High durability mask for dry etch processing of GaAs

A mask is described which enables the fabrication of features in GaAs such as waveguides, channels, facets, mesas, and mirrors by dry etch processing in chlorine containing ambients. The mask consists of an amorphous form of carbon which may contain incorporated hydrogen. The mask can be applied, patterned and removed through dry processing techniques.

FIELD OF THE INVENTION 
This invention relates to the protection of desired regions of GaAs by a 
durable mask during dry etch processing. More specifically, this invention 
relates to the use of a highly durable etch mask to protect desired 
regions of GaAs during the fabrication of features such as waveguides, 
channels, facets, mesas, and mirrors by dry etch processing in chlorine 
containing ambients. 
BACKGROUND 0F THE INVENTION 
Fabrication of opto-electronic devices in GaAs requires the use of etch 
processes to produce features for both microoptical and microelectronic 
components. GaAs as used herein includes all compounds, crystalline and 
polycrystalline, containing gallium and arsenic with or without additional 
elements. Examples of features which are transferred into GaAs by etch 
processes include components such as waveguides, channels, facets, mesas 
and mirrors. To prepare components of this type having small, often 
sub-micron size dimensions, etch processes which provide a high degree of 
anisotropy are required. Wet etch techniques are, in general, unsuitable. 
Etch rates in wet processes are either isotropic or dependent on 
crystallographic orientation. Dry etch techniques, on the other hand, can 
avoid the dependence in rate (under optimum conditions) to 
crystallographic orientation and can provide the anisotropy required. 
The majority of dry techniques used to provide an anisotropic etch in GaAs 
are ion-based processes which utilize chemistry to provide some form of 
reactive assistance. The most common techniques of this type include: 
Reactive Ion Etching (RIE), Reactive Ion Beam Etching (RIBE), and Ion Beam 
Assisted Etching (IBAE) [also known as Chemically-Assisted Ion Beam 
Etching (CAIBE)]. The chemistry utilized by these techniques for reactive 
assistance enhances etch rates, forms volatile etch products, and 
minimizes damage to the GaAs surface by energetic ions, neutrals, and/or 
radicals. In the dry etch processing of GaAs, ambients containing chlorine 
(atoms, molecules, neutrals, radicals) have been found most useful for 
providing suitable reactive assistance. 
Some form of mask is required to protect desired regions of a substrate 
when using dry techniques to etch GaAs. Durable masks are often of 
particular value. The term "durable" as used herein defines the resistance 
of the mask to erosion during an etch process. An ideal mask is durable to 
the extent that it will not erode or change form during an etch process. 
For a mask to exhibit significant durability in the dry etch processing of 
GaAs, the ratio of the etch rate of the GaAs to that of the mask, i.e., 
the selectivity of the etch, must be high. Masks of materials of low 
durability are unsuitable for several reasons. First, the edge quality of 
etch features decreases as mask thickness increases. Second, mask features 
of dimensions smaller than the thickness of the mask are unstable and can 
break away or shift position during processing. Finally, mask erosion 
especially of edges can redeposit mask material into unwanted regions and 
degrade overall etch quality and uniformity. 
In the dry etching of GaAs using chlorine for chemical assistance, few 
materials are known which when applied as thin layers (ca. 0.1 micron) 
offer the durability to survive an etch of from a few to many microns. 
Metals such as nickel (with titanium underlayer) and chromium and salts 
such as aluminum fluoride and strontium fluoride have been used with 
varying degrees of success. Wet techniques are frequently required to 
follow the dry etch processes to effect complete removal of these 
materials. What are needed are masks of high durability which are 
convenient to apply, pattern, and remove. 
SUMMARY OF THE INVENTION 
An object of this invention is to provide a durable mask for the dry etch 
processing of GaAs in chlorine containing ambients. Quite unexpectedly, we 
have discovered that a mask formed of amorphous carbon is highly resistant 
to attack by chlorine during dry etch processing in chlorine containing 
ambients. The amorphous carbon can be applied and removed by dry 
processing techniques. 
A feature of this invention is that no wet etch steps are required 
following lithographic patterning. 
Another feature is that the amorphous carbon mask facilitates the 
fabrication of features such as waveguides, channels, facets, mesas, and 
mirrors by dry etch processing in chlorine containing ambients.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
A mask of amorphous carbon can be prepared by a plasma-assisted CVD 
technique, i.e., by the decomposition of methane in an rf plasma. Other 
hydrocarbons may be substituted for methane and a wide range of process 
conditions may be used. Alternative techniques for depositing this 
material may be used as well. These include variations of the 
plasma-assisted CVD technique, glow discharge (rf and dc) sputter 
deposition, single and dual ion beam sputter deposition, reactive ion beam 
deposition, evaporation, and ion plating. Depending on the deposition 
technique selected, the chemical and physical properties of the carbon 
mask may vary; the chemical properties from amorphous carbon (a-C) to 
hydrogenated amorphous carbon (a-C:H), the physical properties from 
diamond-like to graphite-like. 
Amorphous carbons which serve as highly durable masks are determined easily 
through routine examination using, for example, scanning electron 
microscopy (SEM), and experiments such as those described in subsequent 
Examples I and II. To be considered highly durable, the ratio of the etch 
rate of GaAs to that of the mask should exceed 10:1 and more typically, 
25-50:1. The ambient under which this durability is established may 
include chlorine containing gases or gas mixtures other than those given 
by the examples, e.g. BCl.sub.3, BCl.sub.3 /Ar, etc., as long as the 
ambient is a suitable ambient for the dry etching Of GaAs. 
As shown in FIG. 1a, a layer of amorphous carbon 12 is deposited on a 
substrate 10 of GaAs which may or may not contain an overcoating 11 of a 
material such as SiO.sub.2, Si.sub.3 N.sub.4 or amorphous silicon. An 
amorphous material is preferred for layer 11 to prevent transfer of grain 
boundary features. This overlayer, when present, provides an adhesion 
layer for the amorphous carbon and protects the surface of the GaAs from 
damage by ion bombardment during the deposition of the amorphous carbon. 
As shown in FIG. 1b, a thin layer 13 of a material such as SiO.sub.2, 
Si.sub.3 N.sub.4, or amorphous silicon, is applied over the amorphous 
carbon layer. Again, an amorphous material is preferred for this layer to 
prevent transfer of grain boundary features. A layer 14 of a 
lithographically patternable material, e.g., photoresist or an electron 
beam sensitive resist, is then applied to the surface layer 13 as shown in 
FIG. 1c. Layer 14 is then exposed and developed according to accepted 
practices to provide a pattern as shown in FIG. 1d. Next a Reactive Ion 
Etch (RIE) step is used with an appropriate gas chemistry to selectively 
etch layer 13 through the exposed regions of the pattern in layer 14 as 
shown in FIG. 1e. Alternative dry etch techniques may be substituted for 
any of the RIE steps specified in this description whenever convenient. If 
layer 13 is composed of SiO.sub.2 or amorphous silicon a freon/oxygen 
mixture such as CF.sub.4 /O.sub.2 (96 vol % CF.sub.4, 4 vol % O.sub.2) can 
be used to selectively remove this layer by RIE. Another dry etch by RIE 
is then performed using oxygen (O.sub.2) to selectively transfer the mask 
pattern in layer 13 through the amorphous carbon layer 12 to provide 
openings. This etch may also be used to remove the lithographically 
patternable layer 14 (FIG. 1f). The sample is then dry etched by RIE using 
a suitable selective gas to remove the remaining layer 13 pattern (FIG. 1g 
). By proper choice of materials this RIE step can also be useful for 
patterning layer 11 when present (FIG. 1b). At this point, the processing 
of the amorphous carbon mask with patterned openings which expose the GaAs 
substrate is complete and the wafer is ready for dry etching in a chlorine 
containing ambient. Following transfer of the pattern into GaAs (FIG. 1i), 
the amorphous carbon layer 12 is removed by RIE in O.sub.2 (FIG. 1j). 
Finally, the adhesion/protection layer 11 when present is removed by RIE 
under the same conditions used during pattern transfer (FIG. 1k). 
EXAMPLES 
Two examples are presented here to illustrate the durability of amorphous 
carbon masks in the dry etch processing of GaAs in chlorine containing 
ambients using RIE and CAIBE. For convenience reasons only, the means used 
to pattern the amorphous carbon in these examples deviate from the means 
described in the preferred embodiment. Also, the anisotropy of the etch 
was of no concern in the first example. RIE conditions are presented which 
do not optimize this aspect. 
I. In the first example an amorphous carbon layer of thickness 1.38.mu. was 
deposited onto an evaporated SiO.sub.2 adhesion/protection layer of 
thickness 0.34.mu. on a GaAs substrate. The amorphous carbon was deposited 
by the plasma-assisted CVD technique onto an rf powered electrode at a 
self-bias voltage of 1600 V and a methane pressure of 16 mTorr. 
Photoresist was applied and patterned for application of an 
Imidazole-based photoresist lift-off process. After preparation of the 
appropriate photoresist profile for the lift-off process aluminum was 
deposited to a thickness of 0.1.mu. over the profile. The resultant 
lift-off pattern of aluminum on the amorphous carbon surface was then 
transferred through the amorphous carbon layer by RIE at 300 W in O.sub.2 
at a pressure of 40 mTorr. RIE was again used to transfer the pattern 
through the SiO.sub.2 adhesion/protection layer at 300 W in CF.sub.4 
/O.sub.2 (96 vol % CF.sub.4, 4 vol % O.sub.2) at a pressure of 100 mTorr. 
A wet etchant was next used to strip the aluminum from the sample. 
Features to depths of 13.3.mu. were dry etched through openings in the 
amorphous carbon and SiO.sub.2 mask layers into the GaAs substrate by RIE 
at 330 W in Cl.sub.2 /Ar (25 vol % Cl.sub.2, 75 vol % Ar) at a pressure of 
40 mTorr. Under these conditions the selectivity, i.e., the ratio of etch 
rate of the GaAs to that of the amorphous carbon, was found to exceed 
100/1 (109/1 actual). RIE at 300 W in O.sub.2 at 40 mTorr was used to 
remove the remaining amorphous carbon and RIE at 300 W in CF.sub.4 
/O.sub.2 (96 vol % CF.sub.4, 4 vol % O.sub.2) at 100 mTorr was used to 
remove the SiO.sub.2 adhesion/protection layer. 
II. In the second example an amorphous carbon layer of thickness 0.17.mu. 
was deposited onto an SiO.sub.2 adhesion/protection layer of thickness 
0.27.mu. on a GaAs substrate (actually a GaAs/AlGaAs double 
heterostructure containing substrate). The amorphous carbon was deposited 
by the plasma-assisted CVD technique, this time at a self-bias voltage of 
400 V and a methane pressure of 1 mTorr. The same procedure which was 
followed to pattern the amorphous carbon in the first example was followed 
to pattern the amorphous carbon in this example. Using the CAIBE 
technique, features to depths of 8.46.mu. were dry etched through openings 
in the amorphous carbon and SiO.sub.2 mask layers into the GaAs/AlGaAs 
double heterostructure substrate using 500 V Ar ions at a current density 
of 0.4 ma/cm.sup.2. The flow of Ar into the Kaufman type ion source was 3 
sccm. Cl.sub.2 gas was directed at near normal incidence to the sample 
during etching at a flow of 12.5 sccm. The chamber pressure during etching 
was maintained at 6.times.10E-5 Torr. The base pressure of the chamber 
before etching was allowed to reach 5.times.10E-7 Torr. The ratio of the 
etch rate of the GaAs (including the GaAs/AlGaAs double heterostructure) 
to that of the amorphous carbon in this example was greater than 90/1. The 
amorphous carbon and the SiO.sub.2 adhesion/protection layers were both 
removed by RIE as described above for the first example. The anisotropy 
achieved during this etch was excellent. 
The invention has been described in detail with particular reference to 
certain preferred embodiments thereof, but it will be understood that 
variations and modifications can be effected within the spirit and scope 
of the invention.