Abstract:
A single width bar code exhibiting inherent self clocking characteristics is provided so as to be particularly useful in the identification of semiconductor wafers in very large scale integrated circuit manufacturing processes. The codes described herein are robust, reliable and highly readable even in the face of relatively high variations in scanning speed. The codes are also desirably dense in terms of character representations per linear centimeter, an important consideration in semiconductor manufacturing wherein space on the chips and the wafer is at a premium. Additionally, a preferred embodiment of the present invention exhibits a minimum number for the maximum number of spaces between adjacent bars in code symbol sequences.

Description:
This application is a division of application Ser. No. 08/021,298 filed Feb. 22, 1993, now U.S. Pat. No. 5,552,591 which is a continuation of application Ser. No. 07/796,163 filed Nov. 22, 1991, now abandoned, which was a continuation application Ser. No. 07/351,902 filed May 15, 1989, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention is generally directed to bar code configurations which are useful for representing alphanumeric data. More particularly, the present invention is directed to bar codes in which the bars exhibit uniform single width dimensions and specific periodicity and which are coded to include a subsequence of timing marks to enhance readability. Even more particularly, the present invention is related to high density bar code systems which are employable during the manufacture and processing of semiconductor wafers used in the fabrication of electronic circuit chips, such as those that are produced in very large scale integrated (VLSI) circuit manufacturing processes. 
     Bar code fonts for representing symbol data, particularly alphanumeric symbols, are very desirable since they provide a mechanism for machine readability which does not depend upon optical character recognition (OCR) systems. In general OCR systems tend to be more error prone than bar code systems. However, bar code systems are nonetheless susceptible to certain error conditions. One example of this is the partial covering of the bar code with opaque films or the reducing of the contrast ratio which makes it more difficult to distinguish between wide and narrow width bars. As an example, if the scanning speed in a bar code reader is not absolutely constant a long gap of spaces between bars is more likely to produce a reading error. Bar codes which have a large number of blank spaces between bars are particularly subject to this kind of error. For example, in certain bar codes a 3% speed variation is sufficient to produce reading errors. Accordingly, it is seen that it is desirable to be able to construct bar code systems in which there is a significant reduction in sensitivity to scanning speed variation. 
     A number of bar codes employ bars having a plurality of different widths. However, there are certain disadvantages associated with multi-width bar code fonts. In particular, their density, for example as measured in characters per inch, is not as high as one could obtain in a font which only exhibited a single width bar. Furthermore, in bar code systems employing multiple bar widths, it is necessary that the circuitry discern each bar&#39;s width or at least the width ratio between bars. Modulated bar widths also introduce writing problems when scribing is carried out with a pulsed laser to form the image. Wide bars, tat is, bars with widths greater than the width of a laser spot, yield a lower quality bar image and require a much longer writing time. For this reason, modulated bar width codes also pose a greater risk of damage to the wafer because of the increased laser radiation concentration. Furthermore, when pulsed lasers are used to write on semiconductor wafers in dot matrix fashion, there is a tendency for a trench to form which throws off the laser used for reading the imparted signal. Thus, modulated bar width systems tend to introduce readability problems when there is a writing quality problem, bar image degradation or low contrast such as might occur in the identification of semiconductor wafer serial numbers. 
     Single width bar codes are employed but require the simultaneous use of separate timing marks. The single width bar codes therefore require twice the space, plus dual readers. A typical example of such a code is found on certain envelopes as coded by the United States Postal Service. Dual readers are not only more expensive, but the code that they employ takes up more room on a wafer. This room is much more advantageously given over to a human readable version of the code. 
     While the present invention is generally directed to bar code reading systems having a wide range of applicability to manufacturing, marketing, sorting and identification functions, it is particularly applicable to the identification of semiconductor wafers. In particular, in the manufacture of very large scale integrated circuit devices, that is, chips produced from processed wafers of material such as silicon, it is necessary to employ a code that can be efficiently written and which is robust under the exigencies of processing in unusually harsh environments. Such codes are required to be robust and satisfy the need for representing the full range of alphanumeric characters, A through Z and  0  through  9 . This set constitutes a total of 36 characters. However, the letter “O” is often deleted from the required set leaving a total of 35 characters which usually need to be represented and/or distinguished. Additionally, because of size constraints that exist in the manufacture of semiconductor devices it is very desirable that codes employed exhibit a high density. Thus codes which require a separate set of timing marks are undesirable because of the space that the timing marks require. 
     Furthermore, codes which are suitable for semiconductor chip processing generally should be easy to write on a wafer during processing and should likewise be easily read during wafer or chip processing. Furthermore, the bar code font employed should be such that it is able to withstand processing conditions, namely exposure to sometimes harsh chemicals and high temperature enviroments. Futhermore, it is desirable to employ codes which can be written by means of a pulsed laser. Such robust and highly reliable codes are desired for wafer fabrication processes to facilitate automatic wafer handling, processing and process parameter identification. Furthermore, it is seen that codes that are developed for such processes would also possess a wide range of applicability in other areas in which bar codes are presently employed. 
     SUMMARY OF THE INVENTION 
     In accordance with a preferred embodiment of the present invention a method for marking a substrate with a bar code comprises the step of providing a sequence of uniform width marks at selectively spaced intervals. Each of the marks possesses substantially the same width, with blank intervals between the marks. The blank intervals have a width which is substantially equal to an integral multiple of a second width which represents the width of a single blank space. Nonetheless, the method of the present invention employs a single width bar. Furthermore, in accordance with the present invention, the sequence of marks is such that it includes a subsequence which is spaced apart from adjacent marks in the subsequence by a fixed distance. This provides a means for timing recovery and/or timing compensation. In accordance with a particular embodiment of the present invention each character or symbol is represented by a sequence of 12 marks and/or spaces. This permits the representation of up to 35 or 36 different symbols which is sufficient for alphanumeric representation. Furthermore, the present invention preferably employs bar codes in which the marks are spaced to represent a number of different symbols in such a way that the largest distance between adjacent bars for different symbols is a minimum. In a preferred embodiment of applicants&#39; invention this minimum distance is five bar widths. This latter feature of the invention reduces the occurrence of sequences of symbols in which wide gaps having no bars are produced. Additionally, the present invention is also directed to a substrate marked in accordance with the method described above and includes substrates having such identification marks thereon. 
     Accordingly, it is an object of the present invention to provide a high density bar code system, as measured for example in terms of characters per centimeter. 
     It is also an object of the present invention to provide a high density bar code which is particularly suitable for use in the identification of wafers and/or chips in semiconductor manufacturing processes. 
     It is yet another object of the present invention to devise a single width, self clocking bar code which is both robust and reliable. 
     It is a still further object of the present invention to provide bar codes which are employable in harsh environments and which can be readily written on materials employed in semiconductor manufacture generally and in particular on materials such as silicon. 
     It is still another object of the present invention to provide bar code systems which exhibit single width bars and yet are self clocking. 
     It is also an object of the present invention to provide a bar code which exhibits a minimum distance value from amongst all of the largest distances between adjacent bars in sequences of different symbols. 
     It is a still further object of the present invention to provide a bar code system which is capable of representing up to 35 or 36 different symbols, such as the 36 alphanumeric symbols A through Z and  0  through  9 . 
     Lastly, but not limited hereto, it is an object of the present invention to provide an easily read and easily written bar code having universal applicability to product identification. 
    
    
     DESCRIPTION OF THE FIGURES 
     The subject matter which is regarded as the invention is particularly pointed out and distinctly claimed in the concluding portion of the specification. The invention, however, both as to organization and method of practice, together with further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawings in which: 
     FIG. 1 illustrates a bar code system which is preferred in the present invention; 
     FIG. 2A is an illustration of two single width bar codes with built-in timing marks, in accordance with the present invention; 
     FIG. 2B is an illustration of three single width bar codes without built-in timing marks; 
     FIG. 3A is a planar view of acid etched bar code marks on a semiconductor substrate; 
     FIG. 3B is a view similar to FIG. 3A except illustrating the utilization of an alkaline etch; 
     FIG. 4A is an illustration of a system which is capable of reading codes produced in accordance with the present invention; 
     FIG. 4B is an illustration of signals produced in accordance with the present invention by means of the bar code reading system illustrated in FIG.  4 A. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 illustrates a preferred embodiment of the present invention. In particular, FIG. 1 illustrates a presence/absence single width bar code which is capable of representing the numeric digits  0  through  9  and the alphabetic characters A through z (without the character “O”). This particular code is therefore seen to be able to represent 35 distinct symbols. The flattened U-shaped channel mark at the bottom of each of the bar code symbols is shown only for convenience to point out the fixed distance that each set of bar code symbols occupies. In particular, it is to be noted that each code symbol character occupies the same horizontal distance and is made up of a plurality of single width vertical bars and correspondingly sized spaces between these bars. Furthermore, when compounded together in a sequence of alphanumeric characters, it is seen that the first bar in each code forms part of a subsequence of bars which is capable of providing timing information. Other aspects of the bar code shown in FIG. 1, and designated herein as BC 412 , are more particularly discussed below in comparison with other single width bar codes. 
     In particular, consideration is now directed to FIGS. 2A and 2B which illustrate a variety of single width bar codes including code BC 412  discussed above. In particular, FIGS. 2A and 2B illustrate the presence of a bar in the code in the form of the letter “I”. Spaces or absences of bars are illustrated by hyphens, “-”. In particular, it is noted that code BC 412  shown in FIG. 2A always begins with a “I-” sequence. In this case, these two code symbols constitute two elements of a subsequence which is repeated in any concatenation of bar code symbols. In code BC 412  it is seen that the total number of code elements is 12 so as to make this particular code one which is of fixed length. Furthermore, amongst these 12 slots (elements) where bars could be present or absent there are always a total of 4 bars per alphanumeric character, a fact which is again helpful in establishing or reestablishing timing control. Correspondingly, each symbol includes a total of 8 spaces (bar absences) per character symbol. Code BC 412  is thus seen to be capable of representing 35 characters which is sufficient for most alphanumeric applications. Most importantly for code BC 412 , the maximum number of spaces between bars in any concatenated sequence of code symbols is 5. This means that there are no long runs of blank spaces between adjacent bars. This significantly improves the readability and reliability of such bar codes. There is also a minimum of one space between bars. Therefore the first and last elements a bar and space, respectively. In particular, it is seen that bar codes in which the maximum number of spaces between bars is a minimum is preferred for purposes of controlling sensitivity to speed variation in the reading device. The reason for this is that when a reading device reads the bar/space sequence, the device detects the presence of bars, and derives the number of spaces between the bars from the scanning speed and time. If the scanning speed is not absolutely constant, a long gap of spaces between bars is more susceptible to the production of reading errors. An example of this is the BBC 31  code, as seen in FIG. 2B, having up to 17 spaces between bars. A code such as this is subject to reading errors at a mere 3% speed variation. In contrast however the BC 412  code, with only a maximum of 5 spaces between bars is capable of readability even at an 8% variation in speed. This is a significant advantage for the BC 412  code. 
     Another bar code in accordance with the present invention is illustrated by the code labeled BC 313  in FIG.  2 A. For the same reasons given above with respect to code BC 412 , code BC 313  is also seen to possess timing indicia which are inherent in the code structure. Thus codes such as BC 412  and BC 313 , as shown, do not require the presence of a separate set of timing marks. Code BC 313  however possesses one more element per symbol than code BC 412 . Nonetheless it is fully capable of representing a full alphanumeric character set of 36 different symbols. Code BC 313  is seen to include a total of three bars per character and a total of 10 spaces per character. Additionally, the maximum number of spaces between bars is 8 which is higher than the corresponding maximum distance for code BC 412 . Nonetheless both codes BC 412  and BC 313  represent single width bar codes with built-in timing recovery subsequences. It is further seen that codes BC 412  and BC 313  each possess sufficient inherent and discernible structure so as to readily permit their extension to codes where the number of elements per character represented is greater than twelve. 
     FIG. 2B also illustrates several other single width bar codes namely code BC 411 , code BBC 31  and code BC 311 . While these are all illustrations of single width bar codes, it is seen that they do not possess he desired self clocking feature. Furthermore, code BBC 31  appears to be particularly lacking in the range of different symbols it is capable of representing namely 31, as opposed to 35 or 36. The attributes of the various codes shown in FIGS. 2A and 2B are listed below in Table I: 
     
       
         
               
               
               
               
               
               
             
           
               
                 TABLE I 
               
               
                   
               
               
                   
                 BC411 
                 BC412 
                 BBC31 
                 BC311 
                 BC313 
               
               
                   
               
             
             
               
                 # of elements 
                 11 
                 12 
                 10 
                 11 
                 13 
               
               
                 # Bars/Character 
                 4 
                 4 
                 1 to 5 
                 3 
                 3 
               
               
                 # Spaces/Character 
                 7 
                 8 
                 5 to 9 
                 8 
                 10 
               
               
                 Max. # Spaces 
                 7 
                 5 
                 17 
                 7 
                 8 
               
               
                 between bars 
               
               
                 Timing Bar 
                 No 
                 Yes 
                 No 
                 No 
                 Yes 
               
               
                 # Characters 
                 35 
                 35 
                 31 
                 36 
                 36 
               
               
                   
               
             
          
         
       
     
     It is to be particularly noted that among the four codes in Table I that have a fixed number of bars, code BC 412  has the least number for the maximum number of spaces between bars, that is, five spaces. This is a highly desirable characteristic as noted above. It is also seen that codes such as BBC 31  do not even possess the desirable property of having a fixed number of bars or spaces per symbol character represented. 
     The system of the present invention is particularly applicable to the marking of semiconductor materials for identification during processing. The application of single width character fonts provides a higher code density, simpler writing methods plus easier reading because the reading system only has to recognize the presence or absence of a bar, without having to discern the bar&#39;s width or the width ratio between bars. As applied to silicon wafers, the bar widths are typically between about 0.10 millimeters and 0.05 millimeters for pre-polish mark and post-polish mark writing methods respectively with code densities from 12 to 6 characters per centimeter. Higher densities are possible be writing narrower bars which are read by higher resolution readers. In one embodiment of the present invention, bars having a width of 4 mils are employed. Such bars are disposed at a 10 mil center to center spacing leaving a minimum width between adjacent bars of 6 mils. For these dimensions, the bar width is 4 mils and the space width is 6 mils. 
     The marks are preferably written into the semiconductor substrate by means of a switched Nd:YAG laser operating in dot matrix mode. High quality bars are produced on silicon wafers with non-overlapping dots. Etching plus chemical and mechanical polishing operations transform the marks into more continuous lines. 
     As indicated above, it is highly desirable for the bar code indicia of the present invention to be able to withstand the harsh environments of VLSI processing methodologies. Temporary process overlays can obstruct part of the bar code and/or reduce the contrast ratio thus producing difficult bar code reading situations. FIGS. 3A and 3B clearly indicate that the codes of the present system, when written on silicon semiconductor material, are capable of surviving exposure to both acidic and alkaline chemicals. 
     Next is considered a system which is capable of reading such marks placed on a wafer or similar substrate. In particular, attention is directed to FIG. 4A wherein it is seen that light source and reflection sensor  10  is capable of directing a source of light, preferably laser light, on to the substrate of wafer  18 . The light reflected from wafer  18  is received by the sensor in functional block  10  thus producing an electrical signal such as that shown in FIG.  4 B. This signal is amplified by means of amplifier  12  and sent to both peak detector  14  and comparator  16  which produces the desired binary output which is indicative of the concatenated string of characters appearing on wafer  18  in the form of a bar code such as BC 412  or BC 313 . 
     From the above, it should be appreciated that the bar code system of the present invention is fully capable of satisfying the objects set out herein. In particular, it is seen that the present invention provides single width bar code systems exhibiting inherent self clocking characteristics. Furthermore, it is seen that the codes herein provide for the full range of representation for the alphanumeric character set. Additionally, it is seen that the preferred code herein exhibits a minimum value for the maximum number of spaces occurring between bars. It is further noted that while the codes described specifically herein exhibit an inherent timing subsequence arising out of the leading code bars, that it is also possible to produce codes wherein the subsequence of timing marks occurs either in the middle of the code or in a trailing sequence. Either of these arrangements are possible while still keeping within the spirit of the present invention. Additionally, while the invention herein has been described specifically with reference to the processing of semiconductor wafers and/or chips, it should be noted that the system disclosed herein is generally applicable in whatever systems employ bar codes generally. More particularly, as used herein and in the appened claims, the term “substrate” is not limited to semiconductor materials. A substrate may be paper, such as an adhesively backed label, wood, metal, plastic, compositive or other material capable of being marked or of receiving marking indicia on a portion of its surface. The marking may be provided by ink or by altering surface properties or other physical properties of the material, including but not limited to electrical and/or magnetic properties. 
     While the invention has been described in detail herein in accord with certain preferred embodiments thereof, many modifications and changes therein may be effected by those skilled in the art. Accordingly, it is intended by the appended claims to cover all such modifications and changes as fall within the true spirit and scope of the invention.