Automatic numbers W. JARISCH,
reading of part
For reliable machine recognition of man-readable part numbers, an encoding technique known as theta-modulation is used. Each of the characters to be read is modulated by a grating with a specific angular orientation 0. Ten alpha-numeric characters are encoded by consecutive grids separated by 18”. The numerals are decoded by evaluating the diffraction patterns of the gridencoded digits. The orientation of the diffraction pattern is detected by an array of segmented photo-detectors. This part number reading technique is invariant against translations and was successfully applied to parts with perfectly polished or with coarse surfaces.
Complex manufacturing processes can only be evaluated, optimized, and automated if the individual parts manufactured can be traced and identified throughout the whole manufacturing cycle. Part identification is possible by printing serial numbers onto each part. In an automated manufacturing process the serial numbers must be read by a machine. They must also be readable by people outside the manufacturing line. Much character recognition equipment is commercially available which can read man-readable character sets, such as the machine-printed sets of 1428, OCR-A, and OCR-B, and also hand-printed characters. Generally, a character to be read is scanned by an optical CRT (IBM 1287) or an optical flying spot (Scan Data loo), or it is picked up optically by an integrated array of photo-diodes (IBM 1275, Opscan 288). Curve tracing, matrix and feature matching, and decision operations follow for character recognition. These complex and sophisticated machines are too expensive for an application such as a reading station in an automated line, where many serial number readers (say 20 or 30) are required in one line. Also, the flexibility of these machines is not needed in this particular application. For part identification it is favourable to encode the characters themselves with an easily readable code, which can be recognized with cheap and simple recognition logic. Principle of automatic Grid encoding
The authors are with IBM Deutschland Received 16 July 1973.
The idea of character encoding is to add further information to the characters which does not disturb their shape
and yet is representative of them. This principle is used in General Electric machines (MRS 200/205, DRD-200) which read the COC-5 fount, shown in Fig.1. This fount is made up of vertical bars arranged in a delta-distance mode. In reading, the characters are scanned optically generating a binary signal in the recognition circuitry. Because of the small number of bars, however, the numerals are rather difficult for people to read. In addition, requirements on the print quality are high and severe problems arise when the character height has to be as small as about 1 mm and when the characters have to be printed onto a rough surface. In such conditions we obtained best results by using a grid-encoding method called theta-modulation.
00 10 2
a - complete
set; b -
reading head output
Numerals encoded by ten different Fig.2 with a grid constant of 100 pm
The digits are printed
Theta-modulation was proposed in 1965 by A. Lohmann ’ for colour encoding of black and white pictures, but was not applied up until now for character recognition. We applied the concept of the technique (spatially modulating the characters by grids and encoding them by different angular orientations) for encoding the ten alpha-numeric dig_its used in our part identification process. It does not change the shape of the numerals, especially for small grid constants, and the man-readability of serial numbers is maintained. Each grid orientation represents one of the ten numerals. Consecutive grids are separated by an angle of 18” (Fig.2).
on the perfectly
front side of a silicon wafer
the space available for part serialization, the area may be circular or rectangular. Consecutive grid orientations need not necessarily be identical to the sequence of the numerals. The grid orientation versus numeral relationship can be optimized to achieve the best results for machine and man readability. Decoding method
Theta-modulated numerals can be recognized automatically by determining the directions of the different encoded grid orientations. This decoding can be achieved most easily by evaluating the diffraction patterns of the coded numerals.
The laser method of printing unmodulated serial numbers is described in an article by G. Holzinger et al? Gridmodulated numerals were printed with the same arrangement, but using a modified character mask. The character mask used for printing theta-modulated serial numbers (Fig.3) consists of etched grid-modulated numerals with grating constants of about 500 pm, yielding grating constants of 100 pm on the serialized part after a reduction by five.
The interaction of a grating with an incident light wave products a set of waves diffracted into discrete angular directions relative to the incident beam. The plane of the angular spread is perpendicular to the grid orientation of the numeral. The angular distribution of the diffracted waves depends on the period of the diffraction grating and on the wavelength of light. The equation describing the diffraction of light by gratings for normal incidence is given by:
Compared with numerals meant for reading only by eye, the shape of the theta-modulated numerals was modified slightly to obtain high machine reading performance: that is to get large areas carrying encoded information. The numerals should cover the major part of a definite area which is irradiated in the reading process. Depending on
where utl is the propagation direction of the tlth order diffracted beam, g is the grating constant, h is the wavelength, and n is the diffraction order number (any integer). It is notable that the direction of the diffracted beams is independent of the particular shape of the grating grooves and of the characteristics of the grating material. However, the shape of ten individual grooves determines how the incident light intensity is distributed in the various diffraction orders.3 For generating an optical reading signal, the grid-modulated area of a numeral is illuminated by a parallel light beam at normal incidence through a central hole in a photodetector array (Fig.4). A part of the incident beam is reflected back along the same path; part is diffracted and reflected by the grid-modulated surface.
Character Fig.3 numbers
mask used for printing
At any distance from the serialized part the reflected diffraction pattern, as shown in Fig.Sa, is detectable: it is centred around the incident beam and perpendicular to the grid lines. The zero order diffraction maximum is independent of the grid orientation. The signal to noise ratio of the reading signal can therefore be improved by omitting the centre of the diffraction pattern (FigSb). This is achieved in the experimental reading set-up by the hole in the photo-detector. Since the zero diffraction order is blocked, as much light as possible should be diffracted out of the incident beam. Optimally, a special phase grating should be used which has 100% diffraction efficiency and gives no energy in the reflected zero order.
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Fig.4 Principle arrangement for machine reading of characters as used for wafer and magnetic head identification
Fig.6 Arrangement for machine reading which gives good discrimination between the read-out signals
Fig.5 Optical read-out signal from a numeral on a wafer: a - without blocking of :he zero order intensity; b - with blocking of the zero order intensity
Fig.7 Multiple exposure of the diffraction characters with adjacent grid orientations
As shown in Fig.4, the diffraction patterns of the various grid orientations are detected by an array of sector photoelements. Two opposite elements work in parallel. The diameter of the diffracted beams is given by the diameter and divergence of the incident beam.
precision object adjustment and object motion during the recognition process and provides a high reliability.
By introducing a lens, diffracted beams can be focused to a very sharp diffraction pattern (Fig.6). This increases the number of encodable characters considerably. In our application the number of different characters was only ten and we did not need to do this. An unexpanded He-Ne laser beam with low divergence was adequate as a read-out beam. The ten different orientations of the diffraction pattern can be detected by an array of segmented photodiodes with an 18” discrimination. The electronic signal evaluation is described in a later section. One major advantage of the theta-modulation method is its xy shift invariance, which eliminates the problems of high
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patterns of three
Optical reader Application to wafer identification
Machine readability depends on the surface quality of the serialized parts. Excellent readability was achieved for numerals printed on the perfectly polished front sides of silicon wafers with their surfaces parallel to the crystallographic (111) and (100) planes. Because of the good quality of the grating, the diffraction patterns were well defined. The superimposed diffraction patterns of three symbols with adjacent grid orientations are shown in Fig.7. The picture was generated by a multiple exposure of three different patterns to show the clear discrimination between characters with adjacent grid orientations.
sity of the pattern is plotted WI-sus the angular position u
of the diode. It can be seen that the angular half-width of the diffraction pattern is small compared with the SKIOIangle of 1X”. It is also evident that the signal 10 noise ratio is up tv 20. This is defined as the ratio of the n~.~inlum signal intensity and the background inrcnsir!, r~~c:rsutcd I)!, the detector segments perpendicular to the dil’l‘ractioll pattern. It is possible to encode many I~OI-c tlla11 ten different characters with this type of p;ittern. One ~ultl think of using the capacity for ;I p;~r;~llcl IllllltipIc-11~1IIIct-;ll reading process, where the position of tllc numer:ll is also encoded.
Angular intensity distribution of the diffraction Fig.8 of three adjacent characters; these are about 18” apart
Theta-modulated I mm high digits. printed with ;I ncarl! circular shape, were cvaluatcd by reading with a He one laser beam 1 nm in diameter. According LOthe cspcrimcnta. the maximum reading signal for any ~ncodetl numeral WIS nearly independent of the grid orientation. Fig.0 shows the reading signals of the numeI-al 7 modulated with ten diffcrcnt grid orientations. Their m;Iyimum amplitudes arc equal to within f 3’X although the shape of the character 7 ha\ two preferential directions. The other nine digits. each encoded with ten different grid oricntationb. also gave the same read-out signals to within i 5”; (Fig. IO). Even though the appearance and non-oxidized their diffraction
Electrical read-out signals from a digit 7, printed Fig.9 front side of a wafer, for ten different grid orientations
of characters on oxidked surfaces differs in microscopic structure, patterns are similar. This can be seen in
Fig.1 I. A broken structure. which is more likely to be found on oxidized than on bare silicon. will naturally dcstroy the encoded information almost completely (Fig. 12). Cracks influence the diffraction pattern to a high degree. On the other hand it is not necessary for the signal to 1~ perfect as far as grid resolution is concerned. Fig. 1.3gives a11 example of this. A theta-lnodul~lted symbol was printed
Diffraction patterns and structures of characters on: Fig.1 1 a - a bare silicon wafer; b - a wafer with an oxide layer of 500 nm
Electrical read-out signals from ten digits on the front Fig.10 side of a wafer encoded by ten different grid orientations
For measuring the angular intensity distribution of the diffraction patterns Zn (u), a set-up was used in which a photo-diode could be moved in concentric circles around the zero order of the diffraction pattern in the detector plane. The photo-sensitive area was screened by masking tape leaving a slit with an angular resolution Au of about 3”. The experimental results are shown in Fig.8, where three adjacent characters are evaluated. The relative intcn-
Grid-modulated numeral on oxidized silicon with Fig.1 2 crystallographic cracks: a - structure; b - optical read-out signal
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Theta-modulated wafer areas with different edge Fig.13 definitions caused by having varying focus positions during printing
at different focus positions yielding the three prints a to c. Print a shows the usual quality of the printed crossbars, whereas in b and c the wafer surface is out of the focal plane of the imaging lens. The remarkable result is that even with a grid modulation at the limit of human visibility the diffraction pattern is still machine readable. The relative angular intensity distribution of the three patterns is plotted in Fig.14. Another result of this test is that a character only 0.5 mm x 0.5 mm in size with only six crossbars gave an excellent reading signal.
diagrams related to the patterns seen in Fig.1 3
An important question for implementation of the described reading method in an integrated circuit manufacturing process is that of how the visibility-and the reading signal are changed when the surface is covered with additional layers. To investigate this, a number of characters printed on a nonoxidized wafer were covered with a 500 nm aluminium layer. Apart from a slight overall surface roughness, no change in visibility due to the layer was detected. The surface roughness leads to a slight increase of the reading signal half-width as seen in Fig.1 5. The automatic readability is not affected at all. Therefore the serial number on the wafer will remain readable through all processing steps. Application
L A magnetic head is a part of the IBM 3330 disc store of System /370. Fig.16 shows the head slider. The core with the read-write gap is mounted in a future of barium titanate (white material). The dark material with the two lugs is a ferrite shielding body. The only location which permitted machine reading in the final head assembly was the space on the left and right-hand sides of the lug of the ferrite body.
Angle Read-out diagram of a printed character on bare silicon Fig.15 (dashed line) and after deposition of a 500 nm aluminium layer (solid line)
In order to place a seven digit serial number to give sufficient reading signals, the single characters had to be strongly stylized (Fig.17). The read-out light beam had to be shaped according to the 1.2 mm x 0.4 mm square shape of the digits. This was achieved by 1 : 1 imaging of an illuminated slit on the coded ferrite surface. The spacing between two characters was 0.1 mm. Close to the lug, space was left for an eighth numeral. While the previously described reading method worked excellently on wafers, it did not work in the case of serialized ferrite parts because of the rough surface structure of the material. The diffraction patterns were overlayed by an over strong background of light scattered from the surface. No preferential orientation of the diffraction
Slider of a magnetic head
Fig.1 7 Stylized head sliders
on IBM 3330
patterns was detectable. We therefore had to use optical means to produce optical information about the grid orientation in directions of low scatter. This was achieved by laser machining of grid lines which had the shape of cylindrical mirrors. Fig.1 8 shows an SEM micrograph of the rough ferrite surface and a theta-modulated 0 at three magnifications. The laser printing energy was adjusted to obtain shiny grooves in the ferrite material which act as cylindrical mirrors. The depth of the grooves is about 4 pm. The arrangement for machine reading of serialized ferrite parts is shown in Fig.1 9. The grid-coded digit is illuminated by the parallel reading beam. Superimposed on the diffracte d and scattered light is a reflected light pattern produced by the cylindrical mirrors which has a preferential orientation perpendicular to the grid lines. This preferential orientation was detected reliably by the photo-detector array which has a central hole large enough. to suppress the background of scattered light. Fig.20 shows a seven digit grid-modulated serial number. Fig.21 gives the angular read-out intensity distribution of the character 0; it has a half-width of 17” and a signal to noise ratio of 12 : 1.
Fig.18 SEM micrographs of the grid-modulated number 0 machined into the ferrite body of a magnetic head. The magnification is: a - 100x; b - 1 000x; c - 3 000x
digit of a 3330 magnetic head. In order to improve the signal to noise ratio two opposite photo-elements are connetted in parallel.
The optical part of the reader produces diffraction patterns with &ienta;ions characteristic of each numeral. A-serial number is read digit after digit. The optical reading signals are detected by an array of 20 separated sector photoelements. Fig.22 shows all the discrete electrical signals of the 20 photo-elements measured from ;I ttleta-rnodrll:lted
We measured the readability of a numeral by the discrimination ratio. This is defined as the ratio of the maximum reading signal generated by one of the photo-elements and the signal of an adjacent photo-element. Diffraction patterns from theta-rr~odut:lted digits on wafers are very sharp and
different reflectivity of the serialized parts; and variations of the reader laser output. Reflected
A measuring system was developed which can recognize orientations of diffraction patterns even at low discrimination ratios (of about 1.05) and which works independently of intensity variations of the maximum reading signal. This was achieved with the electronic reading system shown in Fig.23. Each of the ten pairs of silicon photo-elements (BP Y45) of the detector array provides an input signal to the reader electronics. After amplification the ten signals are fed into analogue OR circuits consisting of ten diodes Dr and one common resistor R. These circuits provide a common reference signal independent of the absolute value of the different input signals. The common reference signal adjusts itself to the maximum input signal. The amplified input signals are compared with the reference signals by ten comparators. Only the one comparator whose input signal is equal to the reference signal generates a read-out signal. This is used to address an optical display and is transferred to the computer after passing a DEC to BCD decoder.
Detector Fig.19 slider
reading from a magnetic
therefore have a high discrimination ratio of about 12 : 1. However, numerals on ferrite parts produce optical reading patterns with a broader angular distribution and a discrimination ratio of 2 : 3. Experimental results show that the maximum reading signals and the discrimination ratios differ from digit to digit because of: different sizes of the encoded areas; tolerances of the laser machining process;
serial number 438 0308
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In order to compensate for the diffusion voltage drop of 0.3 V by the germanium diodes Dr of the analogue OR circuits, the amplified input signals are fed to the comparators via identical diodes D,. These compensation diodes increase the discrimination sensitivity of the reader, since they make it possible to discriminate between smaller intensity differences from adjacent photo-elements. Fig.24 shows the reference voltage Vref and the amplified input signal Vsig as a function of the light intensity detected by the photo-elements, with and without the compensation diodes. The reader electronics is able to discriminate between intensity differences of about 5%. This means that all theta-modulated digits which give optical reading patterns with discrimination ratios larger than 1.05 : 1 can be read. Summary The main idea of the serialization method described was to combine human and machine readability by laser imprinting of theta-modulated serial numbers. Because of the easy reading method, which is based on the evaluation of
into the rough surface of the ferrite shield of an IBM 3330
Angular distribution of the optical read-out grid-modulated numeral 0 printed on ferrite materral
signal from a
Fig.24 Reference voltage V,,f and signal voltage Vsig as a function of the light intensity detected by the photo-elements: a ~ without compensation diodes; b - with compensation diodes
wafers and from the diffusely reflecting ferrite material ot magnetic head sliders has been demonstrated. Signal to noise ratios of 20: I and 12 : 1, respectively, have been measured. I
Electrical read-out signals measured from a thetaFig.22 modulated numeral of a 3330 magnetic head
Owing to the high quality of the characters printed OII silicon wafers, about 30 grid orientations or .JO different characters can be distinguished. The characters on wafers cm be as small as 0.5 mm x 0.5 mm in size and still give an excellent reading signal. Thin oxide and aluminium layers deposited after the character imprint do not affect eithcrthe human or the machine readability. Character reading was also possible in the case of scriah/ed ferrite parts. It was achieved by laser machining of gr-id lines which had the shape of cylindrical mirrors. L,ight reflected from these cylindrical mirror grooves rather than from a grating was used for reading. This method is success-
fully used at IBM for identification netic head production.
diffraction patterns of grid-modulated numerals, the serial number reader is simple and inexpensive. It includes an optical light source (laser or bulb), a photo-element detector array, a mechanical fixture and transport of the seriaked part, and an electronic recognition system with an optical display of the recognized numerals. Reliable reading of serial numbers from the polished front side of silicon
of ferrite parts in mag-
In conclusion, the described reading technique can be used for parts with polished surfaces if a diffraction grating C:III be printed onto the material, and for parts with rough surfaces if reflecting cylindrical mirrors cm be machined into the material. References A. W. Theta modulation
Armitage, J. D., Lohmann, ..lppl Opt 4 (1965) 399
Holzinger, G., Kosanke, K., Menz, W. l’rintin~ 01‘ p:rrt nutuhcr~ using a high power laser beam Opt l,ascr Tcrlr~~ol 5 (1973) 256 Born, M.. Wolf, E. Principle\ ol’ Optic\ (I’rrF:rmon I’re\\, 1970)
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