Method of Making Self-Aligned IGFET

This invention relates to an improved self-aligned insulated gate field effect transistor (IGFET) and method for fabricating that device.

Self-alignment techniques are used routinely in silicon technology. These self-aligned IGFETs have reduced source and drain resistance over the non-self-aligned structures, resulting in improved performance. Examples are shown in U.S. Pat. No. by Dennard, 4,182,636 and by Lund et al, U.S. Pat. No. 4,319,395.

There is a need for ultrahigh speed electronic devices, in fields such as airborne radar and communication systems. It is anticipated that GaAs IGFETs will become an important part of these systems. Meiners U.S. Pat. No. 4,268,844 discloses an IGFET capable of operation at frequencies in excess of one GHz. The transistor substrate is a semi-insulating semiconductor which may be formed of Cr-doped GaAs, Tsang U.S. Pat. No. 4,291,327 forms IGFETs by growing a layer of controlled thickness on GaAs. Hughes et al U.S. Pat. No. 4,325,073 and Casey, Jr., U.S. Pat. No. 4,160,261 both disclose method of forming GaAs FETs.

The use of GaAs IGFETs with large interface state densities is described in my copending U.S. patent application Ser. No. 271,754 filed June 9, 1981, now U.S. Pat. No. 4,438,351 issued Mar. 20, 1984.

An object of this invention is to provide GaAs IGFETs having reduced source and drain resistance, to increase the gain and performance.

According to the invention, a self-alignment technique is used to fabricate GaAs metal insulator semiconductor (MIS) structures, also known as IGFETs. The crucial part of this structure is to use a refractory metal or a poly Si/metal silicide gate over a Si.sub.3 N.sub.4 insulator. This gate structure allows subsequent annealing at high temperatures to activate ion implanted species.

FIGS. 1-5 are diagrams showing steps in the fabrication of a self-aligned GaAs IGFET.

FIGS. 6-9 are diagrams showing initial steps in the fabrication of an alternative embodiment.

It is known that source resistance and drain resistance decrease the performance of field effect transistors (FETs). The gain of the FET is reduced by the source resistance R.sub.s according to G.sub.m =G.sub.m.sup.1 /(1+G.sub.m.sup.1 R.sub.s) where G.sub.m.sup.1 is the gain for a FET with R.sub.s =0. Furthermore, the switching speed of a transistor depends on the on-resistance of the transistor with improved speed performance for FETs with small values of on-resistance. The source and drain resistance of these FETs add to the on-resistance. Hence, minimum source and drain resistance is required for optimum performance of the FETs.

Self-alignment techniques have been developed for Si MOSFET technology. The predominantly used technology is the so-called Si-gate technology. Here, the transistor is fabricated up to the gate level. An ion implantation step is then used to form highly conductive areas for source and drain. In this ion implantation step, the gate acts as a mask for the ions and consequently the highly conductive areas extend up to the gate area, thereby reducing source and drain resistance. After implantation, the wafers are annealed for activation of the implanted species.

The preferred embodiment for the self-aligned GaAs IGFET is described now:

The starting material is properly polished, etched and cleaned semi-insulating (100) GaAs 10. Polishing, etching and cleaning methods for GaAs are well known. The GaAs must be selected such that it does not invert upon high temperature processing. The processing steps are shown in FIGS. 1-5. In FIG. 1, the semi-insulating GaAs wafer 10 has been coated with approximately 100 nm of a plasma enhanced deposited Si.sub.3 N.sub.4 layer 12 and a phosphorous-doped 1 .mu.m thick SiO.sub.2 layer 14. The SiO.sub.2 film is then etched off in the areas where the transistors will be located and the SiO.sub.2 is reflown at elevated temperatures to eliminate sharp steps. Techniques for the above described steps are well known. The SiO.sub.2 film allows the metallization pattern to run on a relatively low dielectric constant material (.epsilon.SiO.sub.2 =3.9; .epsilon.GaAs=10.9). Consequently, capacitance effects between lines are minimized. The SiO.sub.2 film also provides alignment marks for subsequent processes. Next, the sample is ion implanted with the SiO.sub.2 film 14 acting as a mask (FIG. 2). The preferred implantation species is Se at 400 keV. The dose required depends on the threshold voltage and conductance values desired. A 2.times.10.sup.12 ions cm.sup.-2 is a typical value. This ion implantation step provides the channel doping. The fabrication process is continued by deposition and delineation of the gate 16. The preferred metal for the gate is Mo with a thickness of approximately 500 nm. A typical gate length is 1 .mu.m. The gate is delineated with commonly used techniques such as lift-off (FIG. 3). Next, the source and drain region are implanted to provide an n+region extending to the gate region from source and drain. For this ion implantation the SiO.sub.2 and the gate act as masks. Again, the preferred species is Se, implanted at 400 keV with a 5.times.10.sup.13 ions cm.sup.-2 dose (FIG. 4). Now the sample is annealed with the originally deposited Si.sub.3 N.sub.4 acting as an annealing cap. The annealing is performed in this preferred process at 800.degree. C. for 30 minutes. This annealing is performed in order to activate the species, implanted during the two previous implantation processes. The FET is completed by etching holes into the Si.sub.3 N.sub.4 for source and drain contacts 18 and 20, to deposit source and drain and to sinter the device for activation of these contacts (FIG. 5). Commonly used contact metal is a Au-Ge-Ni film. The sintering is performed at 400.degree. C. for 1 minute.

There exists many variations to this process. Some are listed below. An n-type epi-material may be used as a starting material and mesa structures are then etched in this sample. This allows the omission of the first ion implantation since the channel doping is already present. Another variation to the process would be the use of different implantation species, such as Si. A further variation to the process would be to remove the Si.sub.3 N.sub.4 over the device area after the first ion implantation and then to deposit a new Si.sub.3 N.sub.4 film. This film may be thinner than the original (say 500A) for reduced threshold voltage. Yet another variation would be to deposit another dielectric film over the entire structure, such as Si.sub.3 N.sub.4 or SiO.sub.2 before annealing. This insulator would provide an insulation for a possible further metallization layer. A final variation would be to use a different gate metal system, e.g., an n-type poly Si or MoSi.sub.2 system. An advantage of this system would be that a SiO.sub.2 film can be grown thermally, which acts as a passivation to the metal system and also forms an insulator between a next level metallization. The oxidation process may be able to activate the implanted species and the annealing process may be eliminated.

The embodiment using an n-type epi-material and mesa structures is shown in FIGS. 5-9 (in place of FIGS. 1 & 2), which is followed by the steps shown in FIGS. 3-5. As shown in FIG. 9, the starting material is a semi-insulating substrate 10a, on which there is a layer 11 of n-type epitaxial material. As shown in FIG. 7, mesas are formed by etching through the epi layer 11 into the semi-insulating layer 10a. The top of each mesa is used for forming one active transistor area. As shown in FIG. 8, the next step comprises coating the substrate with plasma enhanced deposited Si.sub.3 N.sub.4 and a SiO.sub.2 layer. Next the SiO.sub.2 layer is etched away over the transistor area on the mesa, as shown in FIG. 9. The process of forming the transistor then continues as shown in FIGS. 3, 4 and 5.

Thus, while preferred constructional features of the invention are embodied in the structure illustrated herein, it is to be understood that changes and variations may be made by the skilled in the art without departing from the spirit and scope of my invention.