Metallization structure and process for semiconductor devices
&#39; United States Patent 1191 Abraham et al.  
 1451 Apr. 8, 1975 1 1 METALLIZATION STRUCTURE AND PROCESS FOR SEMICONDUCTOR DEVICES [75] Inventors: Howard E. Abraham, Loveland,  
  Colo.; George E. Bodway, San Jose, Calif; Weldon 11. Jackson,  
 Sunnyvale, Calif.; Sanehiko Kakihana, Los Altos, Calif.  
 [73] Assignee: Hewlett-Packard Company, Palo Alto, Calif.  
  22 Filed: June 27,1973  
 1211 Appl.No.:374,230  
 [52] US. Cl. 357/71; 357/68; 357/69 [51] Int. Cl. H011 5/00 [58] Field of Search 317/234, 5.3, 40.13, 5.2  
 [56] References Cited UNITED STATES PATENTS Brewer et al. 29/195 Revitz et a1 317/234 Cohen et al. 219/121 LM Primary E.\&#39;amt&#39;nerMichael .1. Lynch Assistant Examiner-E. Wojciechowicz Attorney, Agent, or FirmRoland l. Griffin; Ronald E.  
 Grubman [57] ABSTRACT A semiconductor device comprising a resistor formed by a region of a layer of tantalum nitride (Ta N), said tantalum nitride layer also serving at another region as an adhesion layer and a barrier diffusion layer for the gold contacts and interconnects of the semiconductor device. A layer of tantalum nitride is also employed to form a mask for the metallization layer of a semiconductor device, the metallization layer thereafter being etched by sputter etching to produce very fine line electrical patterns for the integrated circuit.  
 3 Claims, 5 Drawing Figures WEi-HEBAPR 81975 igure 2 (PRIOR ART) METALLIZATION STRUCTURE AND PROCESS FOR SEMICONDUCTOR DEVICES BACKGROUND OF THE INVENTION In the fabrication of multiple emitter transistors using planar technology, resistors are formed on the surface of the substrate and in series with each emitter to prevent the occurrence of the phenomenon of current hogging. Thus a multiplicity of emitters, e.g., as high as 35 to 50, are formed by diffusion in the common base region of a silicon transistor, and certain ones of these emitters are coupled together by the surface metallization which forms the emitter contacts, and also the surface interconnect. With the common interconnect, the emitters are connected together in a parallel circuit fashion and the current to the parallel emitters is meant to divide equally among the parallel emitter circuits.  
  However, the emission of charge from an emitter and into the adjacent base region is a function of several factors, including temperature. Because of such known factors, there is a tendency for one of the emitter regions to draw more current than the other parallel emitters, and the emitter-base junction for this one region gets hot. The heat at this spot results in drawing more current, called current hogging, and a runaway process takes place which results in the destruction of the transistor.  
  One common technique for preventing such current hogging is to form separate resistors in series with each separate emitter. and it is now common practice to form these resistors with such metals as chromium, metal silicides, nichrome, and tantalum nitride (Ta N). After the formation of such resistors, it is then the practice to form the contacts and circuit interconnections by the process of forming a layer of good electrical conducting metal such as aluminum or gold over the transistor surface, and thereafter forming the metallization pattern by a photoresist and etch process.  
  In the case of aluminum, this metal is not only a good electrical conductor but it adhers well to the silicon surface. However, although gold is a superior electrical conductor, it does not adhere well to silicon. In addition. when heated, gold will diffuse into the silicon material at a high rate and will destroy the device. Therefore, when using gold for the electrical contacts and interconnects, an adhesion layer and a diffusion barrier to the gold is employed in the metallization process. For example, in one known metallization process, a layer of titanium is first placed down on the silicon surface to form a good adhesive layer, followed by a layer of platinum to serve as a diffusion barrier to the gold, followed by the layer of gold. Molybdenum performs well as an adhesion layer and is an excellent diffusion barrier while tungsten is an excellent adhesive and a fair diffusion barrier.  
  Therefore, when fabricating multiple emitter planar transistors. separate metallization depositions take place for the resistor material and the contactinterconnect material, as well as one or two additional materials for the adhesion and gold diffusion barrier layers.  
  It would be most desirable to provide a single metallization for use with the gold contacts and interconnects which would serve as the resistor material, the adhesion material, and the gold diffusion barrier material.  
  Additionally, in forming present day integrated circuits, very fine geometric patterns and circuit delineation is necessary and such fine definition is very difficult to accomplish with the typical forms of wet etching employed following the photoresist masking. Sputter etching as described, for example, in an article entitled RF Sputter Etching-A Universal Etch by P. D. Davidse, Journal of Electrochemical Society, Volume 116, Jan. 1969, pages -103, will provide a very finely defined geometry, especially useful when employed with materials which require strong etchants to remove.  
 SUMMARY OF THE PRESENT INVENTION The present invention provides a transistor structure in which a layer of tantalum nitride (Ta- N) is used in the formation of discrete resistors as well as serving as the adhesive layer and the diffusion barrier layer of gold contacts and interconnects on the transistor.  
  A tantalum nitride layer is also utilized in the formation of a mask for the subsequent sputter etch of the gold and tantalum nitride layer used for the resistors, such sputter etch acting to clear the field while at the same time producing the very fine line electrical patterns desired in state-of-the-art high frequency devices and integrated circuits.  
 BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a plan view of a portion of a typical form of a multiple emitter transistor which may utilize the present invention.  
  FIG. 2 is a cross-sectional view of a transistor of the form shown in FIG. 1 and taken along section line 22 therein utilizing a known resistor structure.  
  FIG. 3 is a cross-sectional view of a transistor of the type shown in FIG. 2 and taken along the same section line as FIG. 2 but showing the resistor structure of the present invention.  
  FIG. 4 is a cross-sectional view of a transistor structure of the type shown in FIG. 3 after the step of metallization in accordance with the present invention and prior to the step of metal removal to form the desired pattern of metal contacts and interconnects.  
  FIG. 5 is a perspective view of the transistor structure of FIG. 4 after clearing the field by sputter etching and subsequent etching to remove the gold over the resistor area.  
 DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to FIGS. 1 and 2, there is shown a top view and a cross-sectional view, respectively, of a portion of a muIti-emitter transistor showing a silicon body having a common base region 11 diffused into a common collector region 12, and three separate emitter regions l3, l4, and 15 diffused into the common base re-&#39; gion l1. Emitter contacts 16, 17 and 18 are formed by known metallization techniques and contact the associated emitter areas l3, l4 and 15, respectively, through suitable openings made in the dielectric layer 19. Resistors 21, 22 and 23 are formed on top of the dielectric layer 19 for electrical connection at one of the ends thereof with the associated contacts l6, l7 and 18, respectively.  
  At their opposite ends, the resistors 21, 22 and 23 are electrically connected with a common interconnect 24 nichrome, or tantalum nitride (Ta N), for example.  
 The electrical contacts l6, l7 and 18 and the interconnect 24 may be formed of aluminum or gold, for example, both being good electrical conductors. In the case of aluminum, it may be applied directly to the surfaces of the dielectric layer 19, the emitters 13, 14 and 15, and the resistors 21, 22 and 23 due to its good adhesion properties and also because it diffuses slowly at the normal operating temperature of the transistors.  
  In the case of gold (Au) contacts and interconnects, however, gold does not adhere well and also diffuses at a high rate into the silicon body when heated. Therefore, when using gold contacts and interconnects, an adhesion layer and gold diffusion barrier must be formed between the gold and the silicon body. A suitable layer or layers (not shown) of titanium and platinum in one case, or molybdenum in a second case, or tungsten as a third example, is formed on the silicon body as a preparatory layer for the subsequent gold metallization interconnects and emitter contacts. Thus, in forming the resistors 21, 22 and 23 as well as the gold contacts and interconnects, at least three and sometimes four separate metallizations are needed, for example, Ta N for the resistors and then titanium for the adhesive, platinum for the diffusion barrier, and gold for the contacts and interconnects.  
  In accordance with the present invention, tantalum nitride (Ta- N) is employed as a single layer 25 under the gold metallization 17, 24 to serve (l) as a contact area between the gold contacts and the contacted areas, e.g., the emitter contacts 17, (2) as an adhesive layer for good adhesion between the gold and the silicon surface, (3) as a diffusion barrier between the silicon body and the gold, and (4) to form the separate resistor elements, e.g., 21, 22 and 23, where needed. This Ta N layer 25 is shown in cross-section in FIG. 3.  
  By utilizing this novel technique, only two metallization layers are needed, Le, a thin Ta N layer and a thicker gold layer. The gold electrical contacts and interconnects as well as the desired circuit resistors such as resistors 21, 22 and 23 are then formed by well known photoresist masking and subsequent metal removal techniques as well as by the novel technique described below where fine line geometry is needed in the formation of the high frequency devices and integrated circuits.  
  One novel technique for forming the resistors and the gold contacts and interconnects will be described with reference to FlGS..4 and 5. After the formation of the emitter areas l3, l4, 15, etc., and the emitter contact openings in the dielectric layer 19, a layer 25 of Ta N is formed overthe entire surface by a typical metallization process such as evaporation or sputtering, followed by a layer 26 of gold. Then a second layer 27 of Ta N is formed over the entire gold surface area. There is thus formed a sandwich of Ta- N, gold, and Ta N. In this particular illustration, the Ta N layer 19 is about 1,000 A thick,-the gold layer 26 is about 8,000 A, and the outer Ta N layer is about 2,600 A thick.  
  By known photoresist techniques, a mask is formed on the upper surface of the Ta N layer 27 which exposes all those areas of the layer 27 that are not in alignment with the resistor areas such as area 21, 22 and 23 and with the areas &#39;of the contacts such as 16, 17 and 18 and the interconnect 24; this exposed area is referred to as the field. The surface is then subjected to a wet chemical etch to remove the top layer of the exposed Ta N over the field until the gold is exposed in those areas. The surface is then exposed to the RF sputter etching to remove the outer layer 27 of Ta N over the emitter finger areas and the interconnect and to also remove the exposed gold and the under layer 25 of Ta N to clear the field. This RF sputter etching provides very clean lines so that the emitter fingers including the resistor area are clearly defined. After this RF sputter etching, the only areas to be thereafter removed is the gold layer over the resistors and this is accomplished with a photoresist masking followed by a wet etching of this gold layer in those resistor areas.  
  If the initial pattern formed in the upper layer 27 of Ta N by the wet etching is not clean or is otherwise improper, the remainder of the layer 27 may be stripped off and a new Ta N layer 27 applied for use in forming a mask with the desired characteristics.  
 We claim:  
 1. In A semiconductor device comprising:  
 a semiconductor body having a first region of a first conductivity type, a second region ofa second conductivity type, and a plurality of separate third regions of the first conductivity type;  
 a dielectric layer over the semiconductor body having an opening at each of the third regions of the semiconductor body;  
 a plurality of metallic electrical interconnections on the dielectric layer, at least some portions of these interconnections extending through the openings in the dielectric layer for electrical connection with associated ones of the third regions of the semiconductor body; the improvement comprising:  
 a single layer of Ta N, including a first plurality of portions of the Ta N layer positioned between said portions of the metallic interconnections extending through the openings in the dielectric layer and the associated ones of the third regions to serve as surface connections as an adhesion layer and as a diffusion barrier between said portions of the metallic interconnections and said associated third regions, a second plurality of portions of the Ta N layer positioned between other portions of the metallic electrical interconnections and the dielectric layer to serve as an adhesion layer and a diffusion barrier therebetween, and a third plurality of portions of the Ta N layer extending between different portions of the plurality of metallic electrical interconnections to serve as a plurality of resistors extending therebetween.  
  2. A semiconductor device as in claim 1 wherein the first region in the semiconductor body comprises a collector region, the second region in the semiconductor body comprises a base region, and each of the plurality of separate third regions of the semiconductor body comprises an emitter region.  
  3. A semiconductor device as in claim 1 wherein the plurality of metallic electrical interconnections comprises a plurality of gold interconnections.