Ion implant mask and cap for gallium arsenide structures

A mask and encapsulating layer suitable for use on gallium arsenide substrates is described incorporating a layer of germanium selenide which is photosensitive and may be exposed and developed to form a mask suitable for ion implantation and which may also remain as a capping layer during an anneal process of ion implanted regions in a controlled atmosphere and temperature furnace wherein the layer of germanium selenide is converted to germanium which may subsequently be removed from the gallium arsenide substrate after the step of annealing.

BACKGROUND OF THE INVENTION 
1. Field of the Invention: 
This invention relates to semiconductor devices and more particularly to an 
ion implant mask and cap during anneal for fabricating submicron gallium 
arsenide structures. 
2. Description of the Prior Art: 
In the prior art, fabrication of gallium arsenide structures may begin by 
applying an organic photoresist layer on the upper surface of a gallium 
arsenide substrate and patterning it in an appropriate manner to form, for 
example, a field effect transistor (FET) active layer mask. The next step 
is to ion implant impurities through the photoresist mask where there are 
windows or openings to form a doped region extending from the surface of 
the gallium arsenide substrate to a predetermined depth. The photoresist 
layer is subsequently removed and a capping layer is deposited over the 
gallium arsenide substrate. 
The material of a capping layer may, for example, be silicon nitride, 
silicon oxide, phosphorus-doped silicon oxide or aluminum nitride. The 
purpose of the capping layer is to reduce the outgassing of arsenic from 
the gallium arsenide substrate when the ion implanted region is annealed. 
The ion implanted region is annealed by raising the gallium arsenide 
substrate to a high temperature such as 800.degree. C. to permit 
recrystallization of the gallium arsenide damaged by the ion implantation. 
During recrystallization, substitution of the ion implanted ions into the 
crystal lattices of the gallium arsenide material occurs. After the ion 
implanted region is annealed, a step also called activation, the capping 
layer is removed and further processing continues. This includes the 
formation of ohmic contacts defining drain and source and deposition of 
material suitable to form the gate of a field effect transistor. 
Other materials suitable as a capping layer to prevent outgassing of 
arsenic from a gallium arsenide substrate during annealing are described 
in U.S. Pat. No. 4,267,014 which issued on May 12, 1981 to J. E. Davey, 
A. Christou and H. B. Dietrich. In U.S. Pat. No. 4,267,014 a method for 
protecting an ion implanted substrate during the annealing process is 
described by covering the ion implanted layer with a suitable encapsulant 
such as a germanium, amorphous gallium arsenide, doped gallium arsenide, 
or gallium aluminum arsenide. The protective capping layer is applied 
subsequent to the step of ion implantation. After the step of annealing, 
the capping layer is removed by selective chemical etching. 
In U.S. Pat. No. 4,058,413 which issued on Nov. 15, 1977 to B. M. Welch and 
R. D. Pashley, a method to prevent dissociation of gallium arsenide during 
the step of annealing was described using a capping layer of aluminum 
nitride which was sputtered onto the gallium arsenide substrate. 
In U.S. Pat. No. 4,330,343 which issued on May 18, 1982 and in U.S. Pat. 
No. 4,263,605 which issued on Apr. 21, 1982 both to A. Christou and J. E. 
Davey, a method of forming a low ohmic contact on gallium arsenide 
material was described by depositing a refractory material such as 
titanium tungsten (TiW) and ion implanting impurities such as silicon, 
selenium or germanium through the layer of TiW. The layer of TiW also 
functions as a capping layer when annealing the implanted structure to 
activate the implanted ions. Other materials suggested as a refractory 
material include tantalum, tungsten, platinum, and molybdenum to a 
thickness of from 400 to 800 Angstroms. 
In U.S. Pat. No. 4,354,198 which issued on Oct. 12, 1982 to Rodney T. 
Hodgson et al., a capping layer of zinc-sulphide is sputtered to a 
thickness of 500 Angstroms onto a GaAs surface. The GaAs surface is heated 
with a laser beam which does not directly heat the zinc-sulphide. The 
zinc-sulphide or group II-VI compound semiconductor acts as a surface 
passivator to reduce or control the recombination of charge carriers at 
the surface of GaAs or other group III-V compound semiconductors. 
In a paper entitled "New Application of Se-Ge Glasses to Silicon 
Microfabrication Technology" by H. Nagai et al. and published in Applied 
Physics Letters, Vol. 28, No. 3, Feb. 1, 1976, pp. 145-147, a film of 
Se.sub.75 Ge.sub.25 on a silicon substrate was selectively etched by an 
alkaline solution. A mercury lamp was used as a light source where the 
wavelength of the lamp is shorter than the absorption edge of Se-Ge. SeGe 
was used as a photoresist on silicon, silicon dioxide and silicon nitride. 
The application of Se-Ge glass films to the patterning of layers for 
silicon device processing and for the fabrication of photo masks were 
investigated. 
In a paper entitled "A Novel Inorganic Photoresist Utilizing Ag Photodoping 
in Se-Ge Glass Films" by Akira Yoshikawa et al. and published in Applied 
Physics Letters, Vol. 29, No. 10, Nov. 15, 1976 pp. 667-679, a 
photoetching procedure is described using an Se-Ge film for positive or 
negative photographic sensitivity. The negative resist would include the 
additional process steps of depositing a thin Ag layer over the Se-Ge film 
prior to photoexposure followed by developing in an acid solution to 
remove the Ag remaining on the unexposed area. The Se-Ge film would be 
etched by an alkaline solution. 
In a paper entitled "Bilevel High Resolution Photolithographic Technique 
for Use with Wafers with Stepped and/or Reflecting Surfaces" by K. L. Tai 
et al. and published in J. Vac. Sci. Technol., 16 (6), Nov./Dec. 1979, pp. 
1977-1979, a tri-level system is described comprising Ag.sub.2 
Se/GeSe.sub.2 over a thick organic polymer resistor a bi-level system is 
described comprising GeSe over a thick organic polymer. The polymer may be 
etched using the GeSe as a mask in an O.sub.2 plasma. The thick polymer 
provides a flat surface necessary for high resolution and good line-width 
control over a wafer surface which may have steps raised as high as 1 
.mu.m above the plane of the wafer surface. 
In a paper entitled "Dry Development of Se-Ge Inorganic Photoresist" by 
Akira Yoshikawa et al. and published in Appl. Phys. Lett. 36 (1), Jan. 1, 
1980, pp. 107-109, a Se-Ge inorganic resist (chalcogenide glass film) is 
described where plasma etching results in a large etch-rate difference 
between Ag photodoped and undoped films. A plasma etch rate of undoped to 
Ag-doped Se.sub.75 Ge.sub.25 films of 370 to 1 was observed with CF.sub.4 
as the plasma source gas. 
In a paper entitled "Submicron Optical Lithography Using An Inorganic 
Resist/Polymer Bi-Level Scheme" by K. L. Tai et al. and published in J. 
Vac. Sci. Technol., Vol. 17, No. 5, Sept./Oct. 1980, pp. 1169-1176, an 
inorganic photoresist system is described. The inorganic resist consists 
of two layers, about 100 Angstroms Ag.sub.2 Se on about 2000 Angstroms 
GeSe. Upon illumination by light, Ag in the Ag.sub.2 Se layer is 
photodoped into the GeSe layer. Ag doped GeSe is less soluble or insoluble 
in the developer. A bi-level resist of GeSe over HPR206 polymer is also 
shown in FIG. 5 over a polysilicon level of a 16K MOS RAM wafer. 
It is, therefore, desirable to provide an inorganic photosensitive material 
amenable to high resolution such as submicron resolution which may be 
deposited on a gallium arsenide substrate and also function as an ion 
implant mask. 
It is further desirable to provide a new material, germanium selenide 
(Ge.sub.x Se.sub.1-x) with respect to gallium arsenide which is 
photosensitive to form an ion implant mask. 
It is further desirable to provide a new material to form a capping 
material on gallium arsenide material during times when ion implanted 
gallium arsenide is annealed at high temperature. 
It is further desirable to use germanium selenide as both an ion implant 
mask and as a capping layer over gallium arsenide material. 
It is further desirable to reduce the number of processing steps in the 
fabrication of gallium arsenide semiconductor devices. 
It is further desirable to provide photosensitive material capable of 
delineating ion implant masking geometries as small as 0.37 micrometers. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, apparatus and methods are 
described for fabricating an ion implant mask out of germanium selenide on 
a gallium arsenide substrate wherein the germanium selenide layer may be 
selectively sensitized, exposed and developed to form the ion implant 
mask. 
The invention further provides an apparatus and method for capping a 
gallium arsenide substrate by depositing a layer of germanium selenide 
over the substrate. 
The invention further provides a photosensitive material which may be 
developed to form a positive or negative ion implant mask and also a 
capping material over a gallium arsenide substrate.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
In FIG. 1 gallium arsenide substrate 14 is shown having an upper surface 
15. Typically, a 2000 Angstrom layer of germanium selenide (Ge.sub.x 
Se.sub.1-x) 16 is deposited by dual thermal techniques on upper surface 15 
of gallium arsenide substrate 14. The dual-filament deposition method and 
film stoichiometry are well known by those in the art. Typical film 
stoichiometry is Ge.sub.0.1 Se.sub.0.9 where this stoichiometry is 
established by dual-filament or co-sputter thin-film techniques. 
The dual-filament deposition method involves co-depositing elemental 
germanium and elemental selenium in an evaporator. The stoichiometry is 
established by varying the filament temperature as determined by the vapor 
pressure of each constituent. 
The co-sputter deposition method involves co-depositing elemental germanium 
and elemental selenium in a sputter system. Here the stoichiometry is 
established by varying the sputter target power as determined by the 
sputter yield of each constituent. 
In each case, the Ge.sub.x Se.sub.1-x layer is optimized by adjusting the 
stoichiometry (either through deposition filament temperature or sputter 
target power), film thickness and purity. 
The germanium selenide film 16 is sensitized and selectively exposed by 
well-known methods. The exposed layer 16 is developed wherein material 
exposed is removed to form window or opening 18 as shown in FIG. 1. The 
maximum distance across window 18 may be as small as 0.37 micrometers by 
optimizing the germanium selenide stoichiometry process and exposure 
method. 
The germanium selenide layer 16 with window 18 may now function as a mask 
for ion implantation with the proper species such as silicon, energy and 
fluence to form n-type region 20 extending from upper surface 15 into 
gallium arsenide substrate 14 below window 18 as shown in FIG. 2. N-type 
region 20 is formed by direct ion implantation. In addition to region 20, 
region 22 below germanium selenide layer 16 can be selectively formed to 
be n- type or n+ type by proper selection of implant conditions which 
includes the ion mass, energy, and fluences and resultant level of recoil 
doping. It is understood that region 22 is formed by implanting ions 
through germanium selenide layer 16. 
Alternately, region 22 may be formed by depositing an encapsulant over 
germanium selenide layer 16 followed by a flash diffusion of selenium from 
germanium selenide layer 16 through upper surface 15 into gallium arsenide 
substrate 14. By this method an n- type or n+ type region 22 may be 
formed. 
The selenium flash diffusion process is known as a recoil doping technique. 
That is, when a .sup.29 Si.sup.+ species is implanted through the Ge.sub.x 
Se.sub.1-x layer at 400 KeV it will "recoil" a .sup.80 Se.sup.+ species 
into the host substrate. It is this Se.sup.+ species that forms the 
shallow n+ implant area. 
N-type regions 20 and 22 are annealed as shown in FIG. 3 with germanium 
selenide layer 16 remaining as a capping layer. The annealing of regions 
20 and 22 are done in a controlled atmosphere technique (CAT) system, a 
diagram of which is shown in FIG. 5. 
Referring to FIG. 5, a CAT system 30 is shown having a housing or chamber 
32 having an opening 33 and decoupling joint 31 suitable for inserting a 
substrate 14 of gallium arsenide material. Opening 44 functions to vent 
gas from chamber 32. Chamber 32 is supplied with gas by gas supply tubes 
34 and 35. Tube 34 is coupled to a valve 36 and through mass flow 
controller (MFC) 37 to a source of gas having the compound AsH.sub.3. Tube 
35 is coupled through valve 38 to MFC 39 and to MFC 40 which in turn are 
coupled to a gas supply of Ar and H.sub.2 respectively. Valve 38 may be a 
two-way valve. Tubes 34 and 35 supply gas to housing 32 which is vented 
through opening 44 resulting in a controlled atmosphere being developed 
within housing 32 depending upon the flow rate of the gasses, furnace 
temperature and composition of the gas from tubes 34 and 35. Around 
chamber 32 may be a six-zone furnace 42 shown in cross section. 
Referring to FIG. 6 curve 43 shows a typical temperature profile for a 
six-zone furnace 42. In FIG. 6 the ordinate represents temperature and the 
abscissa represents distance X along furnace 42. 
During the step of annealing regions 20 and 22 in gallium arsenide 
substrate 14, the atmosphere within housing 32 contains an excess over 
pressure of arsenic generated from supply tube 34 by the decomposition of 
its supply gas AsH.sub.3. The excess arsenic in gas chamber 32 prevents 
loss of arsenic through window 18 shown in FIG. 3 from gallium arsenide 
substrate 14 during the entire annealing heat treatment cycle in chamber 
32. The annealing cycle and gas flow conditions in chamber 32 of furnace 
30 are well known and documented in publications. 
In a typical annealing cycle, GaAs substrate is placed in the cool region 
of the chamber 32 under Ar flow, sealed with joint 41, flushed (greater 
than 1000 SCCM) at high flow rate by H.sub.2 from tube 35 to displace all 
Ar gas from the chamber 32, exposed under the excess As (0.01 to 1 atm) 
generated from the AsH.sub.3 introduced from tube 34 at a flow rate of 1-2 
cm/sec through the chamber 32, which is heated up to 850.degree. C. for 
example for 30 minutes, then retracted to the cool region and removed 
under AR atmosphere at room temperature by decoupling the sealed joint 41. 
In a typical example, gallium arsenide substrate 14 will reach 850.degree. 
C. steady state temperature in one minute from room temperature. At the 
temperature of 850.degree. C., the selenium in germanium selenium layer 16 
will evaporate during the annealing cycle in chamber 32 due to the high 
vapor pressure of selenium at 850.degree. C. The evaporation of selenium 
will transform the germanium selenium layer 16 to an elemental germanium 
layer 48 during the 30-minute anneal. 
Following the step of annealing regions 20 and 22 in CAT system 30, 
germanium layer 48 which was previously germanium selenium layer 16 may be 
plasma etched by well-known techniques. FIG. 4 shows the resultant 
structure with germanium layer 48 removed which leaves exposed regions 20 
and 22 extending from upper surface 15 into gallium arsenide substrate 14. 
The percent of activation of regions 20 and 22 may be controlled by 
properly adjusting the anneal process in furnace 30 and the germanium 
selenium film thickness. 
To properly adjust the anneal process of CAT anneal conditions, the 
temperature, anneal time and the excess As atmosphere can be adjusted to 
control the percent activation of implanted species in regions 20 and 22 
to be electrically active donors or acceptors. 
An apparatus and method has been described for forming a photosensitive 
mask on a gallium arsenide substrate which may be developed to provide 
feature sizes as small as 0.37 micrometers and through which ion 
implantation may occur. 
Further, a layer of germanium selenium which was previously used as a mask 
for ion implantation may be subsequently used as a capping layer to 
prevent outgassing of arsenic during an anneal process wherein the gallium 
arsenide is placed in a furnace having a controlled atmosphere including 
an arsenic vapor pressure. 
Further, the layer of germanium selenide may be converted to elemental 
germanium by vaporizing the selenium. The germanium layer may be removed 
by plasma etching.