Technique for use in fabricating semiconductor devices having submicron features

Abstract: Submicron resolution in the fabrication of transistors is obtained by using sidewall techniques. In the technique described an aperture with a silicon nitride sidewall spacer is formed in a layer (9, 11). Oxide (15) is formed in the aperture and the sidewall spacer is then removed to form an annular opening (17) which can be of a small and precisely controlled width. The opening is then used as a mask for subsequent substrate modification by either diffusion or ion implantation.

Description: Technical Field This invention relates to the field of semiconductor fabrication techniques. Background of the Invention Bipolar transistors are presently still of considerable interest because, for example, their operating speeds are typically higher than are the operating speeds of field effect transistors. As device dimensions decrease, due to a desire for still higher operating speeds as well as greater packing densities, fabrication of bipolar, as well as field effect, transistors becomes more difficult. For example, a high speed bipolar transistor requires fabrication of a small base region which must not only be electrically contacted but must also be accurately aligned with respect to both the emitter and collector regions. A small base region reduces the emitter-base capacitance which is one of the factors limiting device operating speed. A small base width also improves device response time. Additionally, small and low resistance electrical contacts to both the emitter and collector reregions are also required. Similar fabrication problems also arise as the dimensions of field effect transistors decrease. Numerous techniques have been devised for fabricating bipolar transistors. See, for example, U.S. Patents 3,833,429 issued on September 3, 1974 and 4,338,138 issued on July 6, 1982. The fabrication techniques have been developed to a level of sophistication such that bipolar transistors with submicron feature sins have been fabricated. An illustrative submicron feature bipolar transistor is described in International Electron Devices Meeting, 1985, paper 2.1. The technique described by the authors is termed "The S uper S elf-Aligned Process T echnology, " (SST). The fabrication sequence is relatively complicated although only a single patterning step using an optical mask is required. Many materials, e.g., silicon dioxide, silicon nitride and polysilicon, can be used, but the typical sequence for each material comprises depositing the material and then selectively etching it to form the desired pattern with perhaps an intermediate oxidation step. The fabrication sequence is depicted in his FIG. 2 and the device in his FIG. 1. Although useful SST devices were apparently fabricated, there are some undesirable limitations to the technique which also impose limitations on the characteristics of the device disclosed. For example, while the disclosed technique can probably produce line width as small as .1µm, variations in the etch rate probably prohibit a further reduction of the line width by as much as an order of magnitude. Additionally, the distance between the base and emitter metal contacts is determined by optical photolithographic and etching technique limitations. The resulting relatively large spacing is undesirable in the disclosed device because the sheet resistance of the polysilicon used for the base contact is an important factor in determining the extrinsic base resistance. The extrinsic base resistance for the disclosed device will be relatively high thus resulting in increased noise, i.e. it acts as a noise source connected to the base. It will also be appreciated by those skilled in the art that control of the polysilicon sheet resistance will be relatively difficult because the polysilicon grain size is a function of the deposition parameters, annealing conditions, etc., and these parameters may be difficult to control precisly in practice. Variations in both the carrier mobility in the polysilicon film and the extrinsic base resistance may result in variations in device operating characteristics. Doped polysilicon is used as the diffusion source to form the junction between the base and emitter. Process control problems will almost certainly result because the emitter profile is determined by the surface conditions at the single crystal/polycrystalline interface. It is known to those skilled in the art that several layers of an oxide at this interface may dramatically change the emitter profile due to surface segregation effects. EP-A-0 010 624 discloses a method of forming very small mask openings for fabrication of semiconductor devices comprising forming a first dielectric layer, having sidewalls, on a substrate, depositing a silicon nitride layer so as to cover the first dielectric layer, including the sidewalls, etching the silicon nitride layer to form sidewall spacers and leaving portions of the substrate exposed, forming an oxide layer by thermally oxidizing the exposed portion of the substrate and then removing the sidewall spacers to expose portions of the substrate previously beneath them. The substrate is then doped through the small openings to form active device regions. Summary of the Invention According to the invention features are formed in a semiconductor structure by a method as set out in claim 1. The size of the sidewall spacers determines the size of the minimum lithographic dimension. The spacers can be small, and the minimum feature size is also small. In a preferred embodiment, submicron lithographic resolution is obtained by patterning a layer, comprising a dielectric, on a silicon substrate to form openings which expose surfaces of the substrate, depositing a nitride layer in said openings, patterning the nitride layer to form sidewall spacers which expose portions of the silicon surface, depositing silicon, and oxidizing the deposited silicon. Removal of the nitride spacers then again exposes the silicon surface with the openings so formed being accurately spaced from each other and having carefully controlled dimensions. After removal of the spacers has been completed, further steps, such as diffusion or ion implantation may be performed, to modify the exposed silicon surface. The technique, which is expediently used to fabricate bipolar transistors, differs from prior art sidewall spacer techniques for reducing dimensions below those attainable by conventional optical lithography because the sidewall spacers are removed rather than retained. The silicon can be deposited by using selective epitaxial growth to grow a thin silicon layer in the opening. The subsequent oxidation results in growth primarily in the vertical direction so that the integrity of the sidewall spacers is not adversely affected. Brief Description of the Drawing FIGs. 1-6 illustrate steps in a method embodying this invention. For reasons of clarity, the elements of the devices and structures depicted are not drawn to scale. Detailed Description The fabrication technique will be described by reference to a specific embodiment which fabricates a bipolar transistor on a Si substrate. Those skilled in the art will readily appreciate that devices other than the one specifically described may be fabricated and that other embodiments of the growth technique are contemplated. The structure depicted in FIG. 1 comprises a silicon substrate 1 having a first conductivity type on which is grown an epitaxial layer 3 having a second conductivity type. There is additionally a buried layer 5 having a second conductivity type between portions of the substrate and the epitaxial layer. The buried layer provides a reduction in the RC time constant. A low temperature oxide layer 7 is deposited on the epitaxial layer 3 using well known techniques. The oxide layer prevents emitter-collector shorts. A silicide layer 9 is deposited and patterned, and a second low temperature oxide layer 11 is deposited. Any conductor material may be substituted for the silicide. The silicide and second low temperature oxide layers 9 and 11, respectively, are patterned to form openings which expose portions of the first low temperature oxide layer 7. The openings are filled with a nitride plug and are not over buried layer 5. An opening in the nitride plug which exposes the surface of the epitaxial layer and leaves sidewall spacers 13 is now formed. FIG. 1 illustrates the structure after the opening has been formed. The structure depicted in FIG. 1 can be formed by conventional and well known techniques which need not be described in further detail. It will be readily appreciated that the techniques used to form the opening in the nitride plug which leaves sidewall spacers are well known to those skilled in the art and need not be described in detail. Selective epitaxial deposition is now used to grow silicon on the exposed Si in the structure depicted in FIG. 1. The epitaxial silicon is oxidized and the nitride sidewall spacers are removed. Techniques for these steps are well known to those skilled in the art. The thickness of the epitaxial layer should be selected so that all of the layer is oxidized. The silicon will expand primarily in the vertical direction and not into the sidewall spacers. The resulting structure has an oxide region 15 and a space 17 surrounding the oxide region. This structure is depicted in FIG. 2. An exemplary dimension for the opening, prior to nitride deposition, is 1.0 µm, and the resulting openings can be as small as perhaps 10 nm. The minimum opening dimension will be determined by the minimum sidewall spacer dimension. As will be readily appreciated, care must be taken during this oxidation step to preserve the integrity of the sidewall spacers due to their small dimensions. Silicon expands as it is oxidized and the physical integrity of the nitride spacers might be ruined. However, the selective epitaxial growth avoids these problems as expansion during oxidation is primarily in the vertical direction. Successive ion implantations may now be used to form the base region 19 and the emitter region 21. The base and emitter regions have first and second conductivity types, respectively. Polysilicon is now deposited in polysilicon regions 23. The polysilicon is desirably deposited by selective low pressure chemical vapor deposition. The structure is depicted in FIG. 3. Polysilicon region 23 is now partially oxidized to form an oxide cap 25 as depicted in FIG. 4. Etching techniques, such as reactive ion etching, are now used to remove the oxide layer 15 and thereby expose the underlying portion of the surface of the epitaxial layer 3 having a second conductivity type. An ion implantation step is now used to form the passive base region 27 and the resulting structure is depicted in FIG. 5. Well known techniques are used to form openings for the contacts and the metallizations. There are collector, emitter and base contacts 29, 31, and 33, respectively. The finished device is shown in FIG. 6. The ultimate resolution of the described lithographic technique is determined by the thickness of the nitride sidewall spacer remaining after etching. The process is reproducible with respect to dimensions because the etch occurs in the rather wide opening, i.e., the vertical surfaces are the primary surfaces etched rather than the smaller horizontal surfaces. Additionally, the base contact is close, perhaps only 70 nm, to the base, and as a result has only a minimal series resistance. It will be readily appreciated that the emitter contact is made through a low sheet resistance buried silicide layer and that the collector contact is made directly to the epitaxial layer. Both of these contacts minimize resistance. It will be readily appreciated by those skilled in the art that complementary devices, that is, both n-p-n and p-n-p, may be fabricated in the same substrate by using alternating doping and masking processes. Other materials, such as salicides, i.e., self aligned silicides, may be used. Use of such materials in the emitter is contemplated. The base implant can be omitted and a JFET fabricated. Other devices are contemplated. For example, etching down to the n-type epitaxial layer to form a Shottky diode permits fabrication of a Shottky clamped bipolar transistor for high switching speed applications. Also, a long N-Channel JFET can be built in the same process and is useful for BiFet applications. A short N-Channel JFET pair can be fabricated with the channel length determined by a vertical P diffusion or implantation. Additionally, a high speed lateral transistor can be made with the narrow base width being determined by nitride thickness rather than conventional etching.

Claims: A method of fabricating a semiconductor device comprising the steps of: patterning a first dielectric layer (11) on a semiconductor substrate (1) to form an opening exposing a portion of a surface of the semiconductor substrate (1); forming silicon nitride sidewall spacers (13) in said opening, leaving further portions of the semiconductor surface within said opening exposed; forming an oxide layer (15) on the exposed further portions of the semiconductor surface by growing a layer of silicon by a selective epitaxial growth process on said exposed portions of the semiconductor surface; and oxidizing said epitaxially grown layer; removing the sidewall spacers (13) to expose regions of the semiconductor substrate (1) previously underneath the sidewall spacers (13); and doping the exposed regions of the semiconductor substrate (1) to form active device regions (19,21). A method as recited in claim 1 in which said doping is by diffusion or ion implantation. A method as recited in any of claims 1 or 2 in which said doping forms two active device regions of opposite conductivity type, said two active device regions forming emitter (21) and base (19) regions. A method as recited in claim 3 in which said substrate (1) forms a collector region. A method as recited in claim 4 further comprising the step of forming electrical contacts (31,33,29) to said emitter (21), base (19), and collector (3) regions. A method as recited in any of claims 3, 4 or 5 comprising the further steps of removing the oxidized silicon to expose a part of the semiconductor substrate surface and doping said exposed parts to form an active base region (27).

Inventors: FEYGENSON ANATOLY, FEYGENSON, ANATOLY
Applicants: AMERICAN TELEPHONE & TELEGRAPH, AMERICAN TELEPHONE AND TELEGRAPH COMPANY
Application year: 1988
Main classification: H01L21
IPC codes: H01L29, H01L21