PROCESS OF FABRICATING A FULLY SELF- ALIGNED FIELD EFFECT TRANSISTOR

Abstract: A process for the fabrication of "low temperature"-­ gate MESFET structures (10), i.e., gate metal (15G) deposition takes place after annealing of the n⁺-implant that form source- (13S) and drain- (13D) contact regions. The process permits self-alignment of all three important MESFET parts, namely the implanted contact regions (13S, 13D) and both, the ohmic- (14S, 14D) as well as the gate- ­ (15G) contact metallizations. In the process, a multi-layer "inverted-T" structure (30) is used as a mask for the n⁺-implant and for the ohmic- and gate-metallizations. The upper part (23G) of the "inverted-T" is a so-called dummy gate which is replaced by the Schottky gate after ohmic contact metal deposition. The source-gate and drain-gate separations are determined by the shoulders of the lower layer (22G), the shoulders being obtained using sidewall techniques.

Description: The invention concerns a process of fabricating a fully self-aligned field effect transistor (FET), the transistor being formed on a semiconductor substrate and comprising a current channel and associated source, gate and drain electrodes. The process is particularly suited for technologies using III-V compound semiconductors such as GaAs. The FET has become an established element not only in microwave technology but also in switching and data processing system applications. For high speed integrated circuits there is a growing need for submicron FET's, i.e., transistors with gate lengths of below one micron. Furthermore, GaAs structures have received increased attention because of the inherent material properties, particularly the high carrier mobility. A variety of FET structures and processes for their fabrication have already been proposed, among others there are those for metal-semiconductor field effect transistors (MESFET) using GaAs. Basically, there are two types of proposed processes for the fabrication of self-aligned GaAs MESFET's: one type of process uses a refractory gate metal as mask for the implantation of the ohmic contact regions, the other type of process uses a "dummy" gate as mask for the contact implant, the dummy gate being replaced by the Schottky gate after ohmic contact metal deposition. Refractory gate structures and processes have been described by N. Yokoyama et al. in articles "TiW Silicide Gate Self-Alignment Technology for Ultra-High-Speed GaAs MESFET LSI/VLSI's" (IEEE Trans., ED-29, 1541 (1982)) and "A GaAs 1K Static RAM Using Tungsten Silicide Gate Self-Aligned Technology" (IEEE JSC, SC-18, 520, (1983)). The advantage of this type of process is its simplicity. It allows the fabrication of a simple FET structure with a relatively small number of lithographic steps. The prime concern with this process however is the stability of the metal-semiconductor interface on annealing: since the gate deposition takes place prior to the ohmic contact region implant, the Schottky barrier at the gate-channel interface is exposed to high-temperature cycles required to anneal the implanted material and to alloy the ohmic contacts. During these high-temperature processes the metal-semiconductor interface changes due to interdiffusion, lateral diffusion, stress, shrinkage, etc. This affects the Schottky barrier height and the threshold voltage of the transistor. Dummy gate structures and processes have been proposed by K. Yamasaki et al. in articles "GaAs LSI-­ Directed MESFET's with Self-Aligned Implantation for n⁺-­ Layer Technology (SAINT)" (IEEE Trans., ED-29, 1772 (1982)) and "Self-Align Implantation for n⁺-Layer Tech­ nology (SAINT) for High-Speed GaAs ICs" (Electronics Letters, 18, 119, (1982)). The use of a dummy gate makes the fabrication process for MESFET's more complicated compared to the refractory-gate process, however, it offers a number of advantages: - The metal-semiconductor interface is not exposed to a high-temperature cycle and therefore is not distorted since the actual gate metal deposition is done after the contact region implant anneal process. - There is more freedom in the choice of the gate material. The requirements are determined by the final process steps such as chip mounting, packaging, etc., which are significantly less stringent than with the high-temperature implant anneal. - The sequence of process steps permits FET channel measurements (between ohmic contacts) and characteriza­ tion prior to gate metal deposition, i.e., a final adjustment of the channel threshold can be achieved before device completion, e.g., by channel recessing, thereby improving the yield. - The gate metal can be used as the first-level wiring. The SAINT process referred to above is based on the formation of a dummy "T"-gate structure and the use of this T-structure to mask the contact implantation. Embedding of the T-structure in selectively etchable material like SiO₂ and subsequent etching and removal leads to the replication of the foot of the T-structure into a dielectric like Si₃N₄, thus defining the actual gate area. In the resulting MESFET, the gate area is self-aligned with respect to the implanted contact regions, the interdistance being determined by the overhang of the T-shaped dummy gate. There are a few concerns regarding this process: - The gate length of the MESFET is defined by the foot of the dummy T-structure that is formed by an isotropic (or partially isotropic) etching process following the anisotropic definition of the cap of the T. The foot dimension may be hard to control to the extent required by the device design, in particular for dimensions in the submicron regime. - The gate metallization and the ohmic metallization are not self-aligned with respect to the implanted contact regions. Truly self-aligned metallizations are, however, highly desirable to keep parasitic effects, in particular parasitic series resistances, small. Again, this effect will be of increasing importance when re­ ducing the device dimensions. It is a main object of the present invention to provide a MESFET fabrication process which allows self-­ alignment of all three critical device parts, namely the gate and ohmic contact metallizations and the implanted contact regions, with respect to each other. A further object is to disclose a process that provides for improved dimensional control of the gate length and gate separation thereby making the process applicable to the fabrication of submicron structures and permitting a substantial reduction in the required device area. The invention as claimed is intended to meet the above objectives and to remedy the drawbacks encountered with hitherto known FET fabrication processes. It solves the self-alignment problem by forming a multi-layer mask which includes an "Inverted-T" structure for the contact region implantation. The middle part of the inverted T represents the dummy gate that is replaced by the Schottky gate after self-aligned ohmic contact metal deposition. Depending on the embodiment chosen, the source-gate and drain-gate separation distances can be determined either by sidewall spacers formed at both sides of the dummy gate or by the overhang of an initial "double-T" structure. The advantages offered by the invention are mainly that, due to the full self-alignment, the source-to-drain spacing given by the gate length and the gate-source and gate-drain separations can be minimized. In fact, the process can be extended to the fabrication of submicron structures whereby high speed devices and high packaging densities can be achieved. The process furthermore allows GaAs surface passivation throughout the hole process thereby providing for better and more reliable process control. Various ways of carrying out the invention are described in detail below with reference to drawings in which Fig. 1 is a cross-sectional representation of an embodiment of a MESFET produced in accordance with the invention. Figs.2A-2Q are illustrations of the steps of the process used to fabricate the structure shown in Fig.1. Figs.3A-3C are illustrations of alternative process steps that result in graded contact region implant­ ations. Figs.4A-4D are illustrations of process steps resulting in a "double-T" mask structure. Referring now to the drawings and first to Fig. 1 thereof, there is shown a first embodiment of a MESFET produced in accordance with the present invention. The MESFET 10 is formed on a semi-insulating (SI) GaAs substrate 11. The self-aligned device 10 comprises a relatively shallow channel 12G, its doping concentration (resulting in either a n- or n⁻-channel) depending on whether enhancement-type (E) or depletion-type (D) devices are required, i.e., it depends on the desired value of the threshold voltage V th . Arranged on the channel 12G is a metal gate electrode 15G forming a Schottky barrier at the gate-channel interface. Posi­ tioned at both sides of the channel 12G but separated from gate electrode 15G are heavily doped n⁺ GaAs contact regions 13S and 13D forming source and drain regions, respectively. Source and drain electrodes 14S and 14D, respectively, are deposited on their respective n⁺ contact regions forming ohmic contacts therewith. Sec­ tions 15S and 15D of the gate metallization layer cover the underlying ohmic contact metallization (14S, 14D) lowering the ohmic metal wiring resistance. In Figs. 2A through 2Q the successive steps of the process used to fabricate the MESFET of Fig. 1 in accord­ ance with the present invention are illustrated in detail. These successive steps are listed in Table I indicating the correspondence between the steps and the drawings. While the following description is primarily direct­ ed to the fabrication of the MESFET shown in Fig. 1, this description is only exemplary of the inventive method. It should be noted that the thickness and other dimensions, the materials used as well as the process parameters given herein are selected for clarity of illustration and are not to be interpreted in a limiting sense. Most individual process steps that are used to evaporate, to implant, to etch, to pattern, are well known and can be performed by employing conventional equipment and tech­ niques. They are, therefore, not explained in greater detail. Referring now to Fig. 2A, the fabrication process is initiated starting from a semi-insulating GaAs wafer representing the substrate 11. In step 1 of the process its surface is covered with a passivation layer 16 which, in the example, consists of sputter-deposited alluminum nitride (AlN) of 80 nm thickness. This layer is a di­ electric and permits through-implant of ions into the underlying semiconductor substrate 11. Next, photoresist 20 is applied and patterned as shown in Fig. 2B to define the substrate surface area where the channel 12 is to be formed. Subsequently (step 2), the wafer is subjected to an ion beam (arrows 21). The ions, after passing through passivation layer 16, are implanted in substrate 11 forming n-(or n⁻-) channel 12. In the example, Si (29) is implanted at 90 kV, the dose being in the range of 3 to 6 × 10¹² cm⁻², depending on whether a D- or E-type device is being fabricated. After stripping photoresist 20, another dielectric layer 22 is deposited on top of passivation layer 16 as illustrated in Fig. 2C. Silicon nitride (Si₃N₄) is chosen and a layer of 250 nm thickness deposited using a Plasma Enhanced Chemical Vapor Deposition (PECVD) method (step 3). Thereafter, in step 4, a further dielectric layer 23 is evaporated (Fig. 2D) consisting, in the example, of calcium fluoride (CaF₂), its thickness being 350 nm. Subsequently, reference is now made to Fig. 2E, the dummy gate 23G is patterned from layer 23 using photore­ sist lithography (step 5): after applying and patterning the photoresist 24, the exposed portion of CaF₂ layer 23 are sputter-etched, Si₃N₄ layer 22 serving as etch stop. Upon stripping photoresist 24, sidewall spacers 25 are formed at both edges of dummy gate 23G as shown in Fig. 2F. This is done by first depositing an insulation layer consisting of a polymeric material in a Chemical Vapor Deposition (CVD) or sputter process - the thickness of this layer eventially determines the source-gate and gate-drain distances of the completed MESFET device. The layer is then anisotropically etched in an RIE process until the planar surface portions of the underlying structure are exposed. As a result, portions of the insulation layer remain at the vertical edges of dummy gate 23G forming sidewall spacers 25 (step 6). In the next process step 7, the multi-layer mask structure 26 of Fig. 2G is formed which comprises portion 22G of the dielectric Si₃N₄ layer, the dummy gate 23G and sidewall spacers 25. This is achieved by using a further RIE process during which the exposed dielectric layer 22 is etched, the AlN passivation layer 16 acting as etch stop. As will already be apparent now and will become more clear from the following description, some of the process steps rely on a distinct etch selectivity, i.e., the etch rates of some of the materials used (namely that of passivation layer 16, dielectric layer 22, dummy gate 23G and that of the sidewalls 25) must be sufficiently different as to permit preferred etching of one of the materials. A broad spectrum of materials is available for this purpose, examples being poly-silicon, silicon oxide, silicon nitride, silicon oxynitride, alluminum nitride, polyimid, etc. Fig. 2H illustrates step 8, i.e., the n⁺ ion implant (arrows 27) to form the source and drain contact regions 13S and 13D, respectively: after applying and patterning photoresist 28, the Si (29) implant is again through the AlN layer 16, the implant dose is 5 × 10¹³ cm⁻² at 130 kV. The channel region 12G is shielded by multi-layer mask structure 26, the implant is self-aligned to the dummy gate. Subsequently, photoresist 28 is stripped using acetone, the sidewall spacers 25 are dissolved in a photoresist stripper, and the ion implants annealed in a forming gas (N₂/H₂) at 850°C for 20 minutes (step 9). The annealing takes place with the substrate surface pro­ tected by the AlN passivation layer 16 and prior to the deposition of the gate metallization. At this stage, the multi-layer mask structure 26 has been reduced to the "inverted-T" structure 30 as illustrated in Fig. 2I. In the next step 10 those substrate surface areas are defined where the source and drain electrodes forming ohmic contacts with the n⁺ contact regions 13S and 13D are to be deposited. First, photoresist 31 is applied and patterned as shown in Fig. 2K. Thereafter, via holes for the ohmic contacts are etched into passivation layer 16, the patterned photoresist and the inverted-T shaped structure 30 serving as mask. A wet chemical etch process with hydrogen fluoride (HF) is used for this purpose. The remaining photoresist 31 is then stripped. In the succeeding step 11 of the process, the ohmic contact metallization, in the example consisting of an AuGeNiAu layer of 80 nm thickness, is evaporated using a photoresist mask not shown in the drawing. This metalli­ zation is deposited on the planar surfaces forming the various sections 14S, 14SG, 14G, 14GD and 14D as illus­ trated in Fig. 2L. Contacts 14S and 14D are self-aligned to the dummy gate. Alloying takes place at 430°C for 30 seconds. As shown in Fig. 2M, the CaF₂ dummy gate 23G is now removed together with section 14G of the ohmic contact metallization by dissolving the CaF₂ in HCl/H₂O (step 12). This is followed by step 13 during which the self-­ aligned via hole for the gate contact is opened by etching the Si₃N₄ layer 22G (RIE using CF₄) and, subse­ quently, the AlN layer 16G (RIE using CCl₄). This results in the structure shown in Fig. 2N. Next, in step 14, the gate metallization, in the example consisting of a palladium-gold (PdAu) layer of 100 nm thickness, is evaporated using a photoresist mask 17. This metallization forms the various sections 15S, 15SG, 15G, 15GD, 15D as illustrated in Fig. 2P. Gate contact 15G is self-aligned to both, the ohmic contacts 14S and 14D and the n⁺-implant regions 13S and 13D. In a final step 15, photoresist 17 is stripped and the remaining parts 22SG and 22GD of the Si₃N₄ layer are etched off using plasma etching in a CF₄ atmosphere. This results in the MESFET structure of Fig. 2Q which corres­ ponds to that of Fig. 1 except for the thin AlN insula­ tions 16SG and 16GD that separate the gate electrode 15G from the source and drain electrodes 14S and 14D, res­ pectively. These may or may not be removed before com­ pleting the integrated circuit structure, i.e., before adding further insulations and metallizations. The source resistance (n⁺-contact region-channel) of the MESFET of Fig. 1 (or Fig. 2Q) can be further de­ creased by employing a two-step graded contact region implantation. The required process steps are illustrated in Figs. 3A through 3C. In the complete fabrication process, they replace steps 2F through 2H. The successive steps required are listed in Table II where the corres­ pondence between the steps and the drawings are indi­ cated. The modified fabrication process is initiated starting from the structure shown in Fig. 2E. The next process step (step 1′ of Table II) is an intermediate ion implant to form n-implant regions 42S and 42D adjacent channel 12G. This is illustrated in Fig. 3A: after stripping photoresist 24 and applying and patterning photoresist 40, the Si(29)-implant (arrows 41) is through Si₃N₄ layer 22 and through passivation layer 16 with an implant dose of 1 × 10¹³ cm⁻² at 200 kV. The channel region 12G is shielded by dummy gate 23G. The implant is self-aligned to the dummy gate. Subsequently, the multi-layer mask structure 26 of Fig. 3B is formed in virtually the same way as in steps 6 and 7 of the initial process sequence, i.e., after removal of photoresist 40 sidewall spacers are formed (step 2′) and, thereafter, the exposed regions of Si₃N₄ layer 22 are etched (step 3′). The n⁺-implant step 4′, illustrated in Fig. 3C, then corresponds to step 8 of the initial process sequence, i.e., n⁺-contact regions 13S and 13D are formed. The resulting MESFET structure of Fig. 3C differs from that of Fig. 2Q in that n-zones 42S and 42D are arranged inbetween the n⁻-channel 12G and the n⁺-source and drain contact regions 13S and 13D, respectively. In the processes described above, a multi-layer mask structure 26 has been formed and thereafter gradually reduced through selective etching processes to provide the masks needed in the fabrication of the self-aligned gate and of the source and drain electrodes and n⁺-­ contact regions. Mask structure 26 consisted of an inverted-T structure 30 (comprising layers 22G and 23G) and sidewall spacers 25, the thickness of the latter eventually determining the gate-ohmic contact separa­ tions. A different "double-T" multi-layer mask structure 53 consisting of an inverted-T structure (22G and 52) topped by an "extending" thin film 50 may be used instead of the mask structure designated 26 in, e.g., Figs. 2G or 3B. The required process steps for the formation of such "double-T" structure are illustrated in Figs. 4A through 4D which, in the complete fabrication process, replace steps 2E through 2I. The successive steps required are listed in Table II where the correspondences between the steps and the drawings are indicated. This modified fabrication process is initiated starting from the structure in Fig. 2D. The next process step (step 1″ of Table III) provides the thin film 50 that eventually "tops" the double-T multi-layer mask structure. This step is illustrated in Fig. 4A: Film 50 is deposited on top of CaF₂ layer 23 and then patterned using photoresist lithographic techniques involving photoresist 51 and an etch process. Next, layers 22 and 23 are selectively removed (step 2″) in an RIE process using film 50 as a mask. The resulting multi-layer structure is shown in Fig. 4B. In the following process step 3″, the dummy gate 52 is tailored by underetching whereby the double-T structure 53 (Fig. 4C) is obtained. It serves the same purposes as the mask structure 26 in Figs. 2G or 3B - the undercut eventually determines the distance between the gate and the ohmic source and drain contacts. The subsequent ion implant (arrows 54) for forming the n⁺-contact regions (step 4″) corresponds to that described above in connection with Fig. 2H, the diffe­ rence being that the "topping" film 50 of mask structure 53 (Fig. 4D) determines the contact region boundaries rather than the sidewall spacers 25 of mask structure 26 in Fig. 2H. In step 5 film 50 is removed and the ion implant annealed. At this stage, the inverted-T structure 30 illustrated in Fig. 2I is obtained and the fabrication process can proceed with the steps described above with the aid of Fig. 2K to 2Q. One further deviation from the initially described process sequence worth mentioning is a process and device in which the passivation layer 16 is omitted. This layer, providing for better process control and reproducibility, may not be needed in less critical applications and does not form an essential feature of the inventive process. From the above it will be evident that many other alternatives, modifications and variations still falling within this scope and spirit of the present invention will be apparent to those skilled in the art. In summary, the new process allows to self-align both, the n⁺-contact region implant as well as the ohmic contacts with respect to the gate. This minimizes the source-to-drain spacing which is important for high packaging densities. For submicron gate lengths struc­ tures requiring shallow channels it is also of advantage that the ohmic contact can be moved as near as possible to the gate especially when the channel doping is high. Particularly the process employing sidewall spacers for determining the n⁺-implant-to-gate spacing provides for better control of the critical spacing and dimensions than the hitherto known processes.

Claims: 1. A process of fabricating a fully self-aligned field effect transistor, the transistor being formed on a semiconductor substrate (11) and comprising a current channel (12G) and associated source (14S), gate (15G) and drain (14D) electrodes, characterized in that the process comprises the steps of - providing a semiconductor substrate (11) having a defined current channel (12G) at its surface, - forming on said current channel a patterned multi-layer mask (30) consisting of portions of a first (22) and a second (23) dielectric layer, the portion (23G) of said second layer being deposited on and shorther than the portion (22G) of said first layer, the cross-section of said multi-layer mask (30) thus representing an "in­ verted-T", - deposition of an ohmic contact metallization (14S, 14SG, 14G, 14GD, 14D) onto the planar surfaces of the structure resulting from the proceeding steps, - removal of said portion (23G) of said second dielectric layer and of the then exposed center section of said portion (22G) of said first dielectric layer that is not covered by said ohmic contact metallization (14SG, 14GD), whereby a mask for the gate contact is provided, - deposition of a gate metallization (15S, 15SG, 15G, 15GD, 15D), and - removal of the remaining portions (22SG, 22GD) of said first dielectric layer together with those sections of the ohmic contact metallization (14SG, 14GD) and of the gate metallization (15SG, 15GD) deposited on top of it. 2. A process as claimed in claim 1, wherein said multi-layer mask (30) is formed by - coating the surface of said semiconductor substrate (11) having a defined current channel (12G) with said first dielectric layer (22), - depositing said second dielectric layer (23) on said first dielectric layer (22), - patterning said second dielectric layer to form a "dummy" gate (23G), - forming sidewall spacers (25) at the edges of said "dummy" gate (23G), - removing those portions of said first dielectric layer (22) not covered by said "dummy" gate-sidewall spacer-­ structure (23G, 25) thereby providing an intermediate multi-layer structure (26), and by - removing said sidewall spacers (25). 3. A process as claimed in claim 2, wherein said intermediate multi-layer structure (26) serves as a mask in an ion implant process used to form n⁺-contact regions (13S, 13D) within said semiconductor substrate (11) adjacent both ends of said current channel (12G). 4. A process as claimed in claim 1, wherein said multi-layer mask (30) is formed by - coating the surface of said semiconductor substrate (11) having a defined current channel (12G) with said first dielectric layer (22), - depositing said second dielectric layer (23) on said first dielectric layer (22), - depositing and structuring a thin film (50) on said second dielectric layer (23), - directional etching of said first and second dielectric layers using the structured thin film (50) as an etch mask, - underetching said second dielectric layer to form a "double-T" structure thereby providing an intermediate multi-layer structure (53), and by - removing said thin film (50). 5. A process as claimed in claim 4, wherein said intermediate multi-layer structure (53) serves as a mask in an ion implant process used to form n⁺-contact regions (13S, 13D) within said semiconductor substrate (11) adjacent both ends of said current channel (12G). 6. A process as claimed in claim 2, wherein said "dummy" gate (23G) serves as a mask in a first ion implant process that is used to form first contact regions (42S, 42G) within said semiconductor substrate (11) adjacent both ends of said current channel (12G), and wherein, at a later stage of the fabrication process, said intermediate multi-layer structure (26) serves as a mask in a second ion implant process that is used to form second contact regions (13S, 13D) overlapping said first contact regions thereby providing a graded contact implant (42S-13S; 42D-13D). 7. A process as claimed in claim 1 or 2, wherein an additional passivation layer (16) is deposited on the semiconductor substrate (11) prior to the formation of the current channel (12G). 8. A process as claimed in claim 1 or 2, wherein the semiconductor substrate (11) consists of GaAs. 9. A process as claimed in claim 1 or 2, wherein said gate (15G) forms a Schottky barrier at the interface with said current channel (12G). 10. A process of fabricating a fully self-aligned field effect transistor, the transistor being formed on a semiconductor substrate (11) and comprising a current channel (12G) and associated source (14S), gate (15G) and drain (14D) electrodes, characterized in that the process comprises the steps of - providing a semiconductor substrate (11) having a defined current channel (12G) at its surface, - coating the surface of said substrate (11) with a first dielectric layer (22), - depositing and structuring a second dielectric layer (23) to form a "dummy" gate (23G), - forming sidewall spacers (25) at the edges of said "dummy" gate (23G), - removing those portions of said first dielectric layer (22) not covered by said "dummy" gate-sidewall spacer-­ structure (23G, 25), - contact region (13S, 13D) implantation using the "dummy" gate-sidewall spacer-structure (23G, 25) as a mask, with subsequent annealing, - removing said sidewall spacers (25), - depositing an ohmic contact metallization (14S, 14SG, 14G, 14GD, 14D) onto the planar surfaces of the structure resulting from the proceeding steps, - removing said "dummy" gate (23G) and the center section of said first dielectric layer (22G) underlying said "dummy" gate (23G), and - depositing a gate metallization (15S, 15SG, 15G, 15GD, 15D) onto the planar surfaces of the structure resulting from the proceeding steps.

Inventors: GRAF, VOLKER, DR., OOSENBRUG, ALBERTUS
Applicants: INTERNATIONAL BUSINESS MACHINES CORPORATION
Application year: 1985
Main classification: H01L21
IPC codes: H01L29, H01L21