Eackσround of the Invention

1. Field of the Invention

This invention relates to the field of processes for creating Pβtallizaticn patterns on the surfaces of a substrate and more particularly to processes for creating high resolution metallization patterns on the surfaces of quartz cr stals, hybrid circuits or semi conductor devices using photoli hographic techniques which are readily adapted to autcir.ation.

2. Description of the Prior Art

The normally accepted manu acturing process for producing iretailization areas on quartz crystal products is a stenciling process called shadow masking. In this process an apertured ir.ask is placed in contact with a crystal blank which is soir.etimes

polished. The apertures in the mask form a stencil pattern corresponding to places where metallization is desired and will be deposited. The masked blank is placed in a suitable vacuum chamber. Metal is then evaporated within the chamber and adheres to the crystal's surface in the places exposed by the apertures of the stencil. In such a process resolution is moderate at best with dimensional accuracy being only within approximately 1/1000th inch. The resolution is limited by the accuracy with which a εhaaow mask can be manufactured by various machinin ^' g or etching techniques.

If metal is to be deposited on both surfaces of the crystal by shadow masking, alignment of the desired patterns from front to back is difficult to control. This alignment typically varies from one crystal to the next, resulting in inconsistency in the cr stal's motional parameters. The crystal's motional parameters determine the center frequency, passbanά shape, spurious response, and other electrical parameters. The masks are typically made of stainless steel or other metal with thickness of about 3/1000th of an inch. Intricate stencil patterns in this thin metal are easily bent or otherwise damaged.

In addition, due to the stenciling nature of the masks, not all patterns are producible by this method without using two or more iterations of the process. This generally results from the masks being punched or etched from a single sheet of metal. Fatterns which do not connect in some way to the edge of the crystal are impossible to fabricate. In addition, such masks are difficult and time consuming to make. This is a serious drawback to the experimental stages of crystal development.

SUBSTITUTE

In one method for making a tape carrier for manufacturing leads for integrated circuits, an adhesive backed flexible insulating tape is used to carry thin metal sheets of foil on its surface to produce integrated circuit leads. The insulating tape has one entire surface covered with adhesive and has holes punched in its surface which allow access to both sides of portions of the copper foil for processing by photolithographic techniques. Only that portion of the foil exposed by the aperture in the tape carrier is available for photolithographic processing on the surface contacting the adhesive. Bonding of the foil to the tape is accomplished by the adhesive properties of the tape.

Care must be taken to ensure that the finished assembly is not exposed to high temperatures which .would c se the adhesive to lose strength. Care must also be εx ercised in the selection of developing and etching agents used in the processing of the leads also tc ensure that they do nor react adversely with the adhesive. The adhesive is prone to attracting dirt and ust wich can contaminate the chemicals used in further processing the tape carrier assembly.

Although other prior art devices are useful for forming flexible metal leads they are not well suited for processing more brittle vorkpieceε such as ceramic or c artz cryst; ϊcstr* :eε. It is evident that separating such wor piecεs from the adhesive surface of a tape carrier would likely result in damage to a large number of the substrates. This would make production yield low and assembly costs unreasonably high. A further drawback of the tape carrier is that it is obviously usable only once as attempted reuse would degrade the adhesive properties of the tape carrier rendering the adhesive unreliable.

QMPI

It is also known that both sides of a silicon wafer may be exposed simultaneously using double sided photolithography. Such exposures are normally made using an "alligator mask* ^" , which holds photolithographic masks in direct contact with photoresist coated silicon wafers. Other methods of double sided photolithography are known in the art.

Summary of the Invention

It is an object of the present invention to provide an improved method of creating high resolution metallization patterns on one or both surfaces of a substrate.

It is another Object of the present invention to improve front to back alignment and line resolution of metallization patterns on quartz crystals. It is further object of the present invention to provide an improved method of captivating a substrate within a holder during processing.

It is a further object of the present invention to provide a method of captivating a quartz crystal in a holder during photolithographic processing for a single sided or a double sided process.

These and ot er objects of the invention will be¬ come apparent to those skilled in the art upon conεi- - ^■ deration of the following description of the invention.

The present invention is directed to providing a method of captivating a substrate having two opposed surfaces within a holder. The holder has two opposed major surfaces and at least one aperture which is appropriately dimensioned to accept the substrate.

OMPI SHEET

The substrate is placed within the aperture of the holder and a dry film photopolymer resist is laminated to at least one of the surfaces of the holder and the substrate. The photopolymer resist, therefore, captivates the substrate within the holder. The dry film photopolymer resist is then caused to polymerize in desired areas. One of these desired patterned areas should be an uninterrupted area extending from the surface of the holder to the surface of the substrate. The undesired areas of the dry film photopolymer resist are then removed. This leaves a retaining tab ^" of resist in one corresponding uninterrupted desired area extending from the surface of the holder to the surface of the subs-trate. Several such tabs may be formed. The substrate is therefore captivated within the aperture of the holder by the dry film photopolymer resist before it is developed and by one or more retaining tabs after it is developed. The features of the invention believed to be novel are set forth with particularity in the appen¬ ded claims. The invention itself, however, both as to organization and method of operation, together with further objects and ac nt-ages thereof, may ^' be best understood by reference to the following descrip¬ tion taken in conjunction with the accompanying drawings.

Erief Descrition of the Drawincs

FIG. 1 shows an exploded view of the substrate holder showing the relative locations of the .εub- strates and photoresist.

FIG. 2 shows the substrate holder of FIG. 1 after lamination.

FIG. 3 εhows a croεε sectional view of FIG. 2 along line 3-3.

FIG. 4 illustrates the exposure procesε for the dry film photopolymer resist. FIG. 5 εhows a portion of the crosε-εection of FIG. 3 after exposure and development of the photo¬ polymer rεεiεt.

FIG. 6 εhows the cross-section of FIG. 5 after metallization has been etched away. FIG. 7 shows the crystal being retained in the holder by retaining tabs after exposure, development and etching.

FIG. 8 shows the completed quartz crystal.

Description of the Preferred Embodiment deferring to FIG. 1 of the preferred embodiment, the body 15 of a holder 20 is preferably composed of flexible stainless steel shim εtock approximately 3/1000th of an inch thick. In the preferred eriocime t the holder is εta pεd, etched or otherwise formed into a shape similar to 35mm photographic film format. Drive holes 25 are located along each edge and are used to tranεport the holder through a processing system. The preferred embodiment adapts - 35mιr photographic film drive equipment to this purpose. Apertures 30 are of the same shape and of slightly larger dimensions than the substrate which the holder will accommo a e. The aperture and substrate may or may not contain some keying mechanism to ensure the εtb≤trate is inserted with proper orientation if appropriate. In one embodiment, this procesε iε used to produce high resolution metallization patterns on quartz crystalε.

TITUTE SHEET

Similar holders, however, could be manufactured for use with ceramic, metal, or silicon substrates. The pattern of holes 30 may be repeated at regular intervals throughout the length of the holder. Although a reoccurring specific pattern, of eight (8) holes is shown for the preferred embodiment, any convenient number may be used.

In FIG. 1 and again in TIG . 2, one can see that the substrate 35, which is illustrated as a quartz crystal blank un formally coated on one or both sur¬ faces with a metal such as aluminum, is placed within the apertures of the holder. Space 45 between the aperture walls and the crystals is shown exaggerated for illustrative purposes. In the preferred embodi- ment about a 5 mil gap is present between the crystal and the holder. In one embodiment a very tight tolerance is held between the aperture of the holder and the crystal substrate in order to assure good alignment and registration. A dry film photopolymer resist 40 has been laminated-to the bottom surface of the holder.

A dry film photopolymer resist (hereafter called "resist") is a substance which comes in a thin sheet and polymerizes when desired areas are exposed to - - certain wavelengths of light. In the preferred embodiment Ristor.* mace by D pont, may be used, and it has been found that a 1.1 ιr.il thick resist is optimum for this process. Dry film, photopolymer resists have an advantage over liquid photopolymer resists in that the user does not have to be concerned with getting an even thickness with uniform coverage on the workpiece. After polymerization the unpolymerized areas of the dry resist can easily be washed away by immersing the resist in an appropriate developer as recommended by the manufacturer.

As illuεtrated by FIG. 2 the resist is cut into strips somewhat narrower than the width of the holder but wide enough to completely cover all apertures 30 of the holder while not interfering with drive holes 25. The resist is then laminated, using a heat lamination process, to the holder and the substrates are enclosed in the apertures of the holders. In the preferred embodiment resist 40 is actually laminated to the lower surface of holder 20 before the quartz crystals 35 are placed into the apertures 30. But the crystals could be placed in the holder before lamination takes place. The use of heat lamination eliminates the sticky tape used in other processes. Therefore, there is no adhesive to contaminate chemicals use later in the process and handling is much εimpler.

In the next etep of the process a second layer of resist 55 is laminated to the upper surface of the holder thus captivating the substrate on both εideε with the dry resist. A frontal view of this assembly is shown as 60 in FIG. 2.

It should be noted that although the preferred embodiment εhows bothε sides of_holder 20 laminated with resiεt, a εingle si ec process is also possible since only one layer of resist is necessary to capti¬ vate the substrate.

Reεiεt 55 iε alεo of narrower width than the width of the holder 20 but greater width than iε required to cover all apertures 30. Print 55 is also heat laminated to the surface of the holder 20 and the surfaces of the substrate 35. The process may -

S

proceed in the following order: laminate the first side; place substrate in aperture; laminate the second side; the second laminating process for resiεt 55 (the upper side) should cause both the lower and upper layers of reεiεt to adhere to the crystal substrate. The laminating process can be accomplished by paεεing the laminae between heated rollers.

A cross-section of assembly 60 of FIG. 2 along lines 3-3 is shown in FIG. 3. This view more clearly illustrates the captivation of substrate 35 within apertures 30 of the holder 20 by reεiεt layers 55 and 40. In this view, the quartz εubstrate 70 iε shown to have metallization layers 65 and 75 on both surfaces. Although the substrate iε shown to be the same thickness as the crystal, this is not required if a resilient resist iε used. Thiε iε because of a property of the resist known in the art as "tenting". As stated earlier, in the preferred embodiment the εubstrate iε quartz and the metallization is alumi¬ num. The spaces 45 between the holder body 15 and the εubstrate 70 may be greater than shown to allow the reεiεt layers to collapse and touch one another. Thiε makes a more secure bond to hold the substrate in place.

At this ≤tage of the procesε the upper resiεt layer 55 iε tightly bonded to the holder body 15 and the upper metallization 65. The lever resist layer 40 iε tightly bonded to the holder body 15 and the lower metallization layer 75. Although this embodiment shows that the εubstrate is coated with metallization layers 65 and 75, it iε not necessary for the adhesion of the resist. It is merely a function of this embodiment since a metal etching procesε to follow. It will be evident to one εkilled

in the art that an unmetallized εubεtrate could be masked with resiεt to allow selective plating, painting or sputtering of metal in un aεked areas. FIG. 4 shows a photolithographic mask 80. Thiε mask will be used in the next step of this process. The mask allowε light to strike εo e portions of the reεiεt layerε while εhading the light from other portions. In the preferred embodiment the clear areas will allow the light to strike the reεiεt cauεing polymerization. It would be evident, how¬ ever, to anyone εkilled in the art that although thiε embodiment is a positive proceεε the proceεε could also be implimented as a negative process uεing the negative -of thiε ir.aεk and a different type of resiεt and/or developer. One of the many advantages of using a lithographic mask εuch aε ir.aεk 80 iε that any type of pattern which can be drawn and/or photographed can be realized on the surface of the . substrate within the resolution accuracy of the film and resiεt.

In one embodinvent, cryεtal electrode patterns 95 are formed on a quartz substrate by thiε process. Arbitrary code numbers and letter.ε SO may also be formed by thiε proceεε. _-Aε will iε explained later, these numbers and letters, aε shown, could not be manufactured on the finished product with a single shadow masking proceεε. Such photolithographic masks can be made of any number of materials. In the preferred embodiment glass masks are uεed. In the photolithographic proceεs, photolithographic mask 80 is the counterpart of the shadow mask in a shadow masking proceεε. Since it iε manufactured photo¬ graphically itε reeolution iε approximately a factor of ten better than that of a mechanically bored

or chemically etched εhadow mask. This results in a direct improvement in the crystal's electrical parameters.

As stated earlier, εhadow masks are typically made of shim stock of about 3/1000-th of an inch. Due to their very nature, long freε-εtanding pattern members are not possible in a production environment since normal wear and tear will damage or wear out these masks in a very short period of time. Such long members made of such thin material can hardly support their own weight and are very easily damaged. Since the patterns on photolithographic masks are photographic, they exhibit no εuch problem.

In the present invention one or more clear tab areas 85 located at the perimeter of each substrate iε placed on the glass maεk. These tab areas 85 form retaining tabs in the reεiεt material. These retain¬ ing tabs will be attached both to the subεtrate and to the holder aε will be shown in later steps. FIG. 4 further illustrates the process of expos¬ ing the photoresist in a double sided version of the present procεεε. Although FIG. 4 εhows two light sources 100 and 100a, many techniques are known and may be used for exposing both sices using only one source. Any of these techniques could easily serve the same purpose.

In the exposure step, holder assembly 60 iε sand¬ wiched between and placed in contact with photomasks SO and 80A. Since these are optical masks thεy may be readily aligned by any number of m.ethods before assembly 60 iε placed therebetween. The captivated εubεtrate within assembly 60 is then carefully aligned with the pre-aligned masks 80 and 80A. The total assembly iε then held rigidly in place while exposure by light sources 100 and 100A takes place.

O PI

In the preferred embodiment the light sources 100 and 100A are short wave ultra-violet light εourceε. Thiε however, iε a function of the characteriεticε of the dry film photopolymer reεiεt. After expoεure iε complete, assembly 60 is removed from betwεen the glass masks. It should be noted that one distinct advantage of thiε process is that masks 80 and 80A are easily duplicated photo¬ graphically. Thus, if a aεk iε scratched or should it break, it is easily replaced at low coεt. Thiε iε in sharp contrast with shadow masks. Also, new aεkε for experimental designs are quickly and readily fabricated directly from artwork dene in drafting tape. FIG. 5 εhows a portion of the same crcεε-sεction aε that shown in FIG. 3 after exposure has taken place and the undeεired unpolymerized areas of the resist have been removed. Removal of the undesired reεiεt areas iε accomplished by exposing the entire assembly 60 to an appropriate developing agent aε specified by the manufacturer of the reεiεt. It should be noted that holding tabs 115 securely hold the subεtrate in place and further serve to absorb εhockε and stresses if assembly 11C iε flexed. They _. thus provide support and protection to the fragile crystal subεtrate while allowing flexing or rolling of the holder. Thiε allcwε a holder of great length to be rolled onto a reel similar to notion picture film without damaging the substrate. In areas 127 the reεiεt has been dissolved to expose metallization 65 and 75. Resiεt areaε 120 and 125 now maεk areaε of metallization 65 and 75 respec¬ tively which will become upper and lower cryεtal elεctrodeε reεpectively. Thεεe areaε will be protected by the resist during the proceεε steps that

SHEET

13

follow. In the preferred embodiment, it is desired to etch away metallization which was previously deposited to εurf ceε of substrate 70. The chemicals used to perform the etching process should not affect the holder material if the holder iε to-be reuεed.

It will be evident to thoεe εkilled in the art that a plating procesε or other types of proceεεeε can be implemented using this photolithographic masking technique. FIG. 6 shows the same assembly 130 after an etch¬ ing procesε has taken place. The metallization which formerly covered areaε 142 haε been etched away by an appropriate solution, and the bare crystal is exposed. FIG. 7 εhows a frontal view of assembly 130. Cross-section 130 could be taken along line 6-6 of Fig. 7. Here one clearly sees retaining tabs 115 suspending crystal 155 in place within. he holder 20. The tabs 115 are uninterrupted in an area from the surface of the substrate to the surface of the holder. Also shown iε the resist covered electrode 120 and resist covered code letterε and numbers 160. The reεist on these remaining reεists covered areas can now easily be washed away with an appropriate solvent, thereby releasing the substrate from the holder. In the preferred embodiment the solvent iε acetone. Alternatively, removal from the holder can be accomplished by a punching operation, a cutting operation or even cutting away the retaining tabs with a laser. Such a step would normally be followed by dissolving away the remaining reεiεt left on the subεtrate.

FIG. 8 εhows the finished crystal product 190. Metallization areaε 165 remain where the retaining tabs 115 once were. Thiε tab metallization should be either εtratigically placed so as not to interefere

with the operation of the cryεtal (or other device ⁾ , or removed by εome means. Alternatively, the metallization areas 165 can be used aε a portion of the εubεtrate electrodεε or metal patternε themεelveε. In thiε manner they will εerve a multiple function and removal iε not necessary or desired.

Front electrode 170 and backside electrode 175 now constitute exposed metallization areaε where before they were coxiered with photoreεist. For illustrative purpcsεe code lettere and numbers 180 have been shown on the finished product. Note ^' that the freestanding areaε εuch aε 185 of alpha or numeric characterε or 167 of the electrode pattern could not have been produced by shadow maεk stenciling. This iε due to the stenciling nature of the shadow r.aεk process. Similarly, freestanding electrode patterns can be more readily generated using photolithography. Such patternε could only have been generated previously by εhadow maεking with multiple εhadow aεks, or other complicated processes which greatly increase procesε cost and risk of damage to the substrate and masks during handling. Such freestanding areas aε 167 are known in the art to improve εp riouε responses, but were difficult to manufacture until now.

One can therefore see the above method indeed captivates the εubεtratε during proceeεing. The holder can be reuεed many ti eε, and there iε no messy adhesive to contaminate the procesε. In addition, it haε been found that pattern reεolution with the photolithographic proceεε iε approximately 10 timeε better, than with εhadow maεks. Dimensions can be held to within 1/10,000th inch with photolithography compared to 1/1000th inch with

OMPI t.4,-. 1P0

shadow maskε. Also photolithography lends itself to many known methods of attaining good front to back allignment. All these factors contribute to direct improvement in a crystal's electrical performance. Thus it iε apparent that there haε been provided in accordance with the invention a method rhat fully εatisfieε the objects, aims, and advantages set forth above. While the invention haε been described in conjunction with specific embodiments thereof, it iε evident that many alternatives, modificationε and variations would be apparent to those skilled in the art in light of the foregoing description. Accordingly, it is intended to embrace all such alternativee, modif cationε and variations aε fall within the εpirit and broad scope of the appended claims. K Claim:

-BUREA

OMPI