Abstract:
A test circuit, including a vertical NPN, a lateral PNP and a vertical PNP transistor plus a diffused resistor and a thin film resistor, is formed by altogether simultaneous steps with corresponding components of each principal integrated circuit. Four dedicated test pads in each integrated circuit lead to all bases, collectors, emitters and resistor extremities so as to permit substantially unshunted measurements of all basic transistor and resistor electrical parameters.

Description:
BACKGROUND OF THE INVENTION 
     This invention is related to methods for making a large number of identical integrated circuits in a single semiconductor wafer, and more particularly to such wafers having one or more separate test circuits therein that may be probed and electrically analyzed for determining the efficacy of the prior diffusion, metallization and other steps and to predict the ultimate yield after the parts are broken apart, terminated and packaged. 
     Methods for making integrated circuits include a great many sequential steps wherein a subsequence of masking followed by impurity doping is repeated typically from 5 to 15 times depending upon the particular structures being formed. Such a subsequence includes preparing artwork, photographically reducing the art work in the form of a transparency or photographic mask, further optically reducing the image of the photographic mask in a step for exposing to light a portion of a film of photo-lithographic resist that overlies the oxidized major surface of the silicon wafer, selectively etching away only regions of the resist that were not exposed to light, selectively etching away the oxide layer through the holes made in the resist to form a silicon oxide mask having the pattern of the artwork, removing the remainder of the resist and then by diffusing from a hot gaseous atmosphere or by ion implanting, impurities are driven through the holes in the oxide mask into the silicon surface to make a particular element of the electrical components being formed there. To make another element of the electrical components the subsequence is repeated beginning with preparing a different artwork pattern and usually ending with the diffusion of a different impurity. A similar subsequence of steps ends with selectively metallizing to interconnect the integrated circuit elements. 
     For making a large number of identical integrated circuits in a single silicon wafer, each of the steps in the above noted sequence is performed simultaneously for forming all of the corresponding electrical component elements in all of the large number of integrated circuits, except for the one step of illumination exposure of the resist layer. The aptly named &#34;step-and-repeat&#34; exposure method is used whereby a pattern of illumination, corresponding to the art work, is sequentially directed to the resist film portions overlying the first, then the second and each in turn of the large number of integrated circuits to be formed in the wafer. 
     It is known to provide a special integrated test circuit, several of which are substituted for ones of the large number of principal integrated circuits in the wafer. This permits testing of individual integrated circuit components such as a transistor, a resistor, etc., in the test circuit. Here the components are made accessible unlike in a principal integrated circuit that is interconnected so the components are either inaccessible or electrically shunted. Each of the bulk process steps such as oxidizing, etching, diffusing, implanting and metallizing affects the entire wafer; namely all of the integrated circuits of the wafer are affected at the same time. However, because the effect on integrated circuits in one wafer region may be slightly different than in another, the above noted several test circuits are scattered about so each represents one region, and thus lack of uniformity in the processes effects are detectable as well as are the bulk or over-all effects common to all of the integrated circuits on the wafer that can be determined from measurements on the several test circuits. 
     However, it is not unusual in integrated circuits manufacturing that many of the defective integrated circuits are attributable to misregistration of one or more of the oxide masks. During a step and repeat illumination of sequential ones of the integrated circuits, the physical position of the illumination pattern at any particular integrated circuit must be closely aligned with the position of all prior illumination patterns. Such successive illumination pattern alignment, or lack of it, depend upon factors that commonly pertain to an individual integrated circuit and not to a group in a region of the wafer or to the whole wafer. 
     The above noted several test circuits that are substituted for certain of the principal integrated circuits in a wafer do not provide detection or prediction of defects in the principle integrated circuits that result from mask misregistration. 
     It is a primary object of this invention to provide a means for testing at the wafer level, which, how and how many integrated circuits may be defective due to mask misregistrations as well as to bulk process step variables. 
     SUMMARY OF THE INVENTION 
     A plurality of identical integrated circuits are formed on a semiconductor wafer each one of which has a principal integrated circuit and a separate test circuit. Electrical access to each principle circuit includes a set of metal pads whereas each test circuit has another set of metal pads. A transistor of each principal circuit is formed altogether simultaneously with a transistor of a corresponding test circuit especially including the steps of registering photolithographic masks each of which contains pattern portions for forming corresponding parts of the principal and test circuit transistors. For example each illumination exposure step in a step-and-repeat process for forming in a photo resist layer a photolithographic mask includes the simultaneous exposure of the corresponding transistor elements in both the principal circuit and test circuit. 
     Thus every integrated circuit containing a random mask-misalignment induced defect can be detected and so marked to avoid performing the next expensive steps of die bonding, lead attachment and packaging. Final testing of only a small sample then becomes appropriate. The tests of the test circuits can easily be made at extremes of supply voltage, or over an operating temperature range or over a range of signal frequencies. Thus integrated test circuits of this invention provide an improved means of detecting and thus controlling manufacturing processes parameters, an improved means of predicting at an early stage the final yields and provide a reduction in the extent of the tests that will be made of the packaged integrated circuits at the end of the line. Furthermore, a test circuit of this invention will typically occupy only from 1 to 2 percent of each integrated circuit chip. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a magnified top view of part of a silicon wafer having a matrix of identical integrated circuits formed therein, each integrated circuit having formed therewith a test pattern. 
     FIG. 2 shows a further enlarged top view essentially to scale of a portion of the wafer of FIG. 1 containing one test pattern having been selectively metallized. 
     FIG. 3 shows in perspective view the section taken in plane 3--3 of the integrated circuit test pattern of FIG. 2. 
     FIG. 4 shows a circuit diagram representing the test pattern of FIG. 2. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The part of a silicon wafer 10 shown in FIG. 1 reveals a matrix of integrated circuits 12 that have been formed therein, each one of which includes an isolated test pattern 15. Included in each test pattern 15 are four dedicated metal test pads 21, 22, 23 and 24 that are located at a corner of each integrated circuit 12. The scribe lines 26 are formed between adjacent integrated circuits 12 by which they may be broken away and into individual integrated circuit die 28. The portion of such a die 28, that contains a test pattern 15 is seen in FIG. 2. Additional reference to FIGS. 3 and 4 will make the following description of this test pattern 15 more meaningful. 
     A pocket 30 of N-type epitaxial silicon is bounded by a P-type isolation wall 32. Shallow regions 34, 36, 38, 40 and 42 may be doped with P-type impurities by any of several well known and conventional processes including diffusion and ion implantation. The shallower N-type region 44 is likewise doped from the epitaxial surface 46 of the integrated circuit 15. An N-type buried layer 48 is formed at the interface between the epitaxial pocket 30 and the P-type substrate 50. An N+ plug region 52 contacts the buried layer 48. 
     In FIG. 2, the outline of the buried layer 48 is shown by dotted line to underly regions 38, 40, 42 and 52, but does not extend under P-type regions 34 and 36. All the diffused or otherwise doped regions 32, 34, 36, 38, 40, 42, 44 and 52, excepting the buried layer 48, are shown by lines made up of short dashes in FIG. 2. 
     A layer of protective glass 54, a portion of which is seen in FIG. 3, overlies the epitaxial surface 46 except for holes provided therein (e.g. hole 55) permitting contact with the various regions. The holes in the glass, e.g. 55, are delineated by lines composed of long dashes. 
     A thin layer of aluminum is vacuum deposited over the glass 54 and subsequently removed by standard photolithographic and etch steps to provide the separate metal films including test pads 21, 22, 23 and 24. 
     The metal is omitted for clarity from the perspective view of FIG. 3 and instead a diagramatic system of lines representing conductors shows the connections that will be made by metal. 
     The circuit representation in FIG. 4 of the structure depicted in FIGS. 2 and 3, includes an NPN transistor 60 that employs the epitaxial pocket 30 as collector, the region 38 as the base and region 44 as emitter. This conventional vertical double-diffused transistor may be identical to many other NPN transistors that are simultaneously formed in the principle portion of the same integrated circuit. A PNP transistor 62 has a base region consisting of the epitaxial pocket 30. The emitter consists of region 36. A first collector 34 is a commonplace collector used in lateral PNP transistors formed in N-type epitaxial material. The principle part of the integrated circuit may include many such PNP transistors that are formed simultaneously with test PNP transistor 62. The second collector of PNP transistor 62 is the P-type substrate 50. As has been noted, the buried layer 48 has been omitted in this region of the pocket 30 and a vertical PNP is thus formed also. 
     The substrate 50 of FIG. 13 is the conductor 50 in FIG. 4 that connects the second collector of transistor 62 to test pad 24. A hole 64 in the glass layer 54 over the P-type isolation region 32 is provided. Test pad 24 makes direct contact through hole 64 to the isolation wall 32 as well as the substrate 50. 
     When the principle portion of the integrated circuit, e.g. 12, contains thin film resistors formed over an insulative layer, such as glass 54 in FIG. 3, then a thin film resistor may be formed simultaneously in the test pattern 15, e.g. in the place of resistor 42. Such a thin film resistor is described by Miles et al in U.S. Pat. No. 4,225,877 issued Sept. 30, 1980 and assigned to the same assignee as is the present invention. The patent further describes a process for making an ion implanted polysilicon resistor over a glass covered region of an integrated circuit containing inter alia bipolar transistors. That process is appropriate for forming the elements of the test pattern 15 of this invention as well as forming the elements (not shown) of the principle portion of integrated circuit 12, and the Miles et al patent is accordingly incorporated herein by reference. 
     The extension of the metal film of pad 23 seen in FIG. 3 crossing over the resistor 42 represents from a quality standpoint acceptable practice in a test pattern of this invention, which it may not in the principle portion of an integrated circuit 12. 
     The test pattern described above was incorporated in an integrated circuit die measuring 0.150 in by 0.104 in, or 10 square millimeters in area. The test pattern including terminals 21, 22, 23 and 24 occupies 1.25% of that area or 0.125 mm 2 . 
     Thus five different types of components are represented in the test pattern 15 described above. The DC and AC properties of each may be measured at the wafer stage in the manufacturing process by probe contacting only four dedicated contact pads. Each of these measurements may be made essentially unaffected by the presence of the other test pattern components by selecting the appropriate test pads and in some cases applying test voltages of the appropriate polarity. Some of the more important tests are listed in the Table below. Also, the test pads to be used for each test are listed. 
     
                       TABLE______________________________________COMPONENT    TEST PARAMETER  TEST PADS______________________________________Resistor 40  Resistance      21 and 22Resistor 42  Resistance      23 and 24Lateral PNP  BVcer           21, 22 and 23transistor 62        BVebo           21 and 22(collector 34)        V.sub.BE (SAT)  21, 22 and 23        V.sub.CE (SAT)  21, 22 and 23        h.sub.FE        21, 22 and 23Vertical PNP BVcer           21, 22 and 24transistor 62        V.sub.BE (SAT)  21, 22 and 24(collector 50)        V.sub.CE (SAT)  21, 22 and 24        h.sub.FE        21, 22 and 24NPN          BVcer           22, 23 and 24transistor 60        BVebo           23 and 24        V.sub.BE (SAT)  22, 23 and 24        V.sub.CE (SAT)  22, 23 and 24        h.sub.FE        22, 23 and 24______________________________________