US3808067A

US3808067A - Method of controlling an etching process

Info

Publication number: US3808067A
Application number: US00309378A
Authority: US
Inventors: M Brown
Original assignee: Western Electric Co Inc
Current assignee: AT&T Corp
Priority date: 1972-11-24
Filing date: 1972-11-24
Publication date: 1974-04-30
Anticipated expiration: 1991-04-30

Abstract

In an etching process, the degree of etching of a workpiece is controlled by scanning a segment of the workpiece at an intermediate point in the process to derive a measure of the portion of the segment from which a surface layer has been removed. A variable parameter of the etching process, such as the conveyor speed or the etchant flow rate, is adjusted, according to the difference between the derived measure and a standard measure, in a direction tending to reduce the difference.

Description

[451 Apr. 30, 1974 United States Patent [191 Brown METHOD OF CONTROLLING AN ETCHING PROCESS [75] Inventor:

Primary Examiner-William A. Powell Attorney, Agent, or Firm-B. W. Sheffield Martin John Brown, Lambertville, NJ.

[73] Assignee: Western Electric Company,

ABSTRACT Incorporated, New York, N .Y.

Nov. 24, 1972 Appl. No.: 309,378

In an etching process, the degree of etching of a workpiece is controlled by scanning a segment of the work- [22] Filed:

piece at an intermediate point in the process to derive a measure of the portion of the segment from which a surface layer has been removed. A variable parameter of the etching process, such as the conveyor speed or the etchant flow rate, is adjusted, according to the difference between the derived measure and a standard 525 404 3 3 w a 5W 1 3 s w w 5 m l l. .n a m 3 e a 6 u m 5 n 1 mmfim L f C WM st e Umm 11]] 2 8 555 [[l measure, in a direction tending to reduce the difference.

References Cited UNITED STATES PATENTS 3,702,277 156/345 14 9 D'awmg F'gures W D m E F P s T R N A m H E m w E o C an RU TC mm C m mu LN DE R P E L F E R PATENTEHAPR 30 m4 sum 1 or 4 wIII k wozwmwmm H53 SBmdwm ETCHANT PATENTED APR 30 I974 CONTROL CIRCUIT PATENIEDAPR 3o IIIII REFERENCE SHEU 3 III-4 50 I 25 "*gg i SIGNAL I LEvEL NT DISCRIMINATOR ER I 53 1'\ pgNsTAtljTg I R P L E 30 GENERATOR 76 Q' P 56 I [Fa i *SEQUENCER v5? ENABLE COMPARATOR l 28 l DIGITAL I To INITIAL T T 66\ 70 I ANALOG AMPLIFIER VALUE 7| 7 X CONVERTER 3 I DECREMENTI INCREMENT INITIALIzE l I s7- ACCUMULATOR I POwER SOURCE 5 TO PUMP f TO CONVEYOR MOTOR l7 MOTOR 24 5 ;;9 INPUT FROM E 90 '45 5 SENSOR 25 g I OUTPUT TO I AND-GATE 52 I I i i fEBE'! i I m m l l 1 UPPER 9| E ,REFERENCE I 1 93 i i OUTPUT INPUT FROM 1 1 To SENSOR 25 I AND-GATE LOwER j i METHOD OF CONTROLLING AN ETCHING PROCESS BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to methods and apparatus for controlling a spray process for the treatment of workpieces, and more particularly, to methods and apparatus for regulating a spray-etching machine according to an analysis of the light reflected from a segment of the surface of a workpiece being etched.

2. Description of the Prior Art One step in the fabrication of a multi-layered, patterned workpiece, for example, an etched circuit board, is the removal by etching of unwanted portions of a surface layer of the workpiece to produce a desired pattern in the surface layer. Typically, predetermined regions of the workpiece are protected from the etchant by an etch-resistant layer called a resist. The workpiece is immersed in a bath, or conveyed through a spray, of a liquid etchant to remove those portions of the surface layer that are not protected by the resist. After etching is completed, the resist is removed.

An etchant spray typically etches faster than an etchant bath, because the spray constantly projects fresh etchant onto the surface being etched. Therefore, an etching machine preferably includes means for conveying workpieces past one or more spray nozzles, and means for'recirculating etchant from a sump beneath the conveying means to the spray nozzles.

A properly etched workpiece is neither underetched nor overetched. The workpiece is underetched if the etchant does not remove all unwanted portions of the surface layer of the workpiece. Conversely, the workpiece is overetched if the etchant undercuts the resist and removes part of the desired pattern. The degree of etching must be carefully regulated to prevent either underetching or overetching. In a spray-etching machine the degree of etching is typically regulated by varying the rate of etchant flow, and/or by varying the conveyor speed. A machine operator usually adjusts these variables manually, according to his visual inspection of etched workpieces leaving the sprayetching machine.

In a typical spray-etching machine, a batch of etchant is constantly recirculated from the sump to the spray nozzles. The chemical activity of the etchantbatch decreases as it becomes contaminated with the material being etched. Certain etchants also lose strength with time. Means can be provided to regenerate or replenish the etchant, but such means do not compensate completely for variations in etchant strength.

Variations in properties of the surface layer can also effect the degree of etching. For example, a copper surface layer can vary in thickness and in hardness. Different batches of copper can etch at significantly different rates because of such variations. Typically, it has hereto fore been necessary for the machine operator to readjust either the etchant flow or the conveyor speed to conpensate forthe variations in etchant strength and workpiece properties.

Obviously, it would be desirable to automatically control the degree of etching in a spray-etching machine without human intervention. To this end, it becomes necessary to generate a control signal that represents the actual degree of etching of the workpiece.

This control signal may then be used to regulate a variable parameter in the etching machine to achieve the desired degree of etching. Various means have been proposed for generating such a control signal. For example, it is known to sense light that is reflected or transmitted by a workpiece being etched, and to terminat e the etching process when the light reaches a certain intensity. However, it has been found difficult to adapt such light-sensing means to conveyorized sprayetching machines because of fluctuations in the intensity of the light caused by the opacity and reflectivity of the etchant. Such fluctuations in light intensity limit the area of the workpiece that can be satisfactorily sensed. In my invention, however, I disclose methods and apparatus that utilize scanning techniques for automatically controlling the degree of etching of workpieces in a spray-etching machine.

SUMMARY OF THE INVENTION According to the invention, a workpiece having a portion whose area is altered during etching is conveyed through a spray-etching machine having a vari-' able process parameter. A segment of the workpiece is Y scanned, at a location in the etching machine where the portions are being altered in area, to derive an electrical signal that represents the altered portion of the segment. The derived electrical signal is then compared with a predetermined electrical signal that represents the desired area of the altered portion at the scanning 7 location, the variable process parameter is adjusted, ac-

cording to. the difference between the derived and the predetermined electrical signals, in a direction that tends to reduce the difference, thereby regulating the degree of etching.

In a first, preferred embodiment, the variable process parameter is the rate of etchant flow. In a second embodiment, the variable process parameter is the speed at which the workpiece is conveyed through the etching process.

The scanning and comparison steps are accomplished with a reflected-light sensor and a control cit"- cuit. Alternate embodiments of the sensor and the control circuit are disclosed for use with workpieces having various reflectance characteristics, and for continuous workpieces. I

These and other features of the invention will be more fully understood from a consideration of the attached drawings and the following description of the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows a portion of a workpiece which can be fabricated using the methods and apparatus of the invention;

FIG. 2 is a partly schematic, partly diagrammatic representation of a spray-etching machine for use with discrete workpieces, which may be controlled by the method and apparatus of the invention;

FIG. 3 is a partly schematic, partly diagrammatic representation of a spray-etching machine for use with a continuous workpiece, which may be controlled by the method and apparatus of the invention;

FIG. 4 is a more detailed diagram of the reflected light sensor shown in FIGS. 2 and 3;

FIG. 5 is a more detailed diagram of a first, preferred, embodiment of a control circuit shown in FIGS. 2 and FIGS. 6 and 7 are more detailed diagrams of a signal level discriminator shown in FIGS. and 8;

FIG. 8 is a more detailed diagram of a second embodiment of the control circuit shown in FIGS. 2 and 3; and

FIG. 9 shows a modification of the control circuits shown in FIGS. 5 and 8 that may be used with the continuous workpiece shown in FIG. 3.

DETAILED DESCRIPTION Throughout the following description, identical reference numerals are used to identify identical elements in different figures.

FIG. 1 shows a portion of a workpiece 10 that can be fabricated using the instant invention. The workpiece 10 is comprised of a substrate 11 having pattern elements, such as elements 12 and 13 thereon, which are formed by etching away unwanted portions of a surface layer laminated to the substrate 11. A well-known example of such a workpiece is an etched circuit board in which the substrate 11 is an insulating material, such as polyester, phenolic resin, epoxy glass, or the like, and the pattern elements 12 and 13 are a conducting material, such as copper. During the fabrication of such a workpiece, the pattern elements are protected from the etchant used in the etching process by a layer of resist (not shown).

It is sometimes desirable to fabricate double-sided workpieces having pattern elements on both sides of a substrate.

Pattern elements

14 and 15 are indicated to illustrate such a double-sided workpiece. It is selfevident that the instant invention may be used to fabricate double-sided workpieces as well as single-sided workpieces; however, the invention will be described only for use with single-sided workpieces, unless otherwise stated.

Referring now to FIG. 2, the workpieces 10 are carried by a conveyor 18, which is driven by a motor 17, through an etching zone 20, which comprises a plurality of etchant sprays 21. The workpieces 10 can be conveyed horizontally, as shown, or in any other convenient orientation. Etchant from the sprays 21 is collected in a sump 22. An etchant pump 23, which is driven by a motor 24, circulates the etchant from the sump 22 to the sprays 21.

A sensor 25 scans a passing workpiece 10 by illuminating a region 26 of the workpiece, responding to the light that is reflected from the region 26, and transmitting an analog signal, which represents the magnitude of the reflected light, to a control circuit 28. A presence sensor 30 proximate the sensor 25 transmits a digital signal, which is generated by photodetecting means or the like, to the control circuit 28 when one of the workpieces 10 is within the scanning range of the sensor 25. For clarity, the workpiece 10 that is within scanning range of the sensor 26 will be designated as scanned workpiece 16. Each of the workpieces 10 becomes the scanned workpiece 16, in turn, as it is conveyed past the

sensors

25 and 30.

The control circuit 28 processes the reflected light signal from the sensor 25 to determine the degree of etching of the scanned workpiece 16. In a first preferred embodiment, the control circuit 28 then regulates the degree of etching by varying the rate of etchant flow, with a conveyor speed held constant. In a second embodiment, the control circuit 28 then regulates the degree of etching by varying the conveyor speed,

with the rate of etchant flow held constant. The control circuit 28 will be explained more fully in the descriptions of FIGS. 5 and 8.

The workpieces 10 have been shown and described as discrete workpieces. However, it will be clear that the invention can also be used to fabricate continuous, web-like workpieces, such as a continuous workpiece 35, shown in FIG. 3, which may include repeated pattern elements 36. The continuous workpiece is moved from supply means to takeup means (both not shown) and through the etching zone 20 by the motor 17 in an anlogous manner to the movement of the discrete workpieces 10 in FIG. 2.

FIG. 4 is a diagrammatic representation of the sensor 25. A lamp 40 is placed near the end of an outer light pipe 41, which conducts light from the lamp 40 to illuminate the region 26 of the scanned workpiece 16. An inner light pipe 43 is mounted concentrically with a portion of the outer light pipe 41 to conduct light reflected from the region 26 to the photodetector 44. The

light pipes

41 and 43 are preferably fabricated from fused silica (quartz) for maximum light transmission. The inner light pipe 43 is coated with a thin, reflecting film to minimize the direct passage of light from the outer light pipe 41 to the inner light pipe 43. The annular space between

light pipes

41 and 43 is filled with an etchant resistant epoxy resin to prevent etchant from penetrating between the light pipes. The sensor 25 is enclosed within a housing (not shown) which provides means for mounting the sensor at the correct position with respect to the scanned workpiece 16. A circuit 46, which can include signal amplifying and conditioning means, prepares the output signal from the photodetector 44 for transmission as an analog signal to the control circuit 28.

The sensor 25 can be mounted in the path of the etchant sprays 21, as shown in FIG. 2, or in an area protected from etchant. Some etchants, such as ferric chloride, transmit little light; therefore, it is preferable to prevent such etchants from intervening between the sensor 25 and the scanned workpiece 16. Advantageously, the sensor 25 is mounted close to the scanned workpiece 16 with the concentric ends of the

light pipes

41 and 43 positioned about 0.125 inch from the surface of the workpiece. A deflector (not shown) can also be added to direct etchant away from the sensor.

As the scanned workpiece 16 is carried past the sensor 25, the region 26 moves with respect to the workpiece, so that the sensor 25 scans a segment 45 of the workpiece. The sensor 25 is positioned so that the pattern in the segment 45 is representative of the pattern on the workpiece as a whole.

The surface of one of the workpieces 10 typically comprises three types of areas: (I incompletely etched surface layer areas, (2) resist areas and (3) exposed substrate areas. A surface layer area becomes an exposed substrate area when the overlying surface layer is completely etched away. Because these three types of areas typically have different reflectivity characteristics, the sensor 25 can usually be constructed to provide an analog output signal that is within a different range for each type of area.

EXAMPLE 1 A first sensor 25 was constructed to sense an etched circuit card that comprised a light-colored synthetic substrate, a copper surface layer and a solder resist. In this sensor, the lamp 40 was an incandescent lamp operated at low power to enhance red emission, and the photodetector 44 was a Texas Instruments LS-400 photodiode. Since copper reflects red light optimally, compared to the light-colored substrate or the solder resist, and since the LS-40O photodiode is particularly sensitive to red light, the analog output signal from the sensor was within a maximum range when the region 26 was copper, and was within lesser ranges when the region 26 was exposed substrate or the resist. Alternatively, if the incandescent lamp had been operated at full power, a red filter could have been placed in the optical path of the sensor 25, for example, between the lamp and the light pipe 41.

EXAMPLE 2 A second sensor 25 was constructed to sense a memory card that comprised a highly reflective aluminum substrate, a poorly reflective magnetic alloy surface layer, and a slightly reflective resist. In the second sensor 25 the lamp 40 was an incandescent lamp operated at full power, and the photodetector 44 was again a Texas Instruments LS-40O photodiode. The analog output signal from the sensor 26 was within a maximum range when. the region 26 was a substrate area, a medium range when the region 26 was a resist area, and a minimum range when theregion 26 was a magnetic alloy area.

Referring briefly to FIG. 2, the sensor 25 is positioned at a location in the etching zone 20 where some parts of the unprotected regions on the scanned workpiece 16 are exposed substrate, and other parts of the unprotected regions are incompletely etched surface layer. At this location, the areas of the incompletely etched surface layer parts and the exposed substrate parts are being altered by the etching process. The proportion of surface layer area to' exposed substrate area in the unprotected regions is an indication of the degree of etching. If the scanned workpiece 16 is being etched at too fast a rate, a relatively larger proportion of the unprotected regions will be exposed substrate; conversely, if the scanned workpiece 16 is being etched at too slow a rate, a relatively smaller proportion of the unprotected regions will be exposed substrate. The degree of etching can be detennined, therefore, by measuring either the area of exposed substrate or the area of incompletely etched surface layer in the segment 45 of the scanned workpiece 16 at the scanning location.

FIG. 5 is a more detailed block diagram of a first, preferred, embodiment of the control circuit 28 shown in FIG. 2. Referring to FIG. 5, the signal from the sensor 25 is connected to the input of a signal-level discriminator 50. The output of the signal level discriminator is connected to the first input of an AND-gate 52, and the output of a constant-rate pulse generator 53 is connected to a second input of the AND-gate 52. The output of the AND-gate 52 is connected to the count input of a counter 55. The output of the counter 55 is connected by a data path 56 to a first input of a comparator 57. A standard count is stored in a switch register, or other means (not shown) that is connected by a data path 61 to a second input of the comparator 57. A first output of the comparator 57, which is activated when the comparator'is enabled and the accumulated count in the counter 56 is less than the standard count,

is connected by the data path 66 to a decremenf input of an accumulator 67. A second output of the comparator 57, which is activated when .the comparator is enabled and the accumulated count in the counter 56 is greater than the standard count, is connected by the data path 67 to an increment input of the accumulator 67. An initializing means (not shown) is connected by a data path 71 into the accumulator 67. The output of the accumulator 67 is connected by a data path 72 to a digital-to-analog (D/A) converter 73. The output of the D/A converter is connected to an amplifier 75, which, in the preferred embodiment, is connected to drive the pump motor 24. The presence detector 30 is connected to a third input of the AND- gate 52, and to the input of a sequencer 76. First and second outputs of the sequencer 76 are connected to an enable" input of the comparator 57 and a reset input of the counter 55, respectively. A power source 77 is connected to the conveyor motor 17.

It will be necessary to refer to digital logic levels in the remaining description. For brevity, a zero logic level will be designated as 0, and a one logic level will be designated as l.

The signal level discriminator 50 compares the analog signal from the sensor 25 with one or more reference levels that are chosen to define the range within which the analog signal represents the type of surface area to be measured. The output from the signal level discriminator is 1 when the analog signal is within the defined range, and 0 when the analog signal is outside the defined range.

When the defined range is at the upper or lower end of the analog signal range, the signal level discriminator 50 can be a well-known analog comparator 90, as shown in FIG. 6. When the defined range is in the middle of the analog signal range, the signal level comparator 50 can be a combination of two

comparators

91 and 92, as shown in FIG. 7.

In FIG. 6, the analog signal on the input lead is connected to the terminal of the comparator 90, and a reference signal is connected to the terminal of the comparator. The output of the comparator is then 0 when the input signal is less than the reference signal, and 1 when the input signal is greater than the reference signal. Alternatively, if the reference signal were connected to the terminal of the comparator and the input signal were connected to the terminal of the comparator, the output of the comparator would be 0 when the input signal is greater than the reference signal, and 1 when the input signal is less than the reference signal.

For use with the workpiece described in Example 1, the comparator 90 shown in FIG. 5 can be used for the signal level discriminator 50. The reference level is set near the lower end of the maximum range within which the analog signal from the sensor 25 indicates that the region 26 (FIG. 4) is copper. Thus, the output of the comparator 90 is 1 when the region 26 is copper, and 0 otherwise.

For use with the workpiece described in Example 2, the comparator 90 can also be used, but with the reference level connected to the terminal of the comparator and the input connected to the terminal of the comparator. The reference level is set near the upper end of the range within which the analog signal from the sensor 25 indicates that the region 26 is magnetic alloy. Thus, the output of the comparator 90 is 1 when the region 26 is magnetic alloy, and otherwise.

In FIG. 7, the analog signal on the input lead is connected to,the terminal of the comparator 91 and the terminal of the comparator 92. An upper reference level is connected to the terminal of the comparator 91, and a lower reference level is connected to the terminal of the comparator 92. The outputs of the

comparators

91 and 92 are connected to the inputs of an AND-gate 93.

When the analog signal is less than the lower reference level, the outputs of the

comparators

91 and 92 are l and 0, respectively, so that the output of the AND-gate 93 is 0. When the analog signal is between the lower and the upper reference levels, the outputs of both

comparators

91 and 92 are l, and the output of the AND-gate 93 is 1. When the analog signal is greater than the upper reference level, the outputs of the

comparators

91 and 92 are 0 and 1, respectively, so that the output of the AND-gate 93 is again zero.

It will be clear that it is also possible, and may be desirable, to construct a particular sensor 25 and a particular signal level discriminator 50 so that the output of the discriminator 50 is 1 when the area 26 is exposed substrate, instead of incompletely etched surface layer.

Referring again to FIG. 5, in the operation of the first embodiment of the control circuit 28, the power supply 77 drives the conveyor motor 17 at an essentially constant speed. The presence sensor 30 applies 1 to both the AND-gate 52 and the sequencer 76 when the scanned workpiece 16 is within range of the sensor 25, and 0 to both the AND-gate 52 and the sequencer 76 otherwise. For this explanation, assume that the sensor 25 and the signal level discriminator 50 are constructed so that the signal level discriminator 50 applies 1 to the AND-gate 52 when the region 26 is incompletely etched surface layer. The constant-rate pulse generator 53 applies periodically repeated 1 pulses to the AND- gate 52.

As the scanned workpiece 16 (FIG. 2) passes the

sensors

25 and 30, the sensor 25 scans the segment 45 (FIG. 4) and provides a varying analog signal to the signal level discriminator 50. When the region 26 is incompletely etched surface layer, the signal level discriminator applies 1 to the AND-gate 52. When the inputs to the AND-gate 52 from the signal level discriminator 50 and the sensor 30 are both I, the AND-gate 52 enables pulses from the pulse generator 53 to reach the count input of the counter, Since the scanned workpiece 16 is moved at an essentially constant speed, and the repetition rate of the pulses from the pulse generator 53 is constant, the accumulated count in the counter 55 represents the area of the scanned strip 45 (FIG. 3) that is incompletely etched surface layer.

It will be clear that the workpieces must be guided with respect to the sensor by means not shown so that the same portion of the pattern on each scanned workpiece 16 is scanned by the sensor 25.

The standard count is the value that would accumulate in the counter 55 if the scanned workpiece 16 were etched to the desired degree. If the scanned workpiece 16 is overetched, the accumulated count therefor is less than the standard count, because a lesser area of the scanned strip 45 thereon is incompletely etched surface layer. Conversely, if the scanned workpiece 16 is underetched, the accumulated count therefor is greater than the standard count, because a greater area of the scanned strip 45 thereon is incompletely etched surface layer.

When the trailing edge of the scanned workpiece 16 passes the presence sensor 30, the signal from the presence sensor changes from 1 to 0. This change triggers the sequencer 76 to apply a first pulse to the enable input of the comparator 57 followed by a second pulse to the reset input of the counter 55. The comparator 57, when enabled, compares the accumulated count in the counter 55 with the standard count, and activates either an output on the data path 66 to decrement the accumulator 67, if the accumulated count is less than the standard value, or an output on the data path to increment the accumulator 67, if the accumulated count is greater than the standard count. The second pulse from the sequencer 76 resets the counter 55 to an initial count, which is typically zero.

The signals transmitted over the

data paths

66 and 70 to the accumulator 67 can indicate the magnitude, as well as the sign, of the difference between the accumulated count and the standard count, and the accumulator can be incremented or decremented by the magnitude of the difference. Thus, a large difference between the accumulated count and the standard count, indicating a wide variation between the actual degree of etching of the scanned workpiece and the desired degree of etching thereof, results in a large change in the contents of the accumulator 67; conversely, a small difference results in a proportionately smaller change. Alternatively, the signals transmitted over the

data paths

66 and 70 can increment or decrement the accumulator 67 by fixed amounts that are not related to the actual difference between the accumulated count and the standard count.

An initial value is set into the accumulator 67 over the data path 71 to establish an initial etchant flow when the spray etching machine is first started. The initial value is typically estimated by considering the amount of material that is to be etched from the workpieces and the known strength of the etchant.

The digital value stored in the accumulator 67 is converted to an analog value by the D/A converter 73. The analog signal is then amplified by the amplifier 75 to drive the etchant pump 23. The value in the accumulator 67 thus represents the rate of etchant flow.

The resulting operation of the first embodiment of the control circuit 28 is such that the rate of etchant flow is decreased when the scanned workpiece 16 is overetched, as indicated by the accumulated count therefor being less than the standard count; and the rate of etchant flow is increased when the workpiece is underetched, as indicated by the accumulated count therefor being greater than the standard count.

A second embodiment of the control circuit 28 is shown in FIG. 8, which is generally similar to FIG. 5.

However, as shown in FIG. 8, the first output of the comparator 57 is connected by a data path 81 to the increment input of the accumulator 67, and the second output of the comparator 57 is connected by a data path 82 to the decrement input of the accumulator 67. The output of the D/A converter 73 is connected to an amplifier 76 which is further connected to the conveyor motor 17. A power source 78 is connected to the pump motor 24. The output of the amplifier 76 is also connected to a variable-rate pulse generator 54.

In the operation of the second embodiment, the degree of etching is controlled by varying the conveyor speed, and holding the etchant flow constant. The value in the accumulator 67 thus represents the conveyor speed. The power source 78 drives the pump motor 24 to maintain a substantially constant rate of etchant flow, and the amplifier 76 drives the conveyor motor 17 and regulates the repetition rate of the pulse generator 54 to be proportional to the speed of the conveyor motor 17. Thus, the number of pulses generated by the pulse generator 54 is the same for each workpiece, regardless of the speed of the conveyor, and the count accumulated by the counter 55 is independent of the conveyor speed.

The comparator 57 activates either an output on the data path 81, to increment the accumulator 67 when the accumulated count in the counter 55 is less than the standard count; or an output on the data path 82, to increment the accumulator 67 when the accumulated count in the counter 55 is greater than the standard count. Again, the accumulator 67 can be incremented or decremented by either the magnitude of the difference between the accumulated count and the standard count, or a fixed amount.

The resulting operation of the second embodiment of the control circuit 28 is such that the conveyor speed is increased when the scanned workpiece 16 is overetched, as indicated by the accumulated count therefor being less than the standard count; and the conveyor speed is decreased when the scanned workpiece 16 is underetched, as indicated by the accumulated count therefor being greater than the standard count.

If it is desired to construct the sensor 25 and the signal level discriminator 50 so that the output of discriminator 50 is I when the area 26 is exposed substrate, instead of incompletely etched circuit layer, the connections between the comparator 57 and the accumulator 67 must be the reverse of those described above. In this situation, if the spray-etching machine is to be regulated by varying the etchant flow, the first embodiment of the control circuit 28, as shown in FIG. 5, is used; however, the comparator 57 is connected to the accumulator 67 by the

data paths

81 and 82, as shown in FIG. 8. Similarly, if the spray-etching machine is to be regulated by varying the conveyor speed, the second embodiment of the control circuit 28, as shown in FIG. 8, is used; however, the comparator 57 is connected to the accumulator 67 by the

data paths

66 and 70, as shown in FIG. 5.

Referring again to FIG. 3, the workpiece can be replaced, as explained previously, by a continuous workpiece 35. The presence sensor 30 can respond to index marks (not shown) on the continuous workpiece that correspond to each appearance of therepeated pattern, so that similar workpiece areas are repeatedly scanned. Alternatively, the presence sensor 30 can be replaced by a sampling clock 81, in the control circuit 28, as shown in FIG. 9. The clock 81 can be used with either the first embodiment of the control circuit 28, shown in FIG. 5, or the second embodiment of the control circuit 28, shown in FIG. 8. The clock 81 generates periodic sampling pulses, which simulate the signal that is received from the presence detector 30 when either discrete workpieces are etched or index marks are used on the continuous workpiece. However, the 'clock 81 can be used only when the pattern on the continuous workpiece is homogeneous, so that each resulting sampled area of the continuous workpiece comprises essentially the same mix of the three types of surface areas, and a standard count can be established for a typical sampled area.

If it is desired to etch both sides of workpieces 10 in the apparatus of FIG. 2, a second set of sprays (not shown) can be used in conjunction with a suitable conveyor to etch the second side of the workpiece 10 opposite the first side etched by etchant from the sprays 21. Preferably, the degree of etching of the second side is controlled separately from that of the first side. The conveyor speed is obviously common to both sides of the workpieces, so this variable is not convenient for such separate control. Thus, the rate of etchant flow is appropriately used as the variable for control, requiring the addition of a pump, pump motor, sensors and a control circuit (all not shown) for the second set of sprays. Analogously, both sides of a continuous workpiece, as shown in FIG. 3, can be etched.

The elements of both embodiments of control circuit 28 are all well known in the art. The counter 55 is preferably a binary counter, and the comparator 57, accumulator 67, and D/A converter 73 are also preferably binary devices. The

amplifiers

75 and 76 are conventionally designed to drive the pump motor 24 and the conveyor motor 17, respectively. Many possible configurations of such an amplifier are known in the art. The comparators shown in FIGS. 6 and 7 are again well known. The operation and application of elements such as those shown in FIGS. 5-9, inclusive, are described in greater detail in Arithmetic Operations in Digital Computers, by R. K. Richards, D. Van Nostrand Company, Inc., 1955.

In the following claims, a properly etched workpiece is a workpiece that is neither overetched nor underetched.

One skilled in the art may make changes and modifications to the embodiments of the invention disclosed herein, and may devise other embodiments, without departing from the spirit and scope of the invention.

What is claimed is: l. A method of regulating the degree of etching of a workpiece being conveyed through an etching process, the etching process having at least one variable process parameter that affects the degree of etching, the workpiece having a portion whose area is altered during etching, which comprises:

scanning a segment of the workpiece, at a location in the etching process where the portion is being altered, to derive a signal representating the area of the altered portion within the segment;

comparing the derived signal with a predetermined signal representing the desired area of the altered portion at the scanning location; and

adjusting a variable process parameter, according to the result of the comparing step, in a direction that tends to reduce the difference between the derived signal and the predetermined signal, to regulate the degree of etching.

2. The method of claim 1 wherein the scanning step further comprises:

directing a beam of light onto the segment as the segment passes the scanning location;

generating an electrical signal directly related to the intensity of the light reflected from the segment;

generating electrical pulses at a repetition rate proportional to the speed at which the workpiece is conveyed past the scanning location; and

counting the electrical pulses when the reflected light signal indicates that the light beam is reflected from the altered portion, to thereby generate the derived signal.

3. The method of claim 1 wherein the altered portion is a surface layer that decreases in area during etching, the variable process parameter is the rate of etchant flow, and the adjusting step further comprises:

increasing the rate of etchant flow when the derived signal is greater than the predetermined signal, and

decreasing the rate of etchant flow when the derived signal is less than the predetermined signal.

4. The method of claim 1 wherein the altered portion is a surface layer that decreases in area during etching, the variable process parameter is the speed at which the workpiece is conveyed through the etching process and the adjusting step further comprises:

increasing the conveying speed when the derived signal is less than the predetermined signal, and decreasing the conveying speed when the derived signal is greater than the predetermined signal.

5. The method of claim 1 wherein the altered portion is exposed substrate that increases in area during etching, the variable process parameter is the rate of etchant flow, and the adjusting step further comprises:

increasing the rate of etchant flow when the derived signal is less than the predetermined signal, and decreasing the rate of etchant flow when the derived signal is greater than the predetermined signal.

6. The method of claim 1 wherein the altered portion is exposed substrate that increases in area during etching, the variable process parameter is the speed at which the workpiece is conveyed through the etching process and the adjusting step further comprises:

increasing the conveying speed when the derived signal is greater than the predetermined signal, and decreasing the conveying speed when the derived signal is less than the predetermined signal.

7. In an etching process, a method of determining the degree of etching of a workpiece having a surface layer portion that is decreased in area during etching, which comprises:

scanning a segment of the workpiece, at a location in the etching process where the surface layer portion is being decreased in area, to derive a signal representing the area of the surface layer portion within the segment; and

comparing the derived signal with a predetermined signal representing the desired area of the surface layer portion at the scanning location to indicate that, at the scanning location, the workpiece is underetched when the derived signal is greater than the predetermined signal, properly etched when the derived signal substantially equals the predetermined signal, and overetched when the derived signal is less than the predetermined signal.

8. In an etching process, a method of determining the degree of etching of a workpiece having an exposed substrate portion that is increased in area during etching, which comprises:

scanning a segment of the workpiece, at a location in the etching process where the exposed substrate portion is being increased in area, to derive a signal 9. Apparatus for regulating the degree of etching of a workpiece conveyed through an etching process having at least one variable process parameter affecting the degree of etching, the workpiece having a portion whose area is altered during etching, which comprises:

means for scanning a segment of the workpiece, at a location in the etching process where the portion is being altered, to determine a value representing the area of the altered portion within the segment;

means for comparing the determined value with a standard value for the segment; and

means for adjusting a process parameter, according to the output of the comparison means, in a direction tending to reduce the difference between the determined value and the standard value.

10. The apparatus of claim 9 in which the workpiece is conveyed through the etching machine at a constant speed, the variable process parameter is the rate of etchant flow, and the means for scanning comprises:

photodetector means, positioned at the scanning location to receive light reflected from the segment,

for generating an electrical signal directly related to the intensity of the reflected light;

a discriminator connected to the photodetector for generating an output signal when the signal from the photodetector is within a range indicating that the light is reflected from the altered portion of the segment;

a pulse generator for generating pulses at a constant repetition rate; and

a counter, connected to both the pulse generator and the discriminator, for counting the pulses from the pulse generator when the output signal from the discriminator is present, the count accumulated in the counter being the value representing the area of the altered portion within the segment.

11. The apparatus of claim 9 in which the rate of etchant flow in the etching process is constant, the variable process parameter is the speed at which the workpiece is conveyed through the etching process, and the means for scanning comprises:

photodetector means, positioned at the scanning location to receive light reflected from the segment, for generating an electrical signal directly related to the intensity of the reflected light;

a discriminator connected to the photodetector for activating an output signal when the signal from the photodetector is within a range indicating that the light is reflected from the altered portion of the segment; means for generating pulses at a repetition rate proportional to the conveying speed; and

a counter,. connected to both the pulse generating means and the discriminator, for counting the pulses from the pulse generating means when the output signal from the discriminator is present, the count accumulated in the counter being the value representing the area of the altered portion within the segment.

12. Apparatus for determining the degree of etching of a workpiece in an etching process, the workpiece having a surface layer portion that is decreased in area during etching, which comprises:

means for scanning a segment of the workpiece, at a location in the etching process where the surface layer portion is being decreased in area, to determine a value representing the area of the surface layer portion within the segment;

means for indicating that the workpiece is overetched when the determined value is less than the standard value, properly etched when the determined value substantially equals the standard value, and underetched when the determined value is greater than the standard value.

13. Apparatus for determining the degree of etching of a workpiece in an etching process, the workpiece having an exposed substrate portion that is increased in area during etching, which comprises:

means for scanning a segment of the workpiece, at a location in the etching process where the exposed substrate portion is being increased in area, to determine a value representing the area of the exposed substrate portion within the segment;

means for indicating that the workpiece is overetched when the determined value is greater than the standard value, properly etched when the determined value substantially equals the standard value, and underetched when the determined value is less than the standard value.

14. A method of controlling the selective removal of a layer of material from a workpiece comprising the steps of:

subjecting the workpiece to a variable rate etching process,

scanning a segment of the workpiece at an intermediate point in the process to derive a measure of the portion of the segment from which the material has been removed.

comparing the derived measure with a standard measure representing the desired amount of material removal at said intermediate point, and

varying the etching rate in accordance with the comparison at said'intermediate point.

L-566-PT UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 2.808.067 Dated April 30, 197M lnventor(s) Martin John Brown It IS certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

[- In the specification, Column 1, line 55, effect" should l read --affect--. Column 5, line 11, "was exposed should i read --was the exposed". Column 7, line 50, "the count input" should read --the "count" input-H Column 8, lines 7-8, "the enable input" should read -the enable" input---; line 9, "the reset input" should read --the "reset" input--.

In th l Column line n m r Should read --removed,--. I

Signed and sealed this 24th day of September 1974.

(SEAL) Attest:

McCOY M. GIBSON JR. C. MARSHALL DANN Attesting Officer Commissioner of Patents

Claims

2. The method of claim 1 wherein the scanning step further comprises: directing a beam of light onto the segment as the segment passes the scanning location; generating an electrical signal directly related to the intensity of the light reflected from the segment; generating electrical pulses at a repetition rate proportional to the speed at which the workpiece is conveyed past the scanning location; and counting the electrical pulses when the reflected light signal indicates that the light beam is reflected from the altered portion, to thereby generate the derived signal.
3. The method of claim 1 wherein the altered portion is a surface layer that decreases in area during etching, the variable process parameter is the rate of etchant flow, and the adjusting step further comprises: increasing the rate of etchant flow when the derived signal is greater than the predetermined signal, and decreasing the rate of etchant flow when the derived signal is less than the predetermined signal.
4. The method of claim 1 wherein the altered portion is a surface layer that decreases in area during etching, the variable process parameter is the speed at which the workpiece is conveyed through the etching process and the adjusting step further comprises: increasing the conveying speed when the derived signal is less than the predetermined signal, and decreasing the conveying speed when the derived signal is greater than the predetermined signal.
5. The method of claim 1 wherein the altered Portion is exposed substrate that increases in area during etching, the variable process parameter is the rate of etchant flow, and the adjusting step further comprises: increasing the rate of etchant flow when the derived signal is less than the predetermined signal, and decreasing the rate of etchant flow when the derived signal is greater than the predetermined signal.
6. The method of claim 1 wherein the altered portion is exposed substrate that increases in area during etching, the variable process parameter is the speed at which the workpiece is conveyed through the etching process and the adjusting step further comprises: increasing the conveying speed when the derived signal is greater than the predetermined signal, and decreasing the conveying speed when the derived signal is less than the predetermined signal.
7. In an etching process, a method of determining the degree of etching of a workpiece having a surface layer portion that is decreased in area during etching, which comprises: scanning a segment of the workpiece, at a location in the etching process where the surface layer portion is being decreased in area, to derive a signal representing the area of the surface layer portion within the segment; and comparing the derived signal with a predetermined signal representing the desired area of the surface layer portion at the scanning location to indicate that, at the scanning location, the workpiece is underetched when the derived signal is greater than the predetermined signal, properly etched when the derived signal substantially equals the predetermined signal, and overetched when the derived signal is less than the predetermined signal.
8. In an etching process, a method of determining the degree of etching of a workpiece having an exposed substrate portion that is increased in area during etching, which comprises: scanning a segment of the workpiece, at a location in the etching process where the exposed substrate portion is being increased in area, to derive a signal representing the area of the exposed substrate portion within the segment; and comparing the derived signal with a predetermined signal representing the desired area of the exposed substrate portion at the scanning location to indicate that, at the scanning location, the workpiece is overetched when the derived signal is greater than the predetermined signal, properly etched when the derived signal substantially equals the predetermined signal, and underetched when the derived signal is less than the predetermined signal.
9. Apparatus for regulating the degree of etching of a workpiece conveyed through an etching process having at least one variable process parameter affecting the degree of etching, the workpiece having a portion whose area is altered during etching, which comprises: means for scanning a segment of the workpiece, at a location in the etching process where the portion is being altered, to determine a value representing the area of the altered portion within the segment; means for comparing the determined value with a standard value for the segment; and means for adjusting a process parameter, according to the output of the comparison means, in a direction tending to reduce the difference between the determined value and the standard value.
10. The apparatus of claim 9 in which the workpiece is conveyed through the etching machine at a constant speed, the variable process parameter is the rate of etchant flow, and the means for scanning comprises: photodetector means, positioned at the scanning location to receive light reflected from the segment, for generating an electrical signal directly related to the intensity of the reflected light; a discriminator connected to the photodetector for generating an output signal when the signal from the photodetector is within a range indicating that the light is reflected from the altered portion of the segment; a pulse generator for generating pulsEs at a constant repetition rate; and a counter, connected to both the pulse generator and the discriminator, for counting the pulses from the pulse generator when the output signal from the discriminator is present, the count accumulated in the counter being the value representing the area of the altered portion within the segment.
11. The apparatus of claim 9 in which the rate of etchant flow in the etching process is constant, the variable process parameter is the speed at which the workpiece is conveyed through the etching process, and the means for scanning comprises: photodetector means, positioned at the scanning location to receive light reflected from the segment, for generating an electrical signal directly related to the intensity of the reflected light; a discriminator connected to the photodetector for activating an output signal when the signal from the photodetector is within a range indicating that the light is reflected from the altered portion of the segment; means for generating pulses at a repetition rate proportional to the conveying speed; and a counter, connected to both the pulse generating means and the discriminator, for counting the pulses from the pulse generating means when the output signal from the discriminator is present, the count accumulated in the counter being the value representing the area of the altered portion within the segment.
12. Apparatus for determining the degree of etching of a workpiece in an etching process, the workpiece having a surface layer portion that is decreased in area during etching, which comprises: means for scanning a segment of the workpiece, at a location in the etching process where the surface layer portion is being decreased in area, to determine a value representing the area of the surface layer portion within the segment; means for comparing the determined value with a standard value for the segment; and means for indicating that the workpiece is overetched when the determined value is less than the standard value, properly etched when the determined value substantially equals the standard value, and underetched when the determined value is greater than the standard value.
13. Apparatus for determining the degree of etching of a workpiece in an etching process, the workpiece having an exposed substrate portion that is increased in area during etching, which comprises: means for scanning a segment of the workpiece, at a location in the etching process where the exposed substrate portion is being increased in area, to determine a value representing the area of the exposed substrate portion within the segment; means for comparing the determined value with a standard value for the segment; and means for indicating that the workpiece is overetched when the determined value is greater than the standard value, properly etched when the determined value substantially equals the standard value, and underetched when the determined value is less than the standard value.
14. A method of controlling the selective removal of a layer of material from a workpiece comprising the steps of: subjecting the workpiece to a variable rate etching process, scanning a segment of the workpiece at an intermediate point in the process to derive a measure of the portion of the segment from which the material has been removed. comparing the derived measure with a standard measure representing the desired amount of material removal at said intermediate point, and varying the etching rate in accordance with the comparison at said intermediate point.