Treating Articles For A Controlled Duration

Abstract

In a method and apparatus for baking photoresist on semiconductor slices, the length of baking time is controlled independently of the number of slices being baked or the position of the first slice placed on a rotary baking table. The position of the first slice is detected photoelectrically and marked with a magnet. The magnet actuates a combination selector-stepping switch each revolution of the rotary table. Continuity through the switch after a preselected number of revolutions initiates slice removal and, thereby, controls the baking time. BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to methods and apparatus for treating articles for a controlled duration and, more particularly, for baking photoresist on semiconductor slices for a predetermined time. 2. Description of Prior Art In the manufacture of semiconductor devices, slices of semiconductor material must be coated with a photosensitive material or a photoresist which is exposed to an image of the device or devices to be reproduced in or on the semiconductor slice. The photoresist is then developed and baked so that the portions remaining after development become a very adherent mask. This mask protects portions of the semiconductor slice in the shape of a desired image. In prior methods, the photoresist was baked on the slices by placing the slices on a baking means, such as a hot plate which had reached a predetermined stable temperature and setting a buzzer to signal the time when the slices should be removed from the hot plate. That is, the slices were manually loaded on the hot plate, manually timed, and manually unloaded. Such manual handling of slices is undesirable because of potential damage that may be caused by it. Also, this manual handling is costly and manual timer setting is subject to mistakes. It is desirable, therefore, to automate the loading of slices onto the baking means, the control of the baking time, and the unloading of the slices. This is most easily done by making the baking means rotate and yield the advantage of automatically returning any slices placed on it to the starting point to be removed. Some prior art methods control the number of revolutions or length of time by means of a magnet fixed to a disc, a sealed contact, and a counter-timer. The magnet operates the sealed contact and a counter counts during the time interval which a timer permits. However, where it is possible to rotate a baking means at constant speed, the length of baking time may be governed by controlling the number revolutions of the baking means. This yields the further advantage of eliminating the timer as such. As is well known in the semiconductor and thin film art, the photoresist must be baked to achieve the adherence and toughness necessary to withstand the processing involved in the manufacture of thin film and semiconductor devices. The adherence and toughness improve with length of baking time; so much so, that the photoresist becomes difficult to remove if baked too long. There is an advantage, therefore, to limiting the baking to some predetermined amount which provides the best compromise between adherence and ease of removal. This advantage is achieved by identifying the location of the first slice placed on the baking means so that the baking time of it and all other slices following it in sequence, may be precisely controlled. SUMMARY OF THE INVENTION Accordingly, an object of the invention is to provide new and improved methods and apparatus for treating articles for a controlled duration. With this and other objects in view, the present invention contemplates a method for treating articles, such as semiconductor slices coated with photoresist. The articles are automatically loaded on a heated, endless, constant-speed conveyor and the location of the first article on the conveyor is detected. Then, a marker is positioned on the conveyor in response to the detection and in relation to the first article. Next, a selector-stepping switch is actuated by the marker and continuity is established through the actuated switch after a preselected number of cycles of the endless conveyor and, therefore, a preselected treating time has occurred. Finally, both the marker and articles are removed in response to the continuity to terminate the treatment after the controlled duration. A suitable apparatus for practicing the contemplated invention provides a rotary table for transporting the slices through a predetermined number of heat treating cycles. A photocell detector and relay are provided adjacent the table to detect the location of the first slice loaded on the table. A magnet holder, responsive to the photocell detector and relay, is provided for placing the magnet on the table in relation to the first slice to mark or identify it. The magnet actuates a combination selector-stepping switch which is provided for energizing the magnet holder to remove the magnet and initiating slice removal to end the treatment.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

The invention can be understood best from the following more detailed description taken in conjunction with the accompanying drawings:

FIG. 1 is an isometric view, partially cut away, of a semiconductor slice coated with photoresist;

FIG. 2 is an isometric view, partially cut away, of an apparatus for baking the photoresist on the semiconductor slices;

FIG. 3 is a plan view of the apparatus showing how some of the parts are arranged around the center of the apparatus;

FIG. 4 is a plan view of a load station and baking position showing how the slices are conveyed to their baking position on a rotary table;

FIG. 5 is a cross section along line 5--5 of FIG. 4, of the load station and baking position;

FIG. 6 is an electrical schematic showing how a selector switch and stepping switch are connected to form a combination selector-stepping switch, how a sealed contact operates the stepping switch, and how the combination operates to remove the slices and the marker magnet to end the baking;

FIG. 7 is a partial cross section, along line 7--7 of FIG. 3, showing a marker magnet and magnet holder in elevation, for placing the magnet on and lifting it off a rotary table;

FIG. 8 is a partial cross section, along the line 8--8 of FIG. 3, showing the marker magnet on the table and an adjacent sealed contact which operates the stepping switch;

FIG. 9 is a plan view of an unload station and baking position; and

FIG. 10 is a cross section of the unload station and baking position along the line 10--10 of FIG. 9.

DETAILED DESCRIPTION

Methods and apparatus embodying the invention will be described in connection with baking photoresist on semiconductor slices. However, it is to be understood the invention may be used for other articles.

Referring now to FIG. 1, a silicon slice 18 is coated with a photoresist 20 to form a coated slice designated generally by the numeral 22. The photoresist 20 may be that sold by the Eastman Kodak Company under the trade designation"KPR."

The photoresist 20 may be baked best with a rotary apparatus, refer to FIG. 2, in which a base 23 supports a rotary table 24 having an upper level 25 and a lower level 26. The upper level 25 is indirectly heated from below by convection and radiation and its proximity to a plate 27 which is directly heated by a heater 28, e.g., a 1,000 watt Chromalox No. A-90 ring heater. In this way, the coated slices 22 are heated from the bottom and the photoresist 20 is baked outward from its contact with the slice 18 so that gas bubbles are not trapped in it. The upper level 25 is Teflon coated and is rotated at a constant speed, e.g., one r.p.m. The constant speed provides uniform heating of the upper level 25 by the plate 27 and permits the number of revolutions to determine the total heating or baking time in minutes. For example, 20 revolutions will yield a 20 minute total baking time.

Both the upper level 25 and lower level 26 are fixed to a vertical shaft 30 so that they rotate together as a unit, i.e., the table 24, when the shaft 30 is revolved by a drive motor 31.

Referring now to FIGS. 3, 4 and 5, there is shown a load station 34, which is part of a stationary table 35, for loading photoresist coated slices 22 into baking positions 36 on the upper level 25 of the table 24. The slices 22 are handled in carriers 38 and one of these carriers, having the slices to be baked, is inserted in the vertical indexing mechanism of a load station sender, designated generally by the numeral 39, of the type sold for that purpose by the Industrial Modular Systems Corporation. The slices 22 are evenly spaced in the carrier 38 and are removed therefrom by an air conveyor 40 which includes plenum 42, nozzles 44 facing toward the center of rotation, and a solenoid valve 46.

There are a plurality of baking positions 36, e.g., twenty-five, only some of which are shown. The baking positions 36 include guides 48; stops 50; and three plenums, one plenum 52 having nozzles 54 directed toward the center of rotation and two plenums 56 having nozzles 58 directed away from the center of rotation.

Air is admitted to the load station plenum 42 and baking position plenum 52 by the solenoid valve 46 via connections 60 and 61, orifice 62 and port 64; and it flows out through the nozzles 44 and 54, respectively. This air is preheated by passage through tubing 65, attached to the plate 27, refer to FIG. 2, so that it will not cool the upper level 25. Since the nozzles 44 and 54 are directed toward the center of rotation, the slice 22 is conveyed by air from the sender carrier 38 to the baking position 36 where it lodges against the stop 50. The stop 50 is off center, as shown, in order to prevent bouncing of the slice 22. Further understanding of this may be had from L. F. Boyer and A. F. Johnson, Jr. "Air Bearing Stop," technical digest, Western Electric Company, Issue 20, October 1970, pages 13 and 14.

Sequential loading of the slices 22 onto the upper level 25 of the table 24 may start at any baking position 36. However, the baking position 36, into which the first slice 22 is loaded, must be identified so taht the start of baking and the number of revolutions of the table 24, and thereby the total baking time, may be controlled.

The location of the first slice 22 and its baking position 36 may be determined by means of a photocell detector 66. The detector 66, refer to FIGS. 2, 3 and 6, projects light downward toward the upper level 25 and receives any light reflected back again. If enough light is reflected back, as when a slice 22 passes beneath the detector 66, the reflected light activates the detector 66 which operates a photoelectric relay 67, FIG. 6, and causes a magnet 68 to be dropped on the lower level 26 of the table 24. Thus, the magnet 68 marks the position of the first slice 22 above on the upper level 25.

The total baking time for the photoresist 20 is obtained by processing the slice 22 through enough bake cycles (revolutions of the table 24) to add to the total baking time desired. Control of the number of revolutions of the table 24 and thereby, the total baking time is achieved, preferably, by a switching device in the form of a combination selector-stepping switch 70.

Referring to FIG. 6, a selector arm 72 of the switch 70 may be set to contact any one of the plurality of selector contacts 74; e.g., a contact 74h as in FIG. 6, which will yield the correct number of revolutions as determined from the desired baking time and known speed of the table 24. The contacts 74a to 74j are connected in parallel to like stepping contacts 76a to 76j, i.e., the selector contact 74a is connected to the stepping contact 76a, the selector contact 74b is connected to the stepping contact 76b, etc.

A stepping arm 78 is fixed to a ratchet wheel 80 or the equivalent, and a pawl 82, actuated by a solenoid 84, moves the ratchet wheel 80 and, thereby, the stepping arm 78. Each time the solenoid 84 is energized the arm 78 steps from one contact 76 to another.

The magnet 68, refer to FIG. 8, which is placed on the lower level 26 of the table 24 to mark the position of the first slice 22 may be a disc or ring of permanent magnet material, preferably a ring of Alnico V or the like about 7/8 inch O.D. .times. 3/8 inch I.D. .times. 1/4 inch thick, magnetized axially.

When the lower level 26 rotates and the magnet 68 is resting on it, the magnet passes a sealed contact 86 which is sensitive to the flux of the magnet. The sealed contact 86 is a hermetically-sealed, normally-open, flux-sensitive switch, sometimes called a dry reed switch in the art, which is closed by the magnetic flux when the magnet 68 is adjacent to it. Switch closure occurs which the first slice 22 is above the sealed contact 86, because the first slice 22 is on the upper level 25 of the table 24 above the marker magnet 68. Since the contact 86 is positioned along the same radius as an unload station 88, refer to FIG. 3, each closure of the contact 86 signals both: that one revolution of the table 24, and therefore one bake cycle, has been completed; and that the first slice 22 is at the proper location to be conveyed from its baking position 36 into the carrier 38 at the unload station 88.

The magnet 68 is picked up from and dropped into a groove 89 in the lower level 25 of the table 24 by a magnet holder 90. The magnet holder 90, refer to FIG. 7, includes a solenoid 92 having a vertically moving plunger 94, and a nonmagnetic separator plate 96. The holder 90 is in radial alignment with the detector 66, refer to FIG. 3, and its solenoid 92 lifts the plunger 94 from or drops it onto the nonmagnetic separator plate 96. The plunger 94 is magnetic but not permanently magnetizable. When the plunger 94 is on the plate 96, the magnet 68 resting in the annular groove 89 of the lower level 26 of the table 24 will attract itself to the plunger and lift itself a short distance up against the underside of the nonmagnetic separator plate 96. Thus, the magnet 68 will remain suspended free of the lower level 26 as long as the solenoid 92 remains unenergized. When the solenoid 92 is energized, such as when the photocell detector 66 detects a slice 22 and actuates the associated electrical circuit, the solenoid lifts the plunger 94 far enough to make the space between the end of the plunger and the separator plate 96 so large that the force of attraction between the magnet 68 and the plunger becomes too weak to support the magnet. Consequently, the magnet 68 drops into the groove 89 on the lower level 26 of the table 24.

For best results, solenoid 92 should be supplied with the current connected so that the magnetic field of the solenoid is in the opposite direction to that of the magnet 68. This may be accomplished with a rectifier 93. The magnet 68 may be placed at any position in the groove 89 around the lower level 26 by energizing and de-energizing the solenoid 92 at the appropriate time.

It is to be noted that the magnet 68 could be placed on and removed from the upper level 25. However, the use of the lower level 26 removes the contact 86 and magnet holder 90 from the heat above the upper level 25 and, thereby, increases the life of these components.

Although the foregoing method of picking up the magnet 68 is preferred, there are other well known ways to pick it up. For example, a vacuum nozzle may be substituted for the plunger 94 and the vacuum controlled by a solenoid valve to pickup or drop the magnet 68 as desired. In addition, a disc with a reflective surface may be substituted for the magnet 68 and a photocell detector substituted for the sealed contact 86 to achieve results similar to the much less costly sealed contact and magnet.

In any case, when the presence of the first slice 22 loaded on the upper level 25 of the table 24 is detected, the magnet 68, or its equivalent, is dropped on the lower level 26 at the same angular position as the slice, refer to FIG. 3, because the holder 90 is aligned along the same radius as the detector 66. The magnet 68 remains in this position marking or identifying the location above, of the slice 22 on the upper level 25.

The stepping arm 78 of the switch 70 is connected to one side of a voltage source 100, refer to FIG. 6, and the selector arm 72 is connected to the other side through: a solenoid valve 102, a time delay relay 104, and a sealed contact 106; and a relay 108. When the selector arm 72 is set to contact 74h, as in FIG. 6, and the table 24 revolved eight times to cause the magnet 68 to close the sealed contact 86 eight times, the stepping arm 78 is on contact 76h, relay 108 is actuated, normally open contacts 113 are closed, and the solenoid valve 102 is in condition to be operated by closure of the sealed contact 106. The contact 106 may be located any whole number of baking positions 36 away from the center line of the unload station 88. Also, when arms 72 and 78 are on contacts 74h and 76h, respectively, continuity ensues and the relay 108 opens its normally-closed contacts 110 to disconnect the magnet solenoid 92 from the source of voltage. This drops the plunger 94 onto the separator plate 96 so that the magnet 68 attaches itself to the underside of the plate. Thus, the holder 90 removes the magnet 68 in response to continuity through the selector-stepping switch 70.

The valve 102, refer to FIGS. 9 and 10, admits preheated air both to: an unload station conveyor 111, having a plenum 112 with nozzles 114 directed toward the carrier 38, through a connection 116; and the two plenums 56 in the upper level 25 through two connections 117, two orifices 118 and two entrance ports 120. The air is preheated by passage through tubing 121 attached to the underside of the plate 27. Each orifice 118 aligns with its respective port 120 at the same time that the baking position 36 aligns with the unload station 88. Consequently, air flowing out the nozzles 58 and 114 conveys the slice 22 from the baking position 36 to the carrier 38 which is indexed vertically by the index mechanism of an unload station receiver, generally designated by the numeral 122, identical to the sender 39.

A plurality of magnets 123, refer to FIG. 3, are fixed to the upper level 25, one at each baking position 36 approximately on its centerline. The magnets 123 may be 1/8 inch diameter by 1 inch long rod magnets such as type CR-400 - 1 sold by Indiana General Corporation. Each magnet 123 closes the sealed contact 106 as each baking position 36 aligns with the unload station 88. The air valve 102 is made to operate at the precise time that the orifices 118 are aligned with the ports 120 and the baking position 36 is aligned with the unload station 88, by adjusting the time delay relay 104. Thus, the slices 22 are conveyed one after the other, i.e., sequentially, from the baking positions 36.

Although sealed contacts, such as contact 106, operated by the rod magnets 123 are preferred because no rubbing surfaces are involved, the sealed contacts may be replaced by microswitches having followers bearing against the upper level 25 of the table 24 and the magnets may be replaced by rods or the equivalent made of material other than magnet material. As is known in the art, the rods will act as cam lobes which will operate the microswitches. Thus, cam lobes and microswitches will replace magnets and sealed contacts but, of course, are subject to much more wear.

OPERATION

The operation of the baking apparatus may be understood best by: referring to FIGS. 2, 3 and 6; assuming that the speed of rotation of the table 24 is 1 rpm; and assuming that the photoresist 20 must be baked for five minutes. The schematic diagram of FIG. 6 does not show the circuits for the drive motor 31 or heater 28 because these are well known in the art.

The arm 72 of the selector-stepping switch 70 is set to contact 74e, the fifth contact from OFF; the heater 28 and motor 31 are energized; and the upper level 25 of the table 24 is allowed to rotate in the direction of the arrow until thermal equilibrium is reached.

When equilibrium is reached, a sender switch 124 is closed so that the sender 39 and solenoid valve 46 are connected to the voltage source 100 through sealed contacts 126 and 128, respectively. The contact 128 is closed by one of the magnets 123 and operates the valve 46 admitting air to the load station 34 and baking position 36. This removes the first slice 22 from the sender carrier 38. Precise timing so that the valve 46 operates when the load station 34 and baking position 36 are aligned, is achieved by adjustment of a time delay relay 129.

Contact 126 closes, following closure of contact 128, and indexes the sender 39 mechanism one step downward. This lowers the next slice 22 into position to be conveyed or sent into the next baking position 36. The closure of contact 126 occurs after closure of contact 128 because the sealed contact 126 is located one and one half baking positions 36 beyond the contact 128 in the direction of rotation (see FIG. 3). However, if the switch 124 is closed when the table 24 rotation is such that two of the magnets 123 are between the sealed contacts 128 and 126, the carrier 38 in the sender 39 may index downward and break a slice 22 before air is admitted to the sender conveyor 40. This may be prevented by a sealed contact 130, time delay relay 132 and a relay 134 having contacts 136. When switch 124 is closed, the contacts 136 of relay 134 are open until the sealed contact 130 is closed by a magnet 123. This sealed contact is located adjacent the upper level 25 a few degrees in advance of contact 128, e.g., about 5.degree. (or any even number of baking positions plus 5.degree.). Closure of contact 130 energizes the time delay relay 132 which, in turn, energizes the relay 134 for about 1.4 seconds. During this interval the orientation of the magnets 123 with respect to the sealed contacts 128 and 126 is such that they will be operated in that order, i.e., the proper sequence. Thus, the slices 22 are sequentially removed from the sender carrier 38 and placed in adjacent baking positions 36, by sequentially operating the sealed contact 128 first and the contact 126 second. Baking then takes place at the temperature of the upper level 25.

The position 36 of the first slice 22 loaded on the upper level 25 is detected by light reflected from the slice back into the photocell detector 66, which is a part of a photoelectric relay 67. Reflected light seen by the photocell detector 66 causes the relay 67 to energize the magnet-holder solenoid 92 and the plunger 94 to be lifted. The magnet 68, as a consequence, drops to the lower level 26 and marks the position of the first slice 22.

The first slice 22 will have been carried through essentially one revolution and one baking cycle when it arrives at the unload station 88. The sealed contact 86, positioned adjacent the lower level 26 in line with the station 88, is closed by the marker magnet 68 and energizes the stepping-switch solenoid 84 to move the arm 78 to the first contact 76a. This is repeated as the table 24 rotates until, at the end of five minutes, the arm 78 establishes electrical continuity from one side of the voltage source 100 through the relay 108, the arm 78 itself, the contact 76e, the contact 74e and the selector switch arm 72 to the other side of the source. This energizes the relay 108 which opens the normally closed contacts 110, de-energizes the magnet-holder solenoid 92, drops the plunger 94 and lifts;the magnet 68 from the lower level 26. At the same time, the relay 108 resets photoelectric relay 67 and closes its normally open contacts 113 to connect the unload solenoid valve 102 to the voltage source. Thus, each time one of the magnets 123 closes the sealed contact 106, the valve 102 admits air to the baking position 36 and the unload station 88. This conveys the first slice 22 into the receiver carrier 38 and ends the baking of the photoresist 20 for that slice. Shortly thereafter, one of the magnets 123 closes a sealed contact 138, refer to FIG. 3, which energizes the indexing mechanism of the receiver 122 once and raises the receiver carrier 38 one step into position to receive the next slice 22. This is repeated until all slices are removed in sequence. Then, the selector-stepping switch 70 is reset so that the next lot of slices may be baked.

It is to be noted that the slices may be baked starting with the table 24 in any position and a wide range in baking times may be had by proper choice of the speed of the table and the number of contacts for the selector-stepping switch 70.

Thus, a method and apparatus for carrying out the method have been disclosed for treating articles for a controlled duration independently of the number of articles or where the first one is placed on the apparatus.

While specific embodiments have been described in the foregoing specification to illustrate the invention, it will be understood that the invention is not limited to these embodiments. Various changes and modifications may be made without departing from the spirit and scope of the invention.

Claims

What is claimed is:

1. A method of treating articles, wherein the duration of treatment is controlled by a marker, which comprises the steps of:

a. placing a series of the articles on an endless conveyor for treatment;

b. detecting the location of the first article of the series of articles placed on the conveyor;

c. positioning the marker on the conveyor in response to the detection of the location to identify such location;

d. actuating a switching device by means of the marker to count the number of cycles completed by the endless conveyor and to establish continuity through the switching device when a predetermined number of cycles has been completed; and

e. removing the marker and the articles from the conveyor to terminate the treatment, in response to the continuity and beginning with the first article as identified by the marker.

2. The method of claim 1 wherein the marker is dropped on the conveyor.

3. The method of claim 2 wherein the marker is magnetic.

4. The method of claim 1 wherein there is only one article in the series.

5. The method of claim 1 wherein the switching device is a combination selector-stepping switch.

6. The method of claim 5 wherein the articles are semiconductor slices coated with photoresist and the treatment is baking the photoresist.

7. The method of claim 6 wherein the endless conveyor is a rotating table.

8. The method of claim 7 wherein the magnetic marker operates a sealed contact to actuate the switching device.

9. A method of baking photoresist on semiconductor slices, wherein the length of baking time is controlled by a magnetic marker, which comprises of steps of:

a. positioning a selector portion of a selector-stepping switch to a preselected contact representing the number of revolutions of a rotary table which will yield the desired length of baking time;

b. rotating the table to sequentially receive a plurality of slices from a carrier;

c. conveying the slices sequentially from the carrier into each of a plurality of slices positions on the rotary table;

d. heating the table to bake the photoresist on the slices;

e. detecting the position of the first slice;

f. dropping a magnetic marker on the rotary table in alignment with the first slice to mark its position;

g. positioning a magnetic flux sensitive switch adjacent the rotary table, the switch being aligned with a carrier at an unload station and capable of operating the stepping portion of the selector-stepping switch;

h. actuating the flux-sensitive switch by means of the magnetic marker to operate the stepping portion of the selector-stepping switch each revolution of the table until continuity is established through the preselected contact;

i. lifting the magnetic marker from the table in response to the continuity; and

j. removing the slices from their positions on the table in response to the continuity, starting with the first slice, to terminate the baking of the photoresist on each slice at the end of the preselected length of time.

10. An apparatus for treating articles, which comprises:

a. means for transporting the articles through a predetermined number of treating cycles;

b. means for loading the articles sequentially on the transporting means;

c. means for detecting the first article loaded on the transporting means;

d. means for marking the location of the first article;

e. means, responsive to the detecting means, for placing the marking means on the transporting means at the beginning of treatment, to mark the location of the first article, and for removing the marking means at the end of treatment;

f. means, responsive to the marking means, for controlling the number of treating cycles the first article makes and thereby the total duration of the treatment of the articles; and

g. means, responsive to the cycle controlling means, for removing the articles sequentially from the transporting means beginning with the first article marked by the magnet.

11. An apparatus, as recited in claim 10, wherein the means for marking the location of the first article comprises a permanent magnet positioned in relation to the first article.

12. An apparatus, as recited on claim 11, wherein the transporting means comprises a rotary table.

13. An apparatus, as recited in claim 12, wherein the means for detecting the first slice comprises a photocell and a photoelectric relay.

14. An apparatus, as recited in claim 13, wherein the means responsive to the detecting means comprises a solenoid having a plunger which may be raised or lowered to position the marking means in relation to the first article.

15. An apparatus, as recited in claim 14, wherein the means responsive to the marking means comprises a sealed contact and combination selector-stepping switch.

16. An apparatus, as recited in claim 15, wherein the articles are photoresist coated semiconductor slices and the treatment comprises baking the photoresist.

17. An apparatus for baking photoresist on semiconductor slices, which comprises:

a. a base;

b. a rotary table supported by the base and having baking positions for baking the photoresist on the slices;

c. a loading station for loading the slices sequentially into each baking position;

d. a photocell detector to determine the baking position of the first slice;

e. a combination selector-stepping switch for selecting the number of revolutions through which the table is to be rotated, and for counting the revolutions of the table to control the baking time;

f. a marker, positionable on the table to mark the location of the first slice;

g. a switch, operated by the marker once each revolution, to actuate the stepping portion of the selector-stepping switch and count the revolutions of the table;

h. a marker holder, aligned with and responsive to the photocell detector for positioning the marker on the table in line with the first slice and responsive to the selector-stepping switch for removing the marker from the table after the preselected number of revolutions;

i. an unloading station adjacent the table to receive the baked slices; and

j. means for removing the slices sequentially from the table, beginning with the first slice, and conveying the slices into the unloading station in response to the selector-stepping switch, thereby ending the slice baking.

18. An apparatus for baking photoresist on semiconductor slices, which comprises:

a. a base;

b. a rotary table supported by the base for baking the photoresist on the slices, the table having a heated upper level and a lower level;

c. a loading station adjacent the upper level of the table for loading slices sequentially into a baking position for each slice on the table;

d. a photocell detector adjacent the upper level of the table to determine the baking position of the first slice;

e. a combination selector-stepping switch for selecting the number of revolutions through which the table is to be rotated and counting the revolutions to control the baking time, continuity therethrough being established when the preselected number of revolutions have been counted;

f. a magnet, positionable on the lower level of the table, to mark the location of the first slice;

g. a flux-sensitive switch, adjacent the lower level of the table, the switch being closed by the flux of the magnet once each evolution of the table to actuate the stepping portion of the selector-stepping switch to count the number of revolutions;

h. a magnet holder adjacent the lower level of the table and aligned with the photocell detector, the holder being responsive to the photocell detector for depositing the magnet on the lower level of the table, and responsive to continuity through the combination selector-stepping switch for removing the magnet from the table after the preselected number of revolutions;

i. an unload station adjacent the upper level of the table to receive the baked slices; and

j. means for removing the slices sequentially from the table, beginning with the first slice, and conveying the slices into the unloading station in response to continuity through the selector-stepping switch, thereby ending the slice baking.