 137, 1960 (1990).
51. A. J. Tavendale, S. J. Pearton, and A.A. Williams,
Appl. Phys. Lett., 56, 949 (1990).
52. M.W. Horn, Ph.D. Thesis, The Pennsylvania State University, University Park, P A (1989).
3892
J. Electrochem. Soc., Vol. 137, No. 12, December 1990 9 The Electrochemical Society, Inc.
53. J. M. Heddleson, M. W. Horn, and S. J. Fonash, "Semiconductor Fabrication: Technology and Metrology,"
ASTM STP 990, Dinesh C. Gupta, Editor, American
Society for Testing and Materials, Philadelphia, PA
(1989).
54. K. Skidmore, Semicond. Int., J u n e (1989).
55. N. M. Johnson, D. K. Biegelson, and M.D. Moyer,
Phys. Rev., B31, 5525 (1985).
56. S. J. Jeng, G. S. Oehrlein, and G. J. Scilla, Mater. Res.
Soc. Symp. Proc., 104, 247 (1987).
57. A. Rohatgi, M. R. Chin, P. Rai-Choudhury, P. Lester,
R. Singh, and S. J. Fonash, Abstract 320, p. 508, The
Electrochemical Society Extended Abstracts, Washington, DC, Oct. 9-14, 1983.
58. A. Rohatgi, P. Rai-Choudhury, S. J. Fonash, P. Lester,
R. Singh, P . J . Caplan, and E.H. Poindexter, This
Journal, 133, 408 (1986).
59. S. J. Fonash, S. Ashok, and R. Singh, Appl. Phys. Lett.,
39, 423 (1981).
60. R. Singh, S. J. Fonash, P. J. Caplan, and E. H. Poindexter, ibid., 43, 502 (1983).
61. S. Ashok and A. Mogro-Campero, IEEE Electron Device Lett., EDL-5, 48 (1984).
62. F. Ohira, This Journal, 130, 1201 (1983).
63. P. Garoghty and W. L. Smith, Mater. Res. Soc. Symp.
Proc., 68, 387 (1986).
64. J. W. Corbett, T.C. Covelti, V. Desnica, and L.C.
Snyder, in "Microscopic Identification of Electronic
Defects in Semiconductors," N. Johnson, S. Bishop,
and G. D. Wadkins, Editors, Materials Research Society, Pittsburg, PA (1985).
65. S. Samukawa, Abstract 167, p. 238, The Electrochemical Society Extended Abstracts, Los Angeles, CA,
May 7-12, 1989.
Dry Etching of GaAs, AIGaAs, and GaSb in
Hydrochlorofluorocarbon Mixtures
S. J. Pearton,* W. S. Hobson, U. K. Chakrabarti,* G. E. Derkits, Jr.,* and A. P. Kinsella
AT&T Bell Laboratories, Murray Hill, New Jersey 07974
ABSTRACT
Reactive ion etching (RIE) of GaAs, AIGaAs, and GaSb in the hydrochlorofluorocarbons CHC12F and CHC1F2 has been
investigated as a function of etch time, discharge power density, pressure, and additive gas (Q~ or H~). There is no incubation time required for the c o m m e n c e m e n t of etching, and the etch rates are in the range 200 A - m i n - 1 (for A10.3Ga0.TAs)to
1000 A - m i n - 1 (for GaAs) for moderate power densities (0.56 W- cm- 2 ). The etched surfaces have smooth morphologies for
all three materials, and there is no significant lattice disorder introduced into GaAs, as detected by photoluminescence
and diode current-voltage measurements, provided the discharge power densities are kept below -0.7 W 9cm -2. Hydrogen
passivation of the Si dopants in n-type GalAs is detected for both gases, but is more prevalent with CHC1F2. Thin (-< 40/~)
surface residue layers of C1 [comprising 5-9 atom percent (a/o)] and F (0.9-3 a/o) for GaAs and GaSb, and 23 a/o for A1GaAs)
are found on all samples after RIE, but these can be removed by simple solvent cleaning.
The dry etching of Ga-based III-V semiconductors generally utilizes chlorine-containing gas mixtures because of
the high volatitities of the resulting etch products (1).
Dichlorodifluoromethane (CC12F2, better k n o w n as Freon12 or Halocarbon-12) represents a particularly useful dryetching gas for GaAs, AlGaAs, and other compound semiconductors, because of its nonflammable, noncorrosive,
and relatively nontoxic nature relative to pure C12 or SIC14
(2-6). A further advantage is that it contains fluorine, which
provides a natural etch stop in removal of GaAs overlayers
from A1GaAs, due to the formation of the nonvolatile species A1F~ (7-10). Since the first demonstrations of dry etching of GaAs with CC12F2 mixtures, this gas has become accepted as the most widely used for this purpose (2).
However, chlorofluorocarbons (CFCs) are now identified
as among the most potent ozone-destructive chemicals,
and, to protect the environment, both the manufacturers
and users of CFCs have agreed to reduce the production,
sales, and emissions of these gases drastically by 1994 and
to abolish them by the year 2000 (11). This is, ruefully, a severe problem for those who use conventional CFCs in dry
etch processes, because the a m o u n t of these gases released
into the atmosphere from etching is vanishingly small
compared to the world-wide use for other purposes. For
example, in 1988, 0.75 billion pounds of CFCs were used as
refrigerants, 0.48 billion pounds as propellants in aerosol
cans, 0.48 billion pounds for cleaning purposes and 0.73
billion pounds as a foaming agent (12). A total of 0.10 billion pounds of CFCs were used for other purposes, but the
contribution to this from dry etch processes was insignificant. However, a complete halt to the use of all Freon-12,
carbon tetrachloride, methyl chloroform, and so on, m u s t
be planned for.
* Electrochemical Society Active Member.
The best available replacement for conventional CFCs
are the so-called hydrochlorofluorocarbons or HCFCs.
This is controversial i n some quarters because such gases
also contain chlorine and as such still constitute an ozonedepleting chemical. The hydrochlorofluorocarbons, however, do represent a significant improvement over conventional CFCs because they are estimated to have an order of
magnitude lower life time in the atmosphere and an
equivalently lower ozone-depletion potential (11). Even so,
there are moves to phase out HCFCs as well (the date for a
complete b a n ranges from 2010 to 2050, depending on the
advocate). While a completely chlorine-free dry etch chemistry for III-V materials based on C~/H2 does exist (13-17),
the etch rates for GaAs and AIGaAs are very small under
low self-bias conditions, and it seems to make sense to explore reactive ion etching (RIE) of these materials using
hydrochlorofluorocarbons to determine if these gases give
suitable etching characteristics.
In this paper, we report the use of chlorodifluoromethane (CHCIF2, Freon 22) and dichlorofluoromethane
(CHC12F, Freon 21) for RIE of GaAs, A1GaAs, and GaSb. It
is of particular interest to investigate the etching characteristics of these gases, since they contain both CH and C1
units, and it is not clear whether they will behave like
C~2or CC12Frbased mixtures. We have investigated
the dependence of the etch rate on time, discharge composition (relative to 02 or H2 diluent gases), power density,
and pressure. The surface morphologies of the etched features were examined by scanning electron microscopy
(SEM), the introduction of near-surface lattice disorder
monitored by photoluminescence (PL), capacitancevoltage (C-V) and current-voltage (I-V) measurements, and
the surface chemistry after etching obtained from electron
spectroscopy for chemical analysis (ESCA) and Auger
electron spectroscopy (AES) data. We conclude that