Abstract:
A method for making ion implanted resistors in conjunction with transistors and other devices within an integrated circuit semiconductor substrate. The implantation of the resistors is done after a predeposition diffusion of the base region of the transistors but prior to the base drive-in step. The subsequent emitter thermal diffusion, or annealing step in the case of ion implanted emitters, consitutes the annealing step for the ion implanted resistor regions.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to methods for fabricating semiconductor integrated circuit structures utilizing both diffusion as well as ion implantation. In particular, it relates to the formation of ion implanted resistor regions during the fabrication of other devices within the chip, such as transistros having diffused base regions. 
     2. Description of the Prior Art 
     The method of forming semiconductor devices utilizing ion implantation has received a great deal of attention in recent years as a potential substitute for standard diffusion processes. The primary advantage of ion implantation as compared to diffusion is said to be the greater control of the area of the active region to be formed within the semiconductor, as well as the doping level. Thus, while diffusion technology has been satisfactory for the formation of impurity regions within the semiconductor substrate, it is thought that ion implantation will be required for more advanced devices. However, diffusion technology is well established and continues to be used. 
     It has been demonstrated that ion implantation is better than diffusion in the formation of resistor regions within the substrate particularly resistors with high resistivity. Such high valued resistors require low concentration levels, and it is difficult to obtain this with diffusion. Controlling the resistance value of resistors using thermal diffusion is difficult, as the spread of values using selected diffusion parameters is often greater than can be accepted for modern semiconductor circuits. These problems are substantially lessened if resistors are made by ion implantation. 
     However, even with the use of ion implantation for forming all of the impurity regions within a semiconductor substrate, a thermal cycle, commonly termed annealing, is required. For example, the process for forming the emitter region of a transistor with ion implantation is best accomplished by performing what is termed a predeposition ion implantation step followed by an annealing cycle of at least 1000° C. for one hour to rearrange the impurities within the emitter region. It has been recognized that this thermal cycle could cause problems with the resistor regions if they were formed prior to or simultaneously with the formation of the emitter. Thus, it has been the standard practice within the industry to form the resistor region after the formation of all other regions which require thermal cycling for their formation. However, this arrangement requires in general more processing steps due to the need for a greater number of masks. In addition, because the maximum concentration of the implanted ions of the resistor are not at the surface there are problems with regard to the stability of the resistors. 
     In the last few years, those skilled in the art have contemplated using the annealing or diffusion step of the emitter regions to also effect the annealing of previously implanted resistor regions. See, for example, U.S. Pat. No. 3,933,528 issued in the name of B. J. Sloan, Jr. However, these efforts have been confined to simultaneous or successive implantations of the various regions, e.g., the base and resistor regions. It would be desirable to utilize this type of technique in cases where the base or other regions are diffused, rather than ion implanted. In particular, it is desirable that such a process require a minimum number of masks to form the various impurity regions. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of my invention to simplify the fabrication of ion implanted resistors formed within integrated circuit structures in which diffusion is used to form other impurity regions. 
     It is another object of my invention to improve the reliability of such ion implanted resistors. 
     These and other objects of my invention are achieved by ion implanting resistor regions prior to the formation of the emitter region of said transistor. The implantation is preferably done directly into the semiconductor substrate at relatively low energy levels. The resistor is formed after the diffusion of the base region of the bipolar transistor but prior to the base drive-in step. In the preferred process, the base diffusion step also forms contacts for the resistor. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1A-1F are sectional view showing the preferred process for making the present invention. 
     FIGS. 2 and 3 illustate the impurity profiles of the base and resistor regions fabricated in accordance with my invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIGS. 1A-1F inclusive, there are shown the successive steps in the novel method of making an integrated circuit resistor in accordance with my invention. 
     FIG. 1A illustrates a partially-completed integrated circuit which includes an epitaxial layer 4 of N- conductivitiy type which has been deposited atop P- substrate 2. Subcollector regions 5 and 6 have outdiffused into epitaxial layer 4 and P+ regions 8 have outdiffused to function as isolation regions. Region 6 functions as an isolation region for the P type resistor to be formed. Preferably, layer 4 has a thickness of around 2 microns or less and a concentration of from 2.1 to 2.3×10 16  atoms/cm 3 . 
     Base 10 of the transistor is initially formed in layer 4 by the predeposition of BBr 3  atop the substrate. Typically, the predeposition is accomplished in a dry oxygen and argon atmosphere for around one hour to form a 400 A layer of borosilicate glass (BSG) 11. 
     A subcollector reach-through region 12 has been formed which contacts subcollector region 5. Reachthrough region 12 is isolated from the P+ base region 10 by means of an oxide isolation region 14. On the opposite side of base region 10, oxide isolation region 16 separates the active transistor region from region 18 in which is to be formed a resistor. In the present case the resistor is to be of P type conductivity although the principles of my invention are also applicable to N type resistors formed, for example, from arsenic or phosphorus. In that case, the region underneath the N type resistor would be P type and preferably formed simultaneously with isolation regions 8. In addition, it will be evident to those of skill in the art that the conductivity types of various regions previously described and to be described could be reversed and still remain within the scope of my invention. Moreover, not all of the regions illustrated in the drawing are necessary for an operative embodiment. They are illustrated as representing the best mode of practicing my invention. For example, the recessed oxide isolation regions could be replaced by impurity isolation regions. 
     Returning to FIG. 1A, P+ regions 20 and 21 are contact areas for the resistor to be fabricated im the case of P type resistors. These regions are formed in the same steps as are used to form P+ region 10. 
     As shown in FIG. 1B, a photoresist blocking layer 24 is deposited atop the substrate and exposed and developed to open window 25 for the formation of P type resistor region 26. Oxide layer 15 and BSG layer 11 are then etched away over region 18 while mask 24 protects the remainder of the substrate. In the preferred embodiment, the implant species is boron 11 which is implanted at an energy of 70 Kev to a dosage of around 2.2×10 +13  ions/cm 2 . These values of energy and dosage yield a resistor value of around 2000 ohms per square at the completion of the entire process. Although the implantation is shown as being done directly into layer 4, the use of a &#34;screen&#34; oxide of around 300 A thickness is also a practical technique. 
     The dosage selected is that required for the resistor having the highest resistance value. The steps of forming resist masks, exposing selected regions and then ion implanting may be repeated at various selected resistor sites if lower valued resistors are also to be formed. N type resistors could also be formed instead of, or in addition to, P type resistors at this stage. 
     Following the implantation of resistor 26, photoresist layer 24 and BSG layer 11 are stripped. The substrate is then oxidized so as to form a relatively thick oxide layer 28 over the substrate, which is the base drive-in step. This combined base drive-in and reoxidation process is preferably performed in an atmosphere of dry oxygen and steam at 925° C. for around one and one-half hours to form an oxide layer which is around 800 A thick. Different thicknesses are also practical. 
     No resist mask is required at this point. The drive-in causes the depth of regions 10, 20 and 21 to increase slightly. 
     A layer of silicon nitride 30 is then deposited by standard chemical vapor deposition techniques. Nitride layer 30 is formed by conventional techniques, typically using a composition of silane, nitrogen and ammonia gas vapors at a temperature of around 1000° C. to form a layer which is 1600 A thick or less. 
     Openings 32, 33, 34, 35 and 36 are formed in nitride layer 30 by conventional lithographic and wet or dry etching techniques as shown in FIG. 1D. The resist mask used for etching the openings is not shown. In the case of wet etching, a layer of silicon dioxide may be deposited atop the silicon nitride to protect it against the resist etchant. Nitride layer 30 thereby comprises an &#34;all-contacts&#34; mask, with openings defining all contact and subsequent impurity regions to be formed within the substrate, as is well-known to those of skill in the art. 
     Turning to FIG. 1E, openings 32&#39; and 33&#39; are made in oxide layer 28 by blocking off the remainder of the substrate with a photoresist mask (not shown). This serves to expose the substrate over subcollector reachthrough region 12 and that portion of base region 10 in which the emitter of the transistor is to be formed. Emitter 40 is then formed in base 10 by the diffusion of arsenic, preferably from an arsenic capsule source as taught in Ghosh et al, U.S. Pat. No. 4,049,478, which is assigned to the same assignee as the present application. Concurrently, the doping level of subcollector reachthrough region 12 is raised by the diffusion of the same dopant to form a high conductivity region 41. The diffusion of emitter 40 is accomplished in a standard diffusion furnace which is held to a temperature of around 1000° C. for approximately 145 minutes. Alternatively, the emitter could be formed by ion implantation followed by annealing at around the same temperature for 100 minutes. The emitter diffusion process or the annealing step which follows implantation constitutes the annealing step for resistor 26. 
     The basic process is completed by protecting regions 40 and 41 with a resist mask (not shown) and etching away oxide layer 28 from those regions defined by openings 34, 35 and 36 to form openings 34&#39;, 35&#39; and 36&#39;. The resist is then stripped and all of the regions which are to have contact metallization deposited thereon are now exposed as shown in FIG. 1F. Metallization (not shown) is then formed by conventional evaporation or sputtering techniques typically; the metallization would comprise platinum and copper-doped aluminum or platinum, chrome and copper-doped aluminum, etc. This step is not shown since it is well-known to those of ordinary skill in the art and forms no part of my invention per se. 
     FIGS. 2 and 3 illustrate the net impurity profiles in epitaxial layer 4 of base region 10 and implant region 26, respectively, as obtained by the process described above. Having been formed using the same steps as for the base region, resistor contact regions 20 and 21 have the same profile as the base. The curves denoted by the numerals 100 and 102 represent the profile of the P type impurity of base region 10 and resistor region 26 respectively. The curves denoted by the numerals 101 and 103 represent the profile of the N+ subcollector regions 5 and 6, respectively. 
     The significant point in the graphs is the overall similarity of the profiles. The profile of the resistor very much resembles the profile of the base region, thereby assuring that the resistor is highly reliable. The highest concentration of impurities of resistor 26 is at the surface. Thus, the resistive is less susceptible to inversion due to charges in the overlying insulation or potentials in overlying conductive steps. 
     While the invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that the foregoing and other changes in form and detail may be made without departing from the spirit and scope of the invention. For example, with N type resistors, the resistor contact regions would be formed during the emitter diffusion.