Method of Fabricating Up Diffused Substrate FED Logic Utilizing a Two-Step Epitaxial Deposition

The present invention relates generally to integrated injection logic (I.sup.2 L) and more specifically to an improved substrate fed logic (SFL) semiconductor structure.

A typical example of a substrate fed logic semiconductor structure of the prior art is illustrated in FIG. 3b of the U.S. Pat. No. 3,916,218. The structure consists of four layers alternating, namely P1, N1, P2 and N2. The four layers form the complementary, vertical, inversely operated bipolar transistors. This structure has technical drawbacks. For example, by using an epitaxial layer P2 as the base of the NPN transistor, the base width will vary with epitaxial thickness and taper control. This is undesirable in a neutron radiation hardened circuit. Neutron radiation introduces recombination sites in the base and the gain becomes limited by base transport factor vs pre-radiation gain domination by emitter efficiency. Since the base transport factor is strongly related to the base width, the post neutron gains will vary with the epitaxial thickness control. This unpredictable variation in post neutron gain makes the semiconductor structure of U.S. Pat. No. 3,916,218 undesirable for a radiation hardened design. Similarly, P2 has a constant doping profile and hence does not have either an aiding or hindering electric field affecting upwards gain. Similarly, by the use of an epitaxial P2 layer, the P and N.sup.+ diffused regions must be diffused down to and through region P2, respectively. This increases the total diffusion time and, consequently, produces undesirable epitaxial junction movement, which is a function of the square root of the product of diffusion time and temperature.

To increase the inverse gain and f.sub.T of integrated injection logic structures, the prior art has suggested upward diffusing the base of the NPN vertical transistor. The updiffused base structure is discussed in "Integrated Injection Logic" in research disclosure No. 48, Page 67 by Industrial Opportunities Limited of Homewell, England. The structure is formed by outward diffusion of boron through a top epitaxial layer to the top of the said epitaxial layer. The resulting structural has an NPN base width which varies as the top epitaxial thickness varies. This produces the same undesirable results discussed in for U.S. Pat. No. 3,916,218.

The I.sup.2 L structures of the prior art generally are not radiation hardened to three simultaneous environments, namely total gamma dose hardness, neutron hardness, and dose rate hardness. Thus there exist a need for an I.sup.2 L structure which is radiation hardened to three simultaneous environments.

The present invention provides an I.sup.2 L structure having a vertical PNP injector which improves total gamma dose hardness over lateral PNP structures, a thin updiffused intrinsic NPN base region which provides neutron hardness and a P.sup.+ type extrinsic NPN base to provide dose rate hardness. The vertical PNP structure is not subject to total gamma dose induced gain deterioration as lateral PNPs used in most I.sup.2 L structures as base transport is not dominated by surface recombination. The current gain deterioration caused by neutron radiation is minimized in the structure of the present invention by using a thin updiffused base layer. The updiffused layer has a concentration gradient producing an electric field which aids minority carrier transport across the base. Higher neutron levels are then required to bring the base minority carrier diffusion length down to the order of the base width. Since down diffusion of the P layer into the first epitaxial layer also occurs, the impurity level of the first layer is selected higher than that of the second epitaxial layer. The down diffusing P type impurities are then compensated out and the net aiding electric field is obtained. Dose rate hardness is examined relative to when the NPN transistor is in the off logic state. Photocurrent collected by the NPN base region must flow laterally through the base and out of the NPN base contact. The use of a P.sup.+ extrinsic base contact minimizes resultant voltage drops which, therefore, require higher radiation dose rate or ionizing lever before a V.sub.be drop is built up in the base turning the NPN on than if a P.sup.- extrinsic base is used.

The substrate fed logic semiconductor structure of the present invention includes a P.sup.- substrate having a first N type epitaxial layer thereon and a second N.sup.- epitaxial layer on the first N type epitaxial layer. An upward diffused P type buried region lies between the N and N.sup.- epitaxial layers. A diffused P.sup.+ base ring makes contact with the buried, upward diffused P region and a diffused N.sup.+ guard ring makes contact with the first N epitaxial layer. The first N type epitaxial layer has impurity concentration of approximately two orders of magnitude greater than the impurity concentration of the second N.sup.- epitaxial layer. The upward diffused P type region has an impurity concentration substantially equal to that of the second N.sup.- epitaxial layer at their junction. The up diffused P type region has an impurity concentration increasing towards its junction with the first N epitaxial layer. The P.sup.+ substrate, the first N type epitaxial layer and the upward diffused P type buried region form the emitter, base and collector respectively of a vertical inversely operated PNP transistor and the first N type epitaxial layer, the upward diffused buried P type region and the second N.sup.- type epitaxial form the emitter, base and collector respectively of a vertical, inversely operated NPN transistor.

The method of fabricating the substrate fed logic semiconductor structure of the present invention includes epitaxial forming a first N type conductivity type layer of a P.sup.+ type substrate followed by depositing P type impurities on the surface of the first type epitaxial layer either selectively or nonselectively. A second N.sup.31 conductivity epitaxial layer is then grown having a lower impurity concentration than the first N type epitaxially grown layer. Depositing P.sup.+ type impurities in the top surface of a second epitaxial layer to define the base ring and depositing N.sup.+ conductivity type impurities in the top surface of the second epitaxial layer to define a guard ring. Simultaneously diffusing the buried P type impurities, the surface P.sup.+ impurities and the surface N.sup.+ impurities to form a base ring extending from the surface of the first epitaxial layer down to the upward diffused buried P region and an N.sup.+ ring extending down from the surface of the second epitaxial layer down to the first epitaxial layer. An insulating layer is formed and openings provided therein for contacts to the second epitaxial layer, the P.sup.+ base ring and the N.sup.+ guard ring. Separate contact is made to the P.sup.+ substrate.

An object of the present invention is to provide an I.sup.2 L semiconductor structure which is radiation hard to gamma dose, neutrons and dose rate.

Another object of the present invention is to provide an SFL semiconductor structure including complementary, vertical, inversely operated bipolar transistors manufactured with reduced diffusion time.

A further object of the invention is to provide a method of fabricating I.sup.2 L and SFL structures which over comes the problems of the prior art.

Still another object of the invention is to provide a method of fabrication of I.sup.2 L of SFL which produces a device which is radiation hardened to total gamma dose, neutron and radiation rate.

Other objects, advantages, and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.

FIGS. 1-4 are cross-sectional views of a semiconductor structure illustrating the preferred method of fabricating the semiconductor structure incorporating the principles of the present invention using a selectively formed buried P region.

FIGS. 5-8 are cross-sectional views illustrating a preferred method of fabricating the semiconductor structure of the present invention using a nonselectively formed buried P type region.

FIG. 9 is a schematic representation of the semiconductor structures of FIGS. 4 and 8.

FIG. 10 is a graphic representation of the relative impurity concentrations of selective regions of FIGS. 4 and 8.

The substrate fed logic semiconductor structure, incorporating the principles of the present invention, are illustrated in FIGS. 4 and 8 including vertical, inversely operated PNP and NPN transistors. PNP transistor includes an emitter formed from the substrates 12, a base formed from epitaxial layer 14 and a collector formed from upwardly diffused, selectively formed region 16 or nonselectively formed region 18. NPN transistor includes an emitter formed from epitaxial layer 14, a base from upwardly diffused layers 16 or 18 and a collector formed from epitaxial layer 20. Contact point A is made to substrate 12, contact B is made to the N.sup.+ guard ring 24 which contacts the epitaxial layer 14, contact region C makes an ohmic contact with epitaxial layer 20 through an N.sup.+ surface region 26, contact D makes a Schottky contact with epitaxial layer 20 and contact E makes a contact with the buried region 16 or 18 through P.sup.+ ring 22. The schematic is illustrated in FIG. 9.

The method of fabricating the structures of FIGS. 4 and 8 begin with epitaxially forming N type layer 14 on P.sup.+ substrate 12. The substrate 12 may be silicon doped with boron, have an impurity concentration of, for example, 1.times.10.sup..multidot. atoms per cubic centimeter and a bulk resistivity of 0.01 ohm-centimeter. The first epitaxial layer 14 may be grown to a thickness, for example, 3 microns, having an impurity concentration of arsenic or antimony of approximately 10.sup.17 atoms per cubic centimeter and a nominal bulk resistance of 0.1 ohm centimeters. P type impurities are introduced into the top surface of the first epitaxial layer 14 to form selectively regions 16 of FIG. 1 or nonselective layer 18 of FIG. 5. Impurities, for example, boron may be deposited by ion implantation or by deposition having an initial concentration of 3.times.10.sup.12 atoms per square centimeter, for example. The selective depositing of impurities is through a mask on layer 14 having an opening therein. The mask may be a layer of silicon dioxide having an opening formed by etching through a developed layer of photoresist.

A second epitaxial layer 20 is then grown on the first epitaxial layer 14 to a thickness of approximately 2 microns being doped with arsenic or antimony to a impurity concentration of approximately 3.times.10.sup.15 atoms per cubic centimeter and having a nominal bulk resistivity of 1 ohm centimeter, for example. P.sup.+ impurities are then selectively introduced into the top surface of second epitaxial layer 20 to define base region 22. Boron, for example, may be introduced by deposition or ion implantation through a mask as previously described. The structure of at this stage is illustrated in FIGS. 2 and 6.

N.sup.+ type impurities are then introduced to define guard ring regions 24. These impurities, for example, phosphorus, may be introduced by deposition or ion implantation through a mask as previously described. The resulting structure shown in FIGS. 3 and 7 is subject to a diffusion process so as to simultaneously updiffuse the P type impurities of buried layers 16 or 18 and downwardly diffuse base ring 22 and guard ring 24. The diffusion is carried out until the base rings 22 diffuses down to the upward diffusing buried layer 16 or 18. Similarly, the guard ring 24 is diffused down to the first epitaxial layer 14. In the nonselective formation of buried layer 18 of FIGS. 5-8, it is critical that the guard ring 24 is diffused down from through layer 18 and into the first epitaxial layer 14. In the selectively formed region 16 of FIGS. 1-4, the guard ring 24 need not be diffused down to layer 14 and may stop within the second epitaxial layer 20 short of the boundary between the epitaxial layers 20 and 14. Guard ring 24 should at least be diffused to a depth slightly below the top surface of the upward diffusing region 16. In the resulting structures of FIGS. 4 and 8, the base ring 24 has depth of approximately 2 microns and a sheet resistance less than approximately 60 ohms per square and the guard ring 24 would have a depth of approximately 3 microns and a sheet resistance less than approximately 5 ohms per square. A short diffusion cycle at 1100.degree. C. is sufficient for this drive-in.

If an ohmic contact is to be made to the epitaxial layer 20, N.sup.+ impurities must be introduced into the surface as at 26, for example. Arsenic or antimony may be introduced by ion implantation or deposition through a mask to a shallow depth to form a region, having, for example a surface concentration greater than 1.times.10.sup.19 atoms per cubic centimeter. Due to the relatively low impurity concentration of epitaxial layer 14, a metal contact of aluminium to layer 14 will produce a Schottky barrier therebetween.

An insulating layer 28, for example, silicon dioxide, is formed and delineated to provide openings for the contacts to the various regions. Metalization layer 30, for example, aluminum, is applied to the insulator layer 28 and delineated to provide contacts B, C, D and E to regions 24, 26, 20 and 22, respectively. As illustrated in the schematic of FIG. 9, the aluminum contact C forms an ohmic contact to region 26 of the layer 20 which constitutes the collector of the NPN transistor and the aluminum metal contact D forms a Schottky diode contact with the collector layer 20 of the NPN transistor.

The process described provides an upwardly diffused base region 16 or 18 for the NPN transistor. In the impurity profiles illustrated in FIG. 10, the second epitaxial layer 20 has a constant impurity of, for example, 5.times.10.sup.15 atoms per cubic centimeter. The first epitaxial layer 14 is specifically formed to have a constant impurity concentration of, for example, 1.times.10.sup.17 atoms per cubic centimeter. The diffused buried region 16 and 18 have an impurity concentration varying from approximately the impurity concentration of the second epitaxial layer 20, namely, 3.times.10.sup.15 atoms per cubic centimeter at the boundary therewith and increasing towards its boundary with the first epitaxial layer 14 to a level of less than 10.sup.17 atoms per cubic centimeter. Structures of FIGS. 4 and 8 are specifically designed such that the updiffused P layer impurity concentration decreases from the N region 14 which constitutes the emitter of NPN transistor to the lowest impurity concentration epitaxial layer 20 which constitutes the surface collector of the NPN transistor. This critical arrangement of impurity levels in the updiffused P layer provides an electric field in the P layer which aids upward electron flow which is considered critical to the present invention. Processing steps described the high impurity extrinsic base ring 22 and the guard ring 24 as simultaneously diffused, reducing the overall diffusion time and thereby minimizing epitaxial junction movement. Similarly, by using a thin upwardly diffused base region 16 and 18, the amount of diffusion time to form the base ring 22 and guard ring 24 is also reduced.

From the preceding description of the preferred embodiments, it is evident that the objects of the invention are obtained in that an improved substrate fed logic semiconductor structure and method of fabrication is provided. Although the invention has been described and illustrated in detail, it is to be clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation. The spirit and scope of this invention is to be limited only by the terms of the appended claims.