US3791342A

US3791342A - Selective coating

Info

Publication number: US3791342A
Application number: US00260087A
Authority: US
Inventors: L Boyer; A Johnson
Original assignee: Western Electric Co Inc
Current assignee: AT&T Corp
Priority date: 1970-12-07
Filing date: 1972-06-06
Publication date: 1974-02-12
Anticipated expiration: 1991-02-12
Also published as: US3695928A; FR2116568A1; CA948048A; DE2160283A1; JPS4711719A; GB1375738A; FR2116568B1; DE2160283B2; JPS5026919B1

Abstract

A rotatable vacuum spindle supports a substrate thereon. Means coat the substrate on the non-supported face. Means rotate the spindle and substrate thereon at a speed whereby to distribute the coating material. Means disposed about the spindle direct an annular fluid stream outwardly and against the supported face of the substrate whereby to prevent creep of the coating material onto said supported face.

Description

United States Patent Boyer et a1.

[ Feb. 12, 1974 SELECTIVE COATING Inventors: Lynn Forrest Boyer, Reading;

Anderson Forbes Johnson, Jr., Sinking Spring, both of Pa.

Western Electric Company, Incorporated, New York, NY.

Filed: June 6, 1972 Appl. No.: 260,087

Related U.S. Application Data Division of Ser. No. 95,821, Dec. 7, 1970, Pat. No. 3,695,928.

Assignee:

U.S. Cl. 118/52, 118/58 Int. Cl. B05c 11/08 Field of Search 118/58, 63, 52, 56, 48-495;

[56] References Cited UNITED STATES PATENTS 2,386,591 10/1945 Campbell 118/52 2,580,131 12/1951 Rowcll 117/101 2,867,551 1/1959 Kozak 117/38 X 2,946,697 7/1960 Petro 1 18/504 X 2,953,483 9/1960 Torok 118/504 X 3,008,601 11/1961 Cahne 118/52 UX 3,538,883 11/1970 Polin 118/50 X 3,577,267 5/1971 Preston, Jr. et a1 118/52 X Primary ExaminerMorris Kaplan Attorney, Agent, or FirmR. Y. Peters [57] ABSTRACT A rotatable vacuum spindle supports a substrate thereon. Means coat the substrate on the nonsupported face. Means rotate the spindle and substrate thereon at a speed whereby to distribute the coating material. Means disposed about the spindle directan annular fluid stream outwardly and against the sup ported face of the substrate whereby to prevent, creep of the coating material onto said supported face.

7 Claims, 5 Drawing Figures FLUID SOURCE 37 VACUUM SOURCE 38 ROTATING DEVICE PATENTEBFEBIZIBH 3,791,342

SHEEI 2 0F 2 .36 VERTICAL MOVING DEVICE v 33 ROTATING DEVICE FLUID SOURCE I o 37vAcuuM SOURCE SELECTIVE COATING This is a division of application Ser. No. 95,821 filed Dec. 7, 1970, now Pat. No. 3,695,928.

BACKGROUND OF THE INVENTION This invention relates apparatus for selectively coating workpieces, and more particularly, to methods of making diode-array targets from semiconductive substrates for electron-beam charge-storage devices such as television camera tubes.

This invention is particularly suited for use in the manufacture of semiconductive devices or the like. An example of such semiconductive devices are silicon targets having arrays of light-sensitive photo diodes for the storage of electron-beam charges, such as those disclosed in Reynolds Pat. No. 3,01 1,089 and Buck et al., Pat. Nos. 3,403,284 and 3,458,782.

While the invention is adapted to be used in conjunction with the selective coating of objects, it will be particularly described with respect to diode-array targets for electron-beam charge-storage devices such as television camera tubes. Such a diode-array target is basically a flat semiconductive substrate having a closely spaced array of p-n junctions near one surface.

In fabricating the diode array target in accordance with the present techniques, an n-type substrate is prepared by sawing it from a silicon crystal ingot and then etching and polishing it. After a careful cleaning, a layer of silicon dioxide is formed over the entire substrate.

Next, a photoresist masking material is deposited on the substrate which is spun to coat it evenly. The substrate is then exposed with the required diode-array pattern and developed to form apertures in the photoresist masking material. Etching of the substrate through these apertures follows to produce corresponding apertures in the silicon dioxide layer. Through the apertures in the silicon dioxide layer, boron is diffused to form p-type regions, with the diode layer acting as a diffusion mask. These p-type regions in the n-type substrate form the diode array on one side of the substrate.

Following the formation of the diode array, the substrate is mounted with wax to a supporting disc with the diode array side down. The backside of the substrate is first etched to remove any silicon dioxide remaining on it and then etched again to remove any boron-diffused material from the peripheral area of the backside. Next, a wax ring is painted on this peripheral area so that a supporting rim will remain on the backside of the substrate after a subsequent etching step. In that etching step, the substrate is immersed in the etchant and rotated therein for a period sufficient to thin down the substrate to a predetermined thickness. This thickness is considerably less than the diffusion length of minority carriers generated by absorbed light in the ultimate target and limits the amount of lateral diffusion of minority carriers in order to obtain high resolution in the target.

After thinning, the substrate is subjected to several finishing heat treatments. The first of these treatments is a shallow phosphorous diffusion to improve the blue sensitivity of the ultimate camera tube and to reduce its dark current. Next, the boron diffusion glass, which has been left on the array side of the substrate up to this point to protect it against phosphorous diffusion, is re- 2 moved to expose the p-type regions of the diodes. At the same time that the boron diffusion glass is removed, the phosphorous diffusion glass is also removed.

Next, the substrate is annealed in hydrogen at a low temperature to further reduce the dark current of the ultimate camera tube. Finally, a resistive film is evaporated over the diode array and it is ready for evaluation.

In fabricating the diode-array target, it is desirable to eliminate as many processing steps as possible, and at the same time increase the uniformity of the physical and electrical characteristics of the individual diodes in the array. It is also very important that no physical damage or contamination results from any methods used to make the diode-array targets or any apparatus used to handle the substrates.

SUMMARY OF THE INVENTION It is, therefore, an object of this invention to provide new and improved apparatus for selectively coating a workpiece.

Another object of this invention is the provision of apparatus for making from a semiconductive substrate an electron-beam target having a highly uniform diode array.

A further object of this invention is to provide an apparatus for coating a silicon substrate with a masking material wherein a nozzle forces gas under pressure against a portion of the slice to prevent the masking material from contacting a portion of it.

This invention also contemplates an apparatus for selectively coating a workpiece. The apparatus includes a dispensing device for applying a material to the workpiece and an air nozzle for forcing a fluid under pressure against a portion of the workpiece where the material is not desired to prevent the material from coating such portion of the workpiece.

BRIEF DESCRIPTION OF THE DRAWINGS Other objects and advantages of the present invention may be more clearly understood by reference to the following detailed description and accompanying drawings, wherein:

FIGS. 1-4, inclusive, are greatly enlarged perspective views, partially in section, showing a summary of the processing sequence for making diode-array targets and illustrating some problems of the prior art; and

FIG. 5 is an enlarged perspective view, partially in section, of an apparatus for selectively coating a workpiece, illustrating a nozzle for forcing fluid under pressure against a portion of the workpiece where coating material is not desired.

DETAILED DESCRIPTION Diode Array Target Referring now to the drawings and in particular to FIG. 4, a diode array target, designated generally by the numeral 11, is shown. Such target 11 includes an n-type semiconductive substrate 12, preferably silicon, having a layer 13 of silicon dioxide formed on the face of the surface 14 thereof. Through the layer 13 a plurality of apertures 16 are formed. Corresponding to these apertures 16 are a plurality of p-type regions 17 which are formed through the apertures 16 in the surface 14 of the substrate 12. The p-type regions 17 together with the n-type substrate 12 form an array of light-sensitive photo diodes.

The substrate 12 has an extremely thin central portion 18 for reducing the distance minority carriers must travel to the diodes on the opposite surface 14 of the substrate 12 and for limiting the lateral diffusion of these carriers. The substrate 12 also includes a relatively thick rim 19 on the periphery thereof for supporting it.

In using the diode array target 11 in a television camera tube application, the light being sensed impinges on the central portion 18 of the substrate 12 producing minority carriers that travel to the diode-array in the opposite surface 14, while an electron beam from a cathode scans the diode array in the surface 14.

Typically, the substrate 12 has the configuration of a disc with a diameter of about 850 mils and with the central portion 18 being about 760 mils in diameter and about 0.6 mils thick. The ring portion 19 typically is about mils thick. Usually there are about one million individual diodes in the array in the surface 14 of the substrate 12, and about 650,000 of these are used when the target 11 is mounted in a television camera tube. Obviously, the relative proportions in FIGS. 1-4 have been exaggerated to more clearly illustrate the target 11.

Fabricating Method The present invention can best be illustrated by first briefly describing the prior art method of fabricating the diode-array target 11 (FIG. 4) and some of the problems in connection therewith.

In fabricating the target 11 by the prior art method, the substrate 12 (FIG. 1) is formed by sawing a slice from a silicon crystal ingot and then etching and polishing it. After a careful cleaning, the silicon dioxide layer 13 is formed by thermal oxidation over the entire substrate 12.

Next, the substrate 12 is positioned for spin coating on, and securely held by, a vacuum chuck (not shown). Then, the top surface 22 (FIG. 1) of the silicon dioxide layer 13 is flooded with a photoresist masking material, and the substrate 12 is allowed to sit for a short time to allow the masking material to become distributed over the entire top surface 22 and form a layer 23 with meniscus 24. The substrate 12 is then rapidly accelerated to a high spinning velocity and is left spinning for a short time. The spinning of the substrate 12 breaks down and scatters the layer 23 of masking material and forms a thin uniform coating 26 (FIG. 2) of the material on the top surface 22 of the layer 13 with a portion of the coating 26 overlying part of the edge of the layer 13, as shown in FIG. 2.

A major problem with this prior art method is that as the substrate 12 is spun. the photoresist masking material often creeps over, or splashes on, the backside or bottom surface 28 (FIG. 2) of the silicon dioxide layer 13 of the substrate 12, and forms undesirable coatings 29 at various locations on such surface 28, as shown in FIG. 2.

It has been found that these coatings 29, which overlie and protect portions of the silicon dioxide layer 13 during a first etching operation, cause these portions to remain on the bottom surface 28. Consequently, a second etching operation is required to remove them. After the removal of these coatings 29 and of these underlying portions, the diode array in the surface 14 of substrate 12 opposite these coatings 29 has different electrical characteristics than the diode array not opposite such coatings 29. Hence, such diode array lacks uniformity.

While the exact reason for this lack of uniformity is not known for certain, it has been theorized that these portions of the silicon dioxide layer 13 underlying the coatings 29 cause internal stresses in the crystal lattice of the substrate 12 even after the removal of the coatings 29 and the underlying portions of the layer 13. These stresses which are located in these areas where the coatings 29 and underlying portions had been, apparently result in certain electrical characteristics in the diode array formed in the surface 14 of the substrate 12;. Because of its extreme thinness (about 0.6 mils thick at the central portion 18), the substrate 12 is especially susceptible to such stresses. These electrical characteristics are different from those that are exhibited by the-diode array where there are no such stresses; hence, uniformity of the diode array is decreased as a result of the creep-over problem. Significantly, this decrease in uniformity often renders the targets 11 defective.

It is believed that the creep-over problem is caused, at least in part, (a) by the overflowing of the masking material to the edges and the bottom surface 28 of the substrate 12 during the flooding of the surface 14 with the masking material, (b) by the breaking down and scattering of the layer 23 of masking material by the rapid acceleration of the substrate 12in spinning it, and (c) by leakage from the vacuum chuck that holds the substrate 12, tending to draw the scattered material toward the center of the surface 28 and to the chuck.

In the making of the target 11 by the prior art method, the next step is the forming of a plurality of apertures (not shown) in the. layer 26 (FIG. 2) of the photoresist masking material. The apertures are formed by conventional photolithographic techniques in accordance with the desired array of diodes.

After the forming of these apertures in the layer 26, the apertures 16 (FIG. 3) are formed in the silicon dioxide layer 33 by the first etching operation, wherein the substrate 12 is immersed in hydrofluoric acid. Those portions of the silicon dioxide layer 13 that are not protected by the

layers

26 and 29 of photoresist masking material are etched away, thereby forming the apertures 16 in the layer 13 and removing thoseportions of the layer 13 which do not have their bottom surface 28 underlying the coatings 29, as shown in FIG. 3.

Then, the

coatings

26 and 29 of the photoresist masking material are removed by conventional dissolving and washing techniques. leaving on the substrate 12 those portions of the silicon dioxide layer 13 that had been underlying

such coatings

26 and 29.

Next, through the apertures 16 in the silicon dioxide layer 13, boron is diffused to form the p-type regions 17 in the n-type substrate 12, with the layer 13 acting as diffusion mask. The p-type regions 17 in the n-type substrate 12 form the diode array on the surface 14 of the substrate 12.

Following the formation of the diode array and in accordance with the prior art method, the substrate 12 is mounted with wax to a supporting disc (not shown) with the silicon dioxide layer 13 and the surface 14 with the diode array facing down on the supporting disc. The second etching operation is then performed. In this operation the backside of the substrate 12 is etched in hydrofluoric acid to remove those portions of the silicon dioxide layer 13 of the bottom surface 28 thereof that had been underlying the coatings 29 of the photoresist masking material. Such portions of the layer 13 must be removed to provide a clear path to the central portion 18 of the substrate 12, and, as mentioned above, such removal in accordance with prior art techniques leaves the array deficient in uniformity, possibly due to the development of internal stress in the extremely thin substrate 12.

After the removal of the silicon dioxide layer 13 underlying the coating 29, the peripheral area of the backside of the substrate 12 is etched in accordance with the prior art method in nitric, hydrofluoric, and acetic acid saturated with iodine. This etching removes any boron-diffused material on this area of the substrate 12.

In the prior art method, a wax ring (not shown) is then painted on the peripheral area of the backside of the substrate 12 to define the rim 19 (FIG. 4) for supporting the substrate 12. This rim 19 is left on the substrate 12 after a subsequent etching step wherein the thin central portion 18 is formed in the substrate 12. The substrate 12 is then immersed in the aforementioned iodine saturated acid and rotated therewith for a period sufficient to thin down the substrate 12 to form the central portion 18 with a thickness of about 0.6 mils.

Coating Apparatus and Method With Air Nozzle In accordance with the present invention, the aforementioned problem of creep-over with the resultant lack of diode uniformity is overcome by the use of the selective coating apparatus of FIG. 5. The use of this apparatus also reduces the number of processing steps required to make the target 11.

In using this apparatus, the substrate 12 which already has the silicon dioxide layer 13 formed thereon is positioned on and held by a free end 31 of a hollow tubular member 32 having a passageway 33 therein. The tubular member 32 also has an end 34, opposite the free end 31, which is associated with several conventional expedients (shown diagrammatically in FIG. 5) including a vertical moving device 36, a vacuum source 37, and a rotating device 38. While the device 38 is shown associated with the tubular member 32 for moving it vertically, it should be understood that the device 36 may instead be associated with a platform 39 for moving it vertically. (Some of these expedients are incorporated in Model No. 6604 of an Automatic Photoresist Coater manufactured by Industrial Modular Systems Corporation, Cupertino, Calif.)

The vacuum source 37 is connected to the passageway 33 of the tubular member 32 to produce a vacuum at the free end 31 thereof to securely hold the substrate 12 on such free end 31 during a subsequent spin coating operation. For increasing the ease of positioning the substrate 12 on the free end 31, the tubular member 32 is vertically movable by the device 36. Also, the tubular member 32 is rotatable by the device 38 for the subsequent spin coating operation.

An annular fluid projecting vent or nozzle 41 (FIG. 5) is located circumferentially about and spaced from the tubular member 32 and is formed by an upper beveled flange 42 of an inner tubular member 43 and by a coacting chamfered portion 44 of an outer tubular member 47. The nozzle 41 is spaced from the substrate 12 so as not to interfere with the substrate 12 as it is spun by the rotating device 38, and the tubular member 43 is fixed to the platform 39. Of course, if the vertical moving device 36 is associated with and moves the platform 39, as previously described as an alternative, then the tubular member 43 is slidably mounted in the platform 39.

The inner and outer

tubular members

43 and 47 forming the nozzle 41 also form a cavity 48 communicating with the annular nozzle 41 and also communicating, through a sealed fitting 51 and a tube 52, with a source 54 of a fluid under pressure.

The nozzle 41 forces the fluid, which may be a liquid or gas, but preferrably is either nitrogen or air, from the source 54 in an annular pattern against and radially outward from the bottom surface 28 of the silicon dioxide layer 13 of the substrate 12. The fluid after striking the bottom surface 28 of the substrate 12 is forced radially outward away from the substrate 12.

In carrying out the present invention, the photoresist masking material is applied to the top surface 22 (FIG. 1) of the silicon oxide layer 13 of the substrate 12 by a conventional dispensing device 61'(FIG..5). Advantageously, the top surface 22 is flooded with the material and the substrate 12 is allowed to sit for about 20 seconds to allow the material to become distributed over such surface 22 to form the layer 23. Preferrably, the material is applied to the surface 22 after the nozzle 41 forces the fluid from the source 54 against the bottom surface 28 of the substrate 12, thereby preventing any such material from contacting such bottom surface 28. However, it should be understood that the fluid can be forced against such surface 28 at the same time that the material is applied.

Next, the rotating device 38 is energized to rapidly accelerate the substrate 12 to a spinning velocity in the range of about 3,000 to 6,000 rpm and it is left spinning for about 15 seconds. The spinning of the substrate 12 is sufficient to break down, and scatter the layer 23 (FIG. 1) and to form the uniform coating 26 (FIG. 2) of the masking material on the top surface 22 of the layer 13 with a portion of the coating 26 overlying part of the edge of the layer 13, as shown in FIG. 2. A conventional retaining cup 62, supported by a ring 63 and the outer tubular member 47 surrounds the substrate 12 and collects the photoresist masking material thrown from the spinning substrate 12 and forced radially outward by the fluid from the nozzle 41. After the spinning, the coating 26 has a thickness of about 1 micron.

Although the layer 23 of the photoresist masking material is broken down and scattered by the rapid acceleration of the substrate 12 as it is spun and the leakage of the vacuum at the free end 31 of the tubular member 32 tends to draw the scattered material toward the center of the bottom surface 28 of the layer 13, the masking material does not creep-over or splash on, the bottom surface 28 of the layer 13 and does not form any coatings 29 (FIG. 2). This is because the nozzle 41 of the selective coating apparatus of FIG. 5 forces the fluid from the source 54 against the bottom surface 28 of the layer 13 as the substrate 12 is spun. As a result, the creep-over problem of the prior art with the resultant lack of uniformity of the diode array is solved. Further, the selective coating apparatus of FIG. 5 does not physically damage or contaminate the substrate 12.

Certain adjustments to the apparatus of FIG. are necessary. For example, the vacuum of the source 37 is adjusted to securely hold the substrate 12 at the free end 31 of the tubular member 32. Also, the volume and the pressure of the fluid of the source 54 are adjusted so that the force of the fluid against the bottom surface 28 of the substrate 12 is sufficient to prevent the masking material from being conveyed to the bottom surface 28. It has been observed that the fluid from the source 54 being forced against the bottom surface 28 tends to aid the vacuum from the source 37 in securely holding the substrate 12 to the free end 31 of the tubular member 32. This additional holding action by the fluid 12 is believed to be due to the Bernoulli principle.

After the coating 26 of the photoresist masking material is formed on the silicon dioxide layer 13 in accordance with the present invention, the aforementioned thinning operation is performed, except that the aforementioned first etching operation of the backside of the substrate 12 with hydrofluoric acid to remove any remining silicon dioxide is not eliminated.

After the thinning operation, the substrate 12 is removed from the support disc and subjected to several conventional finishing heat treatments. The first of these treatments is a shallow phosphorous diffusion to improve the blue sensitivity of the ultimate camera tube and to reduce the dark current. The boron diffusion glass, which was left on the surface 14 of the substrate 12 up to this point to protect the diode array against the phosphorous diffusion, is now removed by immersing the substrate 12 in an etchant of hydrofluoric acid. The removal of this boron diffusion glass exposes the p-type regions of the diode array in the surface 14. At the same time that the boron diffusion glass is removed, the phosphorous diffusion glass is also removed.

Next, the substrate 12 is annealed in hydrogen at a low temperature to further reduce the dark current of the ultimate camera tube. Finally, a resistive sea layer is applied to the surface 14 having the diode array thereon. This layer is produced by evaporating antimony trisulphide in a bell jar vacuum system. After the application of this layer, the target 11 is completed and ready for evaluation.

While the invention has been described in connection with semiconductive substrates such as diode array targets, it should be understood that the invention may also be used in connection with other articles.

What is claimed is:

1. An apparatus for selectively coating a workpiece, comprising:

a hollow tubular member having a free end;

means for producing a vacuum in the tubular member to hold one side of the workpiece on the free end thereof;

means for applying coating material to the opposite sideof the workpiece;

means for rotating the tubular member to spread the material on the workpiece;

means, positioned around the tubular member, for

forcing a fluid under pressure, in an annular pattern, against the held side of the workpiece to prevent the material from coating such held side of the workpiece.

2. The apparatus of claim 1, wherein the means for forcing the fluid against the workpiece is an annular fluid projecting vent positioned adjacent the portion of the workpiece.

3. An apparatus for coating with a masking material a substrate having a face and a backside opposite its face, comprising:

a hollow tubular member having a free endfor holding the backside of the substrate;

means for producing a vacuum at the opposite end of the tubular member to hold the substrate on the free end thereof;

means, positioned around the tubular member, for

forcing a gas under pressure, in an annular pattern, against the backside of the substrate;

means for flooding the face of the substrate with the masking material; and

means for rotating the tubular member to spin the substrate to thereby spread the masking material on the face thereof, the gas under pressure preventing the material from contacting the the backside of the substrate and aiding the vacuum in holding the substrate to the free end of the tubular member.

4. An improved apparatus for coating a substrate having a face and a backside opposite the face, wherein a hollow tubular member has a free end for holding the backside of the substrate, wherein means produce a vacuum at the end of the member opposite the free end to hold the substrate on the free end thereof, wherein a material is applied to the face of the substrate, and wherein the improvement comprises:

means for rotating the tubular member to spread the material on the workpiece; and

means positioned around the tubular member, for

forcing a fluid under pressure, in an annular pattern, against the backside of the substrate, the fluid preventing the material from contacting the backside of the substrate and aiding the vacuum in holding the substrate to the free end of the tubular member.

5. The apparatus of claim 4, wherein the fluid is gas.

6. The apparatus of claim 4, wherein the means for forcing the fluid against the backside of the substrate is a fluid projecting vent formed circumferentially about the tubular member.

7. The apparatus of claim 5, wherein the fluid projecting vent is a nozzle located circumferentially about and spaced from the tubular member and is formed by an upper beveled flange of an inner tubular member and by a coacting chamfered portion of an outer tubular member.

UNITED STATES QPATENT OFFICE A CERTIFICATE OF CORRECTION 3,791,342 Dated February l2,' 1974 Patent No.

lnvemods) Lynn F. Boyer and Anderson -F. Johnson, Jr.

It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 1, line 40 (Spec. page 1, line 31) "diode" should be "dioxide". H

Colurnn' 2, line 61 (Spec. page 4, line 16) "of" should be --o,ir"-- Q I C l I 7 Column 4, line 41 (Spec. page 7 line .16) "33" should be l3---. I 7 7 Column 5, line 25 (Spec. page 8, line 30 "therewith" should be -therein--. I 7

Column 7 line 21 (Spec. page 12, line 15) "not" should be -now--. v

Signed and sealed this 9th day of July 1971+.

(SEAL) Attest:

McCOY M. GIBSON,JR. c. MARSHALL DANN A L Attesting Officer Commissioner of Patents

Claims

1. An apparatus for selectively coating a workpiece, comprising: a hollow tuBular member having a free end; means for producing a vacuum in the tubular member to hold one side of the workpiece on the free end thereof; means for applying coating material to the opposite side of the workpiece; means for rotating the tubular member to spread the material on the workpiece; means, positioned around the tubular member, for forcing a fluid under pressure, in an annular pattern, against the held side of the workpiece to prevent the material from coating such held side of the workpiece.

3. An apparatus for coating with a masking material a substrate having a face and a backside opposite its face, comprising: a hollow tubular member having a free end for holding the backside of the substrate; means for producing a vacuum at the opposite end of the tubular member to hold the substrate on the free end thereof; means, positioned around the tubular member, for forcing a gas under pressure, in an annular pattern, against the backside of the substrate; means for flooding the face of the substrate with the masking material; and means for rotating the tubular member to spin the substrate to thereby spread the masking material on the face thereof, the gas under pressure preventing the material from contacting the the backside of the substrate and aiding the vacuum in holding the substrate to the free end of the tubular member.

4. An improved apparatus for coating a substrate having a face and a backside opposite the face, wherein a hollow tubular member has a free end for holding the backside of the substrate, wherein means produce a vacuum at the end of the member opposite the free end to hold the substrate on the free end thereof, wherein a material is applied to the face of the substrate, and wherein the improvement comprises: means for rotating the tubular member to spread the material on the workpiece; and means positioned around the tubular member, for forcing a fluid under pressure, in an annular pattern, against the backside of the substrate, the fluid preventing the material from contacting the backside of the substrate and aiding the vacuum in holding the substrate to the free end of the tubular member.

5. The apparatus of claim 4, wherein the fluid is gas.