SELECTIVE DEPOSITION PROCESS

Abstract: In the process for depositing metallic films in spatially selective regions on a semiconductor substrate (1), the substrate is illuminated with coherent photon energy which is selectively absorbed by patterned regions (6) of the substrate (1), which patterned regions (6) have accentuated absorption characteristics suitable to cause localized heating and pyrolytic reaction with selected ambient gases. The reactant gases are transparent to the illuminating radiation. Matching of the coherent photon energy wavelength with the photon absorption characteristics of the selected regions (6) in combination with pulsed operation of a laser (19) providing the photon energy can further localize the spatial selectivity. Optical free carrier absorption occurs preferentially in heavily doped regions, which regions are readily defined using known techniques for patterned doping. The deposited films thereafter act as nucleation sites for more conventional forms of chemical vapor deposition.

Description: SELECTIVE DEPOSITION PROCESS Technical Field This invention relates to a process for selectively depositing a film of a particular material on a substrate. The invention has a particular application to the field of selectively depositing a metal film, such as tungsten, during fabrication of a semiconductor device. In particular, the invention finds use in the fabrication of VLSI semiconductor devices in which the resistivity of doped polycyrstalline silicon, the commonly used lower level interconnect and electrode material, is too great for the speeds or power consumption levels required. A layer of high conductivity material, for instance tungsten, provided on the polycyrstallic. silicon, alleviates this ' problem. Background Art An article in SOLID STATE TECHNOLOGY, June 1985, pages 220-227; Solanki et al: "Laser-Induced Chemical Vapor Deposition", discloses the laser- induced deposition of thin films by pyrolysis, evaporation and photolysis. When utilizing pyrolysis, a laser beam is directed onto a substrate to heat the substrate locally in the beam area. The laser wavelength is chosen such that it is absorbed by the substrate and is transparent to the reactant gases. Since the deposition area is related to beam size, this technique is inefficient for use in a volume production process. U.S. Patent No. 4,395,433 discloses a process wherein a silicon dioxide film is formed on a substrate and etched to expose an area of the substrate surface. The structure is placed in a reaction chamber at a temperature of 600°C and exposed to silane gas. In one arrangement, a laser beam is applied to the whole substrate surface. Silicon is deposited on the exposed surface of the substrate but not on the insulating film because the relatively high thermal conductivity of the substrate conducts heat away from the substrate surface, whereas the relatively low thermal conductivity of the silicon dioxide causes heat to concentrate at the surface thereof, such that silicon to be deposited is scattered, failing to form a silicon layer. Disclosure of the Invention It is an object of the present invention to provide an efficient process for the selective deposition of a material on a substrate surface. Therefore, according to the present invention, there is provided a process for selectively depositing a film of a particular material on a substrate, characterized by the steps of: rendering a selected region of the substrate to have accentuated radiation absorption characteristics at a selected wavelength; subjecting the substrate to an environment containing a gaseous compound which is adapted to pyrolytically deposit a film of the material on a region of elevated temperature; exposing the substrate to radiation at the wavelength for which the radiation absorption characteristics of the selected region are accentuated; and illuminating the substrate with the radiation until the selected region is sufficiently elevated in temperature to cause a localized pyrolytic reaction of the gaseous compound. According to one practice of the invention, colli ated coherent photon energy, commonly in the form of a laser beam, is used to blanket illuminate the upper surface of a partially completed semiconductor substrate while the substrate is situated within a reactant gas environment. The wavelength of the laser, the composition of the reactant gas, and the composition of the various regions of the exposed surface of the substrate are selected to ensure that photon energy absorption rate is materially greater in those regions where pyrolytic reaction is intended. The amorphous or polycrystalline silicon regions of accentuated absorption are selectively heated to elevate the local temperate and initiate a pyrolytic reaction with the metal bearing gaseous compound ambient, while the adjacent lower temperature regions are substantially unaffected. Thereby, the pyrolytic deposition of the metals coincides with the pattern of the high photon energy absorption coefficient silicon. The selectivity of the photon absorption can be further accentuated by matching the wavelength of the photon energy source and the thicknesses of the selected regions to adjust the optical reflection or absorption, or by changing the wafer temperature to alter the optical free carrier absorption. According to an alternate embodiment, the selectivity, namely the differences in photon absorption coefficient, is further accentuated by aligning the collimated photon energy beam to be precisely perpendicular to the surface of the region having high photon energy absorption characteristics. The use of a short pulse duration photon energy source then localizes the regions of elevated temperature to the upper surface of the layer. Thereby, pyrolytic reaction and the associated spatially selective deposition is limited to the upper surface of the selected substrate regions. Upper surface selectivity can be enhanced by using very short wavelength, short pulse duration photon energy sources and by altering the photon energy absorption characteristics of the upper surface relative to the bulk of the patterned region. Typically, this would involve a surface - 4 - doping for a silicon semiconductor by shallow ion implantation or the like. Optical free carrier absorption in such doped regions would be materially larger than for undoped regions. The use of these concepts allows one to deposit, with a high degree of spatial selectivity, low resistance shunting materials such as tungsten onto patterned amorphous or polycrystalline silicon, either on the whole or on merely the top surfaces thereof, for gate electrodes or interconnect electrodes of integrated circuit devices. The process can also be applied to selectively deposit on exposed monocrystalline silicon, such as in the heavily doped source/drain regions of field effect transistors, or on the polycrystalline silicon gate electrode layers. Furthermore, the selectivity can be practiced in such a way as to avoid undesired pyrolytic deposition of refractory metal on the undoped or very lightly doped regions which are intended to remain of high resistance. Brief Description of the Drawings Fig. 1 is a schematic cross-section of a field effect transistor structure in which the accentuated photon energy absorption of the gate electrode is employed to pyrolytically deposit a metallic layer thereover. Fig. 2 is a schematic similar to that in Fig. 1, wherein the thickness of the gate electrode is matched to the wavelength of the photon energy. Fig. 3 depicts another arrangement for spatially selective deposition, the figure diagrammatically depicts the use of a short wavelength pulsed photon energy source collimated and aligned to limit the pyrolytic reaction to the upper surface of the gate electrode. Fig. 4 schematically illustrates the use of a thin doped region at the surface of a polycrystalline silicon gate electrode layer to spatially control the deposition to the upper surface via the significantly higher optical free carrier absorption in this region. Fig. 5 schematically depicts a field effect transistor structure in which spatially selective deposition by photon energy absorption coefficient differences is used to selectively deposit a metal over the source/drain regions and gate electrode while avoiding gate to source/drain shorts over the sidewall oxides. Fig. 6 schematically illustrates the structure in Fig. 5 extended to include a high resistance polycrystalline silicon region, which by selectivity of the photon absorption coefficient is not subject to a pyrolytic deposition. Low free carrier absorption occurs because of the low doping levels. Fig. 7 is a schematic illustrating apparatus suitable to practice the spatially selective deposition process to which this invention pertains. One aspect of the invention may be appreciated by considering the field effect transistor application depicted in Fig. 1 of the drawings. According to that practice, a semiconductor substrate 1 comprised of a material such as monocrystalline silicon is processed according to known techniques to form spaced-apart field oxide regions 2 and 3, preferably composed of 500-1000 nanometers thick silicon dioxide. The active region 5 of substrate 1 situated between field oxides 2 and 3 is covered by gate oxide 4, preferably composed of silicon dioxide to a thickness in the range of 20 to 50 nanometers. Situated over the gate oxide 4 and generally centrally within the exemplary active region is gate electrode 6, typically patterned from doped polycrystalline silicon and having.a thickness in the range of 30 to 50 nanometers. On either the side of gate electrode 6 and within the structure of substrate 1 are regions 7 and 8 generally representing the transistor source and drain electrodes. The practice of the invention as depicted in Fig. 1 is directed to forming a spatially selective deposition of low resistance, shunting, conductive layer 11 which fully encloses gate electrode 6 while eliminating spurious formations of the conductive material (forming conductor 11) on gate oxide 4 or field oxide regions 2 or 3, wherein the practice thereof obviates the use of photolithographic masking. Optical free carrier absorption which is much stronger in heavily doped regions will encourage selective heating and hence selective deposition. Following one practice of the invention, as schematically depicted in Fig. 7, substrate 1 is situated on heater 12 and fixed by target holder 13 within vacuum chamber 14. The heater provides a temperature bias to optimize selectively in the free carrier absorption. Reactant gases enter the chamber by inlet 16, and the residuals thereof are removed by vacuum level exhaust 17. One wall of vacuum chamber 14 includes a photon energy transmissive window 18, the composition of which is selected to match the fundamental wavelength of the coherent photon energy beam provided, in this case, by laser 19. Laser beam 21 is shown according to this embodiment to be directed through window 18 by reflection off mirror 22. Table A sets forth suitably wavelength matched sets of laser types and window materials. Applying the apparatus shown in Fig. 7 to the processing of the substrate 1 in the form depicted in Fig. 1, it is possible to selectively deposit conductive layers such as 11 in Fig. 1, preferably composed of a refractory metal such as tungsten, onto the exposed surfaces of a patterned polycrystalline silicon layer, such as gate electrode 6. The selectivity of the pyrolytic reaction by which conductor 11 is deposited is attributable to a number of process conditions. First, the wavelength of the coherent photon energy is matched to the selective photon absorption wavelength of polycrystalline silicon gate electrode 6, while being of such wavelength that field oxides 2 and 3, gate oxide 4 and monocrystalline silicon substrate 1 are substantially transparent thereto. Alternatively, the combination of the substrate temperature and localized doping can cause selective absorption of free carriers. Additionally, the wavelength of the photon energy 23 is selected to minimize absorption by reactant gases 24 through which the photon energy beam is projected. A specific set of parameters by which such process may be practiced will be introduced at a later point. The central processing feature which Fig. 1 conveys is the application of selective photon energy absorption to initiate spatially selective, localized, pyrolysis in the pursuit of selectively depositing materials created by the pyrolytic reaction. The source of the photon energy 23 for the practice of the deposition depicted in Fig. 1 is either a CW or a pulsed laser providing a blanket illumination of substrate 1. Fig. 2 of the drawings depicts a variation to the invention described with reference to Fig. 1, wherein photon energy 23 is further matched to accentuate the selective absorption characteristics of gate electrode 6. According to this embodiment, the thickness "X" of gate electrode 6 is selected to be a multiple of one-fourth the wavelength of the coherent photon energy source, the laser. The thickness of gate electrode 6 would then have an optical path length which maximized absorption for internally reflected photon energy. The temperature rise of gate - 8 - electrode 6 would thereby be maximized for the energy received. In contrast, the thicknesses of the regions which preferably transmit or reflect photon energy 23 would be selected to be multiples of one-half the photon energy wavelength, to minimize energy absorption. Again, conductive layer 11 would be selectively deposited by the temperature responsive pyrolytic reaction of gases 24 in the immediate proximity of gate electrode 6. A further variation to the fundamental concept depicted in Fig. 1 is schematically represented in Fig. 3. In this case, polycrystalline silicon gate electrode 26 is patterned to have substantially vertical sidewalls, photon energy beam 28 is collimated in a direction substantially perpendicular to plane 30 of substrate 1, and the photon energy source is of pulsed rather than continuous wave character. For ' instance, a laser operating in the ultraviolet radiation wavelength region and with a pulse duration of 10 nanoseconds or less and a repetition rate of 100-500 pulses per second or less will provide further absorption selectivity or localization, in this case limiting the pyrolytic temperature to upper surface 29 of polycrystalline silicon gate electrode 26. Under such conditions, the photon energy heating is limited to the top 100 nanometers of gate electrode 26. The pulse duration and repetition rate ensures that heat generated at the surface is sufficient to support pyrolysis, yet inadequate to materially elevate the temperature of the bulk substate. Therefore, only the surface 29 of gate electrode 26 acquires the temperature necessary to initiate pyrolytic reaction with reactant gases 24, and the selective deposition of conductor layer 31 is so limited. Another technique for providing enhanced spatial selectivity, where again the desire is to limit conductor deposition to the top surface such as depicted in Fig. 4, employs the combined use of a pulsed photon energy source with a non-uniformly doped polycrystalline silicon layer. According to one practice, patterned polycrystalline silicon layer 32 is doped, by diffusion or ion implantation, to have a significantly higher impurity concentration 33 at the upper surface than exists deeper within the gate electrode 32. The structure is then subjected to pulsed photon energy at the infrared radiation end of the spectrum. The doped surface region photon absorption characteristics at such wavelengths due to free carrier absorption are materially greater than the absorption characteristics of the undoped sidewall. If the photon energy source 34 is pulsed with a pulse duration and repetition rate suitable to minimize heat transfer to the bulk substrate 1, the temperature rise, pyrolitic reaction, and deposition will be localized to the upper surface region 33 of gate electrode 32. It will no doubt be appreciated that this technique can be combined with that described with reference to Fig. 3 to further control the deposition pattern of the conductor, such as upper surface conductor 36 on gate electrode 32 in Fig. 4. The various techniques and refinements described with reference to Figs. 1-4 can also be applied to selectively deposit conductive materials on heavily doped source/drain regions of field effect transistors, such as regions 37 and 38 depicted in Fig. 5. According to this practice of the invention, gate electrode 39 has deposited thereupon a shunting conductive layer 41 and remains electrically isolated by sidewall silicon dioxide regions 42 from similarly overlying conductive layers 43 and 44, respectively shunting source/drain regions 37 and 38. A background discussion of this field effect transistor structure appears in U.S. Patent No. 4,503,601. - 10 - A further refinement of the process to which this invention pertains leads to the structure depicted in Fig. 6. As shown in Fig. 6, field oxide region 3 has formed thereupon a composite polycrystalline silicon layer 46-47-48 configured to perform the function of a resistor. Resistive region 46 can be intrinsic polycrystalline silicon or polycrystalline silicon lightly doped in situ or by post-formation implant, and is connected at either end to heavily doped polysilicon interconnect regions 47 and 48. By applying the concepts of optical free carrier absorption to selectively heat a designated pattern and thereby initiate spatially localized pyrolysis and deposition, conductive layers 41, 43, 44, 49 and 51 are formed respectively over heavily doped polycrystalline silicon gate electrode 39, heavily doped monocrystalline silicon source/drain regions 37 and 38, and heavily doped polycrystalline silicon interconnect regions 47 and 48, while lightly doped or undoped polycrystalline silicon layer 46 remains uncovered to retain its resistive character. The practice of the particular features which characterize the present invention to create the structure depicted in Fig. 5 follows the formation of source/drain regions 37 and 38 associated sidewall silicon dioxide regions 42-42, for example in accordance with the practice set forth in U.S. Patent No. 4,503,601. Thereafter, substrate 1 is inserted in chamber 14 (Fig. 1 ) and elevated to a temperature of approximately 100-3000 C by heater 12. The chamber is evacuated to maintain a pressure in the range between 0.1 and 1 Torr by a vacuum pump connected to exhaust 17. A 1:5 to 1:200 mixture by volume of WF5 and H2 is introduced into the chamber through- gas inlet 16 while maintaining the prescribed chamber pressure. Beam 21 is then generated by to have a power of 50 watts and a beam cross section of 1 cπr at the substrate surface for a Cul laser operable at 510 nanometers (green), or at a power of 50 watts and a beam cross section of 5 cm 2 for a XeF laser operable at 358 nanometers (uv), and projected through window 18 comprised of either SUPERSIL II or MgF2» Both wavelengths are transparent to the WF5 reactant gas and provide photon energy which is selectively absorbed by silicon. Furthermore, both wavelengths of photon energy are transparent to silicon dioxide, and as such, ensure that any photons impinging on substrate 1 at locations other than the polycrystalline silicon gate electrode 39 or monocrystalline silicon source/drain regions 37 and 38 are transmitted into and absorbed by the bulk substrate. In contrast, the photons illuminating silicon regions 37, 38 and 39 elevate the local region temperatures sufficiently to initiate pyrolysis, and locally disassociate the tungsten from the reactant gas mixture to deposit it directly on the locally heated silicon. A nominal 100 nanometer thick deposition will result if the Cul or XeF lasers are used for approximately 5-10 minutes. The further selective deposition as depicted in Fig. 6 of the drawings may be practiced with CO2 or HF/DF lasers operating, respectively, at 10.6 and 3 micrometers wavelengths, 500 and 300 watts, and 10 cm 2 beam areas. In such cases, the apparatus depicted in Fig. 7 elevates the substrate to a temperature of approximately 400-6000 C. Preferably, the chamber pressure is maintained at approximately 0.2 Torr while a 1:5 to 1:200 mixture by volume of Fs and H2 reactant gas is being supplied. The substrate is illuminated for approximately 1-5 minutes with the CO2 laser and approximately 2-8 minutes with the HF/DF laser to deposit tungsten layers 41, 43, 44, 49 and 51 of approximately 100 nanometers thick over monocrystalline source/drain regions 37 and 38, and heavily doped polycrystalline silicon regions 39, 47 - 12 - and 48. Note, however, that intrinsic or lightly doped polycrystalline silicon region 46 remains uncovered. This ability to differentiate between doped and undoped silicon regions is unique to the spatially selective free carrier absorption technique of the present invention. In addition, the absorption depth of the free carriers is amenable to further variation in relation to the bias temperature of the substrate. Conventionally practiced techniques for the selective deposition of tungsten are unable to differentiate, and would, consequently deposit a shunting tungsten layer over all exposed polycrystalline or monocrystalline silicon regions. The onset of pyrolysis and selective deposition of the conductive layer, preferably refractive tungsten, inherently alters the selective photon absorption characteristics of the region upon which it forms. For tungsten deposited to form the structure depicted in Fig. 5 using a Cul or XeF laser, the tungsten layer is self-limited to approximately 100 nanometers in thickness. Nevertheless, the metallic films or layers formed by selective pyrolysis according to the present invention form nucleation centers for subsequent, more conventional chemical vapor depositions of the conductive layers to greater thicknesses. For instance, when tungsten is the refractory metal being deposited and the selective pyrolysis deposition is self-limited to approximately 100 nanometers, the nucleation center characteristics of the thin tungsten layer allows for subsequent selective CVD deposition of tungsten in substantially conventional manner to a combined thickness of approximately 1 micrometer. It should be understood that the concepts which are central to .the present invention are not limited to applications involving monocrystalline or polycrystalline silicon semiconductors, but are applicable with equal effect to structures employing amorphous silicon regions as sites for pryolitic reaction. In like manner, the concepts of the present invention are applicable to other semiconductor materials used in the fabrication of intergrated circuit devices, including but not limited to gallium arsenide, indium phosphide, aluminum gallium arsenide, and indium gallium aluminum phosphide.

Claims: CLAIMS 1. A process for selectively depositing a film of a particular material on a substrate (1), characterized by the steps of: rendering a selected region (6) of the substrate (1) to have accentuated radiation absorption characteristics at a selected wavelength; subjecting the substrate (1) to an environment containing a gaseous compound which is adapted to pyrolytically deposit a film of the material on a region of elevated temperature; exposing the substrate (1) to radiation at the wavelength for which the radiation absorption characteristics of the selected region (6) are accentuated; and illuminating the substrate (1) with the radiation until the selected region (6) is sufficiently elevated in temperature to cause a localized pyrolytic reaction of the gaseous compound. 2. A processing according to claim 1, characterized in that the radiation wavelength and the gaseous compound are selected to minimize direct photolytic reaction. 3. A process according to claim 1, characterized in that the optical path length thickness of the selected region (6, Fig. 2) of the substrate (1) is approximately a multiple of one- fourth of the radiation wavelength. 4. A process according to claim 2, characterized in that the radiation is collimated, aligned to be substantially perpendicular to the plane of the illuminated surface of the substrate (1), and pulsed (Figs. 3, 4). 5. A process according to claim 4, characterized that the wavelength of the radiation is short in relation to the thickness of the selected region (26) of the substrate (1). 6. A process according to claim 4, characterized in that the pulse duration and repetition rate of the radiation are matched to further localize the heating for pyrolytic reaction to the illuminated surface of the selected region (26, Fig. 3). 7. A process according to claim 4, characterized in that illuminated surface of the selected region (33, 37, 38, 39, 47, 48) is heavily doped to have a further accentuated radiation absorption characteristic via optical free carrier absorption. 8. A process according to claim 1, characterized by the further step of: subjecting the substrate (1) to a chemical vapor (CV) deposition in which the material deposited by localized pyrolytic reaction serves as a nucleation region for the CV deposition. 9. A process according to claim 2, characterized in that the radiation is comprised of coherent photons generated by a laser. 10. A process according to claim 9, characterized in that the. substrate temperature is adjusted to further accentuate the photon energy absorption characteristics via optical free carrier absorption. 11. A process according to claim 1, characterized in that the selected regions (6) having accentuated photon energy absorption characteristics include polycrystalline or amorphous silicon. 12. A process according to claim 11, characterized in that the deposited material is a refractory metal. 13. A process according to claim 12, characterized in that said refractory metal is tungsten. 14. A process according to claim 13, characterized in that said gaseous compound is tungsten hexafluoride.

Inventors: COLLINS GEORGE JOSEPH, METZ WERNER ADAM JR, COLLINS, GEORGE, JOSEPH, METZ, WERNER, ADAM, JR.
Applicants: NCR CO, NCR CORPORATION
Application year: 1986
Main classification: H01L21
IPC codes: C23C16, H01L21, H01L23, H01L29