Method of fabricating a bipolar transistor

Abstract: A high performance bipolar transistor and a method of fabrication. Base resistance is reduced by a self-aligned silicide formed in the single-crystal region of the extrinsic base, thereby eliminating the polysilicon to single-crystal contact resistance as well as shunting the resistance of the single-crystal extrinsic base region. Oxide from the sidewall of the polysilicon local interconnection is selectively removed prior to silicide formation. Therefore, selected sidewalls of the poly interconnect layer also becomes silicided. This results in significant reductions in resistance of the interconnection, particularly for sub-micron geometries. Improved techniques for forming field oxide regions and for forming base regions of bipolar transistors are also disclosed.

Description: The present invention relates to the field of semiconductor devices and their manufacture. More specifically, the present invention provides a method of fabricating a high performance bipolar transistor. Bipolar semiconductor devices and their methods of manufacture are well known. Such devices are described in, for example, U.S. Patent No. 4,609,568 (Koh et al.) and U.S. Patent No. 4,764,480 (Vora). Certain problems have arisen in the manufacture of semiconductor devices according to the prior art. however. For example, field oxide encroachment increases device area and, therefore, parasitic capacitance and resistance of devices. Further, previous single polysilicon bipolar devices have had high extrinsic base resistance and large collector-base capacitance. Still further, previous bipolar devices have silicide only on the upper portion of a polysilicon region used for a local interconnect. This results in particularly unacceptable resistance when polysilicon line widths are decreased. Still further, thermal cycles in existing processes create relatively deep junctions which limit the speed of devices. Still further, the packing density of previous bipolar devices has been limited by the area required for interconnect. Accordingly it is seen that improved methods for the manufacture of semiconductor devices are desired. US-A-4,824,794 discloses a method of fabricating a self-aligned bipolar transistor. An improved isolation process may provide reduced oxide encroachment which increases density, reduces capacitance, and increases reliability. Base resistance may be reduced by a self-aligned silicide formed in the single-crystal region of the extrinsic base, thereby eliminating the polysilicon to single-crystal contact resistance as well as shunting the resistance of the single-crystal extrinsic base region. This also allows the polysilicon overlap onto the extrinsic base to be reduced, resulting in a reduction of extrinsic base area and, therefore, a reduction in base to collector capacitance. A method of fabricating a bipolar transistor having emitter, base, and collector regions is disclosed. The method according to the invention is defined in claim 1. A further understanding of the nature and advantages of the inventions herein may be realized by reference to the remaining portions of the specification and the attached drawings. Figs 1a and 1 b are cross-sections of a bipolar device manufactured according to two embodiments of the invention; Figs. 2a to 2t illustrate fabrication of the device shown in Figs. 1a and 1b; and Fig. 3 is a gummel plot for devices fabricated according to one aspect of the invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS CONTENTS I. General Description II. Description of Fabrication III. Performance Characteristics I. General Description Fig. 1a is a cross-section of a bipolar transistor 2 and a resistor 4 manufactured according to a first embodiment of the invention. While the method according to the invention is illustrated primarily with regard to an n-p-n device, it will be readily recognized by those of skill in the art that the role of n- and p-type conductivity regions could readily be reversed without departing from the scope of the invention herein. The bipolar transistor 4 includes an n+ buried layer 6 in a p- substrate 8. The buried layer 6 serves as a low resistivity path for mobile charge carriers in the collector region of the bipolar transistor structure. In preferred embodiments the peak dopant concentration of the buried layer 6 is between about 10 17 and 10 20 /cm 3 , although dopant concentrations between about 10 18 and 10 19 /cm 3 are preferred. The resistivity of the substrate is preferably between about 30 and 50 ohm cm. Adjacent the bipolar transistor, field oxide regions 10a and 10b are underlain by p+ channel stops 12a and 12b. The field oxide regions and channel stops serve to isolate the transistor from adjacent transistors and at the same time create a very high threshold voltage for the parasitic field effect structure formed by the substrate, the field oxide region and any overlying interconnect regions. In preferred embodiments, the peak dopant concentration in the channel stops is between about 10 17 and 10 18 /cm 3 . Oxide region 10c separates collector sink 14 from the active region of the transistor. The collector region of the transistor is comprised of not only the buried layer, but also the collector sink region 14, which has a dopant concentration of the same order of magnitude as the buried layer. The sink extends to and is in contact with a heavily doped n+ collector contact 16 which acts as an interconnect to other devices on the structure. Buried layer 6 contacts a base region 18 of the bipolar transistor via a more lightly doped n-type collector region 20 which preferably has a peak dopant concentration of between about 1x10 16 and 1x10 17 /cm 3 . The base region includes a shallow, lightly doped region 18a which is substantially self-aligned with an n+ emitter contact 22. The lightly doped region of the base is especially shallow and extends only about 180 to 220 nm (1,800 to 2,200 Å) from the emitter contact. Adjacent the lightly doped region of the base, moderately doped regions 18b (about 10 18 /cm 3 ) extend under the sidewall oxide regions 28. Adjacent the moderately doped regions, more heavily doped regions 18c extend from under the sidewall oxide and are self-aligned with p+ polycrystalline silicon (polysilicon) base contacts 24a and 24b. The more heavily doped regions 18c are preferably doped to a concentration of between about 5x10 18 and 5x10 19 with a preferred range of from 8x10 18 to 2x10 19 . Regions 18b and 18c preferably extend between about 0.20 and 0.25 µm into the substrate from the bottom of the emitter. Regions 18d are diffused from the overlying p+ base contacts. Regions 18b, 18c, and 18d collectively form the extrinsic base region of the transistor. Refractory metal silicide contacts 26a, 26b, 26c, and 26d, such as titanium silicide contacts, are used for the base contacts, the emitter contact, and the collector contact, respectively. The base contact silicide 26a/26c extends along the upper portion of the base contact poly 24a/24b, down its sidewall, and along the upper portion of the single-crystal base region 18 to sidewall oxide 28 on the emitter contact. The silicide contact 26b on the emitter contact extends along the upper surface of the emitter contact 22 from one sidewall to the opposite sidewall. The collector contact 16 is covered along its upper surface and sidewall with silicide region 26d. Silicide contacts as shown herein provide significantly reduced resistance to the polycrystalline and monocrystalline features such as the emitter, base, and collector contacts, as well as to the base region itself. The resistor 4, base contact 24a, emitter contact 22, base contact 24b, and the collector contact 16 are all made from a single layer of polysilicon. The resistor 4 and other selected regions include silicide exclusion oxide 29 and the entire surface of the device is provided with cap oxide layer 30. Contact metal layer 32 extends along the cap oxide layer at selected locations for interconnection of the device. In preferred embodiments, tungsten plugs 31 fill the apertures in the cap oxide. Fig. 1b illustrates an alternative embodiment for forming the interconnect. According to this embodiment, the contact metal includes a layer of AlSi 34 on a layer of TiW 36. The contact layer extends through the apertures in the oxide layer 30, with TiW 36 contacting selected silicide regions 26. II. Description of Fabrication Figs. 2a to 2t illustrate fabrication of a bipolar transistor according to one embodiment of the invention. As shown in Fig. 2a, a layer of initial thermal oxide is formed on a p-type substrate. In preferred embodiments the p- substrate has a resistivity of between about 30 and 50 ohm-cm. A denuded zone (depleted of oxygen contaminants and the like) 10 µm to 20 µm into the substrate and thin screen oxide region 40 are then formed simultaneously using conventional thermal processing. A mask 38 is then formed on the substrate to define the buried layer 6 of the transistor. It should be noted that by "mask" it is intended to mean a material formed on a substrate for protection of selected regions (as opposed to the lithographic element used to form the mask on the substrate). An n+ implant using arsenic forms the buried layer of the bipolar transistor and the mask 38 is then removed. In preferred embodiments the buried layer is doped to a concentration of between about 10 19 and 10 20 /cm 3 with a dopant concentration of about 5x10 19 /cm 3 preferred. The buried layer implant uses an implant energy of between about 75 and 150 keV with an implant energy of between about 75 and 100 keV preferred. Referring to Fig. 2b, the buried layer is thereafter annealed at a temperature of about 1100°C for a time of about 80 minutes and another layer of oxide 42 is formed on the surface of the device. Due to differential oxidation rates of n+ and p- silicon, the oxide thickness grown over the buried layer is about two times the thickness over the remaining p- portions of the surface. This creates a step in the silicon which permits alignment of subsequent layers. As shown in Fig. 2c, a mask 44 is then formed on the device so as to define the channel stops of the device and an implant is performed to provide p+ channel stops 12a and 12b, preferably using boron. In preferred embodiments the p+ channel stops have a dopant concentration of between about 10 16 and 10 19 /cm 3 with a dopant concentration of about 10 17 /cm 3 preferred. The p+ implant used to form the channel stops uses an implant energy of between about 100 and 200 keV with an implant energy of between about 125 and 175 keV preferred. In Fig. 2d the mask 44 and the oxide layer 42 on the substrate are removed. An n-type epitaxial silicon layer 46 having a dopant concentration of about 5x10 15 to about 5x10 16 /cm 3 is grown on the device and an oxide layer 48 is formed thereon, also diffusing the n+ and p+ dopants into the epitaxial layer from the underlying substrate. In preferred embodiments, the n-epi layer has a dopant concentration of between about 1x10 16 and 3x10 16 /cm 3 . A layer of nitride 50 is deposited on the oxide layer 48, preferably having a thickness of about 150 nm (1500 Å). In preferred embodiments the epitaxial silicon layer has a thickness of between about 0.8 and 1.5 µm with a thickness of between about 1.0 and 1.2 µm preferred. Referring to Fig. 2e, an active area mask 49 is formed on the surface of the device which protects regions of the device that will ultimately form the base and collector sink of the transistor. An etch of the exposed nitride, oxide, and silicon is then performed to provide the structure shown in Fig. 2e. The silicon is preferably etched about 0.3 µm below its original surface. After removal of the mask, a grown oxide/ nitride/deposited oxide sandwich is then formed on the device. Referring to Fig. 2f, the thermally grown oxide 51 is preferably about 40 nm (400 Å) thick, while the deposited nitride 53 is preferably about 60 nm (600 Å) thick, and the deposited oxide 55 is originally 180 nm (1800 Å) thick. An unmasked etch of this oxide/nitride/oxide sandwich leaves the oxide/nitride/oxide sandwich on the sidewall of the exposed silicon etch and the first nitride/oxide over the previously masked regions. The silicon is preferably etched about 75 nm (750 Å) below its previous level with a plasma etch. Fig. 2f shows the resulting structure. The remaining oxide spacer 55 is then removed using BOE, providing the structure shown in Fig. 2g. Referring to Fig. 2h, a field oxidation is then performed by high pressure (e.g., 10 atmospheres) steam oxidation. The top of the field oxide is grown to be substantially planar with the top of the base/collector sink. The field oxidation preferably uses a temperature of about 1000°C such that about 1 micron of oxide is formed or until the field oxide is level with the surface of the substrate. Remaining exposed nitride is then stripped from the device. Referring to Fig. 2i, a screen oxide 52 is then formed on the base and collector regions of the bipolar transistor. The device is masked with photoresist to expose only the sink region 14 of the device and an n+ implant is performed with phosphorus so as to dope the sink region to a concentration of between about 1x10 19 and 1x10 21 /cm 3 using an implant energy of between about 100 to 200 keV. The sink mask is then removed. In an alternative embodiment, an additional mask is formed so as to expose only selected emitter regions. An additional n+ implant is performed to raise the epitaxial layer doping under the emitter contact to between about 10 16 to 10 17 /cm 3 . This increases the performance of the transistors when operated at high current densities. Further, this implant permits formation of transistors optimized for low current and high current operation on the same wafer. In Fig. 2j, an intrinsic polysilicon layer 54 is formed on the surface of the device. An unmasked p implant is performed using boron across the entire surface of the device. The p-type material is annealed in an oxygen environment so as to form oxide 56 and p-type material is diffused from the polysilicon into the single-crystal base region such that the average dopant concentration in the base region 18 is between about 1x10 17 and 2x10 18 /cm 3 with an average dopant concentration of about 1x10 18 /cm 3 preferred. The base implant uses an implant energy of between about 20 and 60 keV with an energy of between about 30 and 50 keV preferred. In preferred embodiments, this diffusion step results in a base thickness below the bipolar emitter of between about 0.15 and 0.25 µm with a thickness of about 0.20 µm preferred. This implant is also used to form lightly doped regions of polysilicon to act as, for example, resistors. As shown in Fig. 2k, a mask is then formed on the device so as to define base contact regions 24a and 24b, and contact portions of the resistor 4. A p+ implant is performed using boron to dope these regions to a concentration of about 1x10 20 /cm 3 . The p+ mask is removed and an n+ mask and implant is used to form the emitter contact region 22 and the collector contact region 16. The n+ mask is then removed and a nitride layer 57 is deposited. The n+ and p+ implants preferably use an implant energy of between about 40 and 100 keV. The device is then masked so as to protect the p+ resistor/base contact 24a, the base contact 24b, the emitter contact 22, and the collector contact 16. The exposed poly is etched to provide the poly structure shown in Fig. 21. The resist is then removed and the exposed silicon areas are oxidized to provide an oxide layer 59 having a thickness of about 15 nm (150 Å). This thermal cycle also serves to diffuse the n+/p+ dopants from the poly emitter/base contacts and form emitter region 23. The poly overetch is controlled to be slightly greater than the emitter diffusion depth in order to reduce emitter-base capacitance. In preferred embodiments, the emitter 23 extends about 100 nm (1000 Å) into the epi layer while the silicon is over etched about 120 nm (1200 Å) from the surface of the epi layer. A p-type implant is then performed using BF 2 to form a moderately doped link base region which is self-aligned to the emitter contact and extends from the emitter contact to the base contact. The nitride over the polysilicon serves to mask this implant. The doping level of the exposed base region is adjusted to minimize base resistance and improve the emitter-base breakdown characteristics. In preferred embodiments, the region between the emitter and base contacts is doped to an average dopant concentration of between about 1x10 17 and 1x10 19 /cm 3 with a dopant concentration of about 5x10 18 /cm 3 preferred. This base implant uses an implant energy of between about 30 and 50 keV with an implant energy of about 40 keV preferred. The base region 18a below the emitter remains very shallow (on the order of about 0.15 to 0.25 µm in preferred embodiments) while the link base 18c is thicker (on the order of 0.25 to 0.30 µm in preferred embodiments). Referring to Fig. 2m, an extrinsic base cap oxidation is performed to anneal implant damage. The nitride over the polysilicon is then removed by a selective etch. A layer of oxide 60 having a thickness of about 0.4 µm is deposited on the surface of the device and a mask 61 is formed to protect areas of the oxide where it is desired not to ultimately produce a silicide interconnect (e.g., over the center portion of the resistor) and a spacer etch is performed using means well known to those of skill in the art to leave oxide sidewalls on the base contacts, the emitter, and the collector contact. The mask 61 is then removed. The oxide is also masked so as to leave oxide on regions of the wafer (not shown) which are not covered by polysilicon (except for the active transistor areas). This increases the ultimate oxide thickness between metal layers and the substrate reducing device capacitance, as well as producing an approximately planar surface for the formation of subsequent levels of oxide/interconnect. In Fig. 2n, a resist mask 62 is formed on the device which protects the oxide silicide exclusion regions on the resistors and the like, as well as the oxide on the sidewall of the emitter regions of the bipolar transistors. A spacer removal etch is then performed so as to remove oxide from the sidewalls of the base contacts 24 and the collector contact 16, leaving oxide sidewalls on the emitter contact 22, but not on the sidewalls of the base contacts and collector contact. It should be recognized that the inventions herein have been illustrated primarily with regard to the use of a mask to selectively remove oxide from the sidewalls of the poly contact layers, but selective growth and etching of p+ and n+ contacts may also be used. The mask shown in Fig. 2n is then removed and an additional blanket p+ implant is performed as shown in Fig. 20. The implant is masked from the resistors by the silicide exclusion oxide and serves to increase the concentration of the resistor ends and in the extrinsic base region, thereby reducing the ultimate silicide to silicon/polysilicon contact resistance. The implant uses BF 2 at about 40 keV and results in an exposed surface dopant concentration of about 1x10 19 to 1x10 20 /cm 3 and a junction depth of about 0.25 to 0.30 µm. A final anneal is then performed for about 20 seconds at about 1,050°C. The use of a rapid thermal anneal improves dopant activation and minimizes encroachment of the extrinsic base under the emitter. In Fig. 2p, a silicide preclean step is performed to remove oxide from regions which are to be silicided. A layer of refractory metal 64 such as titanium is thereafter deposited on the surface of the device. As shown in Fig. 2q, a first silicide reaction step is then carried out in which TiSi is formed over exposed silicon/polysilicon. Excess titanium is then stripped from the device and a second silicide reaction step at a higher temperature (e.g., 800°C) is carried out in which the silicide sheet resistance is reduced. This leaves silicide straps 26a and 26c on the upper portion and sidewall of the base contacts 24 which further extend along the surface of the extrinsic single-crystal base region up to the sidewall oxide of the emitter. Silicide 26b is left on the upper surface of the emitter contact 22 extending from one sidewall to the opposite sidewall. Similarly, silicide strap 26d is formed on the sidewall and the upper surface of the collector contact. Fig. 2r illustrates the formation of a contact oxide layer 30. The oxide layer is formed by deposition of a CVD oxide, and SOG (Spin On Glass) spincoat using techniques well known as in, for example, U.S. Patent No. 4,806,504. Thereafter, a planarization etchback is performed to planarize the surface of the device. A chemical vapor deposition cap oxide is then formed and a contact mask is formed so as to define contact regions of the device. Fig. 2s illustrates the contact scheme shown in Fig. 1a, along with its method of formation. In Fig. 2s, tungsten is deposited and etched back by plasma etching to form tungsten plugs 65. This permits contacts/vias with aspect ratios (height-to-width) of greater than about 2 and hence reduces the required contact size and permits the use of thicker intermetal oxide for reduced capacitance. The overall planarity of the surface is improved, and metal step coverage issues are eliminated. Thereafter, a layer of AlSi (aluminum silicon) 26 is deposited and etched to form the first layer contact metal. Fig. 2t illustrates formation of the contact scheme shown in Fig. lb. According to this embodiment, a layer of TiW 34 is formed on the device, followed by a layer of AlSi (aluminum silicon) 36. This two-layer metal interconnect provides a low sheet resistance interconnect. A metal mask is formed on the device to remove shorts between regions of the metal contacts and a metal etch is performed. Multiple metal interconnection layers (e.g., five levels of metallization) are, thereafter, formed by sequential deposition of oxide, formation of contact holes, deposition of metal, and metal etching. III. Performance Characteristics Devices exemplary of the above-described inventive method were fabricated. The devices had field oxide regions with reduced bird's head and a minimum active area dimension of 1.6 µm for a 1.0 µm-thick field oxide. The co-diffused emitter-base profile provided a narrow [70 nm (700 Å)] neutral base width with excellent uniformity across a 6-inch wafer with negligible I ceo leakage. The extrinsic base region had a lateral profile which was self-aligned to the emitter poly, minimizing the extrinsic base encroachment under the emitter. Spacer oxide from the base poly was selectively removed in order to provide direct connection to the extrinsic base region by the silicide. This allows for reduction of the extrinsic base area as well as lower base resistance. The product of R b *C jc was reduced approximately 25% resulting in a 13% reduction in gate delay. High tolerance on poly resistors was maintained with reduced resistor size by minimizing encroachment and end-effect variation. Three poly resistors combined with diffused resistors of similar values provided simplified circuit design and layout. The metal capacitance of 4 fF/mil translates to 1.0 pS/mil at 600 µA switching current and 2.5 pS/mil at 200 µA for metal 1, metal 2, and metal 3. The process, with three values of poly resistors, requires 17 masks to metal 4. Table 1 illustrates the overall characteristics of the fabricated devices. Fig. 3 shows Gummel plots for devices with a 0.8x2.5 µm 2 emitter. Near ideal device characteristics and a beta of approximately 100 are obtained. NPN Transistor Parameters Emitter Area 0.8 µm x 1.6 µm Device Area 1.6 µm x 6.1 µm Base Contacts 1 Current Gain 90 E-B Breakdown 6 V C-B Breakdown 16 V C-E Breakdown 8 V E-B Capacitance 8.7 fF C-B Capacitance 2.9 fF C-S Capacitance 10.5 fF Emitter Resistance 86 ohms ECL Ring Oscillator <50 pS @ 1 mA/gate Speed Power Product at Low Currents <50 fJ Gate Delay 8.3 nS @ 2 watts A 16-bit multiplier which was originally designed using the "ASPECT-I" process (2 µm feature size) and which has approximately 6000 equivalent gates, was used as a point of comparison to demonstrate the scalability of design using the inventions herein. The layout was shrunk by software manipulation without the need for additional custom layout. The final layout area is 40% of the original. A multiply delay of 8.3 nS at 2 watts was obtained using the inventions herein. This was an improvement of 25% in speed and 50% in power as compared to the original layout, consistent with the scaling of the power/delay product. It is to be understood that the above description is intended to be illustrative and not restrictive. Many variations of the invention will become apparent to those of skill in the art upon review of this disclosure.

Claims: A method of fabricating a bipolar transistor having emitter, base, and collector regions comprising the steps of: a) on a silicon substrate having a first surface, implanting dopants of a first conductivity type in a first region (6), said first region forming a buried portion of said collector, said substrate having a second conductivity type; b) forming an epitaxial silicon layer (46) on said first surface, said epitaxial layer doped to said first conductivity type; c) forming a first oxide layer (48) on said epitaxial layer; d) forming a first nitride layer (50) on said first oxide layer; e) masking (49) said first nitride layer above said buried portion to protect regions that will ultimately form said base region and a collector sink region; f) etching said oxide, nitride and epitaxial layers in regions not protected by said mask to form recesses in said epitaxial layer having substantially horizontal surfaces and sidewalls; g) removing said mask on said first nitride layer; h) forming a second oxide layer (51) on the resulting surface, i) forming a second nitride layer (53) on said second oxide layer and a third oxide layer (55) on said second nitride layer; j) without a mask, etching to remove said second and third oxide and said second nitride layers from all horizontal surfaces but not from said sidewalls, said base and collector sink regions remaining covered by said first oxide and nitride layers; k) oxidizing the exposed horizontal surfaces to form field oxide regions, and removing said first oxide and nitride layers from said base and collector sink regions; l) heavily doping said collector sink (14) with dopants of said first conductivity type; m) forming a polysilicon layer (54) on said substrate; n) implanting dopants of a second conductivity type into said polysilicon layer; o) diffusing said dopants of said second conductivity type into said base region of said epitaxial layer; p) implanting dopants of said first conductivity type into at least a portion of said polysilicon region over said base region (18) to form an emitter contact region of said first conductivity type; q) etching said polysilicon region to form said emitter contact (22), base contacts (24a, 24b), and a collector sink contact (16); r) implanting dopants of said second conductivity type so as to increase a dopant concentration in said base region between said emitter and said base contacts; s) forming oxide (60) on sidewalls of said emitter contact; t) forming metal silicide on exposed regions of silicon along an upper surface of said device, said metal silicide (26a, 26c) extending along an upper surface of said base region up to said oxide sidewalls; u) forming a fourth oxide layer (30) having contact apertures to said contacts on said device; v) filling said apertures with tungsten (65), or forming a layer (34) of TiW on said device extending through said apertures; and w) forming an aluminium silicon contact layer (26) on said fourth oxide layer. The method as recited in claim 1 wherein said step (k) of oxidizing is carried out for a time sufficient to form a surface of said field oxide regions substantially coplanar with said first surface. The method as recited in claim 1 wherein the step (n) of doping said polysilicon layer is a step of doping substantially all of said polysilicon with said dopant of said second conductivity type and wherein said step (p) of forming an emitter region compensates said polysilicon to said first conductivity type. The method as recited in claim 1 further comprising the step of heavily doping a second portion of said polysilicon region to said second conductivity type, said second portion forming said base contact of said bipolar transistor. The method as recited in claim 1 wherein said step (n) of doping said polysilicon simultaneously forms lightly doped regions of resistors. The method as recited in claim 3 wherein said steps of doping said polysilicon with a dopant of a second conductivity type provides a net dopant concentration in said base region of between about 1x10 17 /cm 3 and 2x10 18 /cm 3 . The method as recited in claim 1 wherein a base thickness below the emitter of said bipolar transistor is between about 0.15 and 0.25 µm.

Inventors: BASTAMI BAMDAD, BISWAL MADAN, BLAIR CHRISTOPHER S, BOUKNIGHT JAMES L, DAVIES TAD, DELONG BANCHERD, GANSCHOW GEORGE E, ILDEREM VIDA, IRANMANESH ALI, JEROME RICK C, LAHRI RAJEEVA, LEIBIGER STEVE M, SOLHEIM ALAN G, BASTAMI, BAMDAD, BISWAL, MADAN, BLAIR, CHRISTOPHER S., BOUKNIGHT, JAMES L., DAVIES, TAD, DELONG, BANCHERD, GANSCHOW, GEORGE E., ILDEREM, VIDA, IRANMANESH, ALI, JEROME, RICK C., LAHRI, RAJEEVA, LEIBIGER, STEVE M., SOLHEIM, ALAN G.
Applicants: NAT SEMICONDUCTOR CORP, NATIONAL SEMICONDUCTOR CORPORATION
Application year: 1991
Main classification: H01L21
IPC codes: H01L21, H01L29