Method of forming a via in a substrate

A method of forming a via in a substrate is provided. The method generally includes laminating a support to the substrate, forming the via in the substrate, and then stripping the support from the substrate. The support is preferably a photoresist that collects any debris generated by the via formation so that the debris is removed from the via and substrate surface as the photoresist support is stripped.

BACKGROUND OF THE INVENTION
 1. Technical Field
 The present invention generally relates to a method of forming a via in a
 substrate, and more particularly, to a method for forming a via in a
 substrate using a support.
 2. Background Art
 In the production of substrates, such as printed wiring boards or laminated
 chip carriers, the formation of vias, through holes, or the like is often
 necessary. As such, various drilling methods are employed. However, a
 problem exists when the vias are formed in that the entry and exit
 portions of the vias are often left distorted by the drilling operation.
 In addition, debris from formation of the vias remains disposed along the
 entry and exit portions of the vias. This debris is not only adverse to
 the performance of the substrate, but is also time consuming and costly to
 remove. Although a number of methods of forming such vias exists, no
 previous method eliminates these problems, hereinafter referred to as
 "volcanoing."
 Heretofore, many have attempted to develop suitable methods of forming
 vias, but have fallen short. One such example of this is shown in Japanese
 Publication No. 58218387 JP A1, in which plastic powder is packed as a
 backing material on a workpiece as a laser drills a hole therethrough.
 However, because the plastic powder is not laminated to the workpiece,
 additional components, such as a piston, are necessary to maintain the
 contact between the backer and the workpiece. In addition, the plastic
 powder does not aid in the removal of the debris generated from drilling
 the workpiece. In contrast, upon removal of the plastic powder, the debris
 is still present on the workpiece.
 Another example of a backing material is shown in Japanese Publication No.
 58058987 JP A1, which teaches the use of a metallic plate as a backer
 material. Specifically, a metallic plate is closely adhered to a workpiece
 which is then pierced by a laser to form holes therein. By, adhering a
 metal plate to the workpiece, however, additional process steps are
 required to remove the backing plate. Moreover, the rigid nature of a
 metallic plate fails to collect the debris created from drilling the
 workpiece, thus, leaving the debris disposed along the surfaces of the
 workpiece.
 Yet another example of a backer material is shown in Japanese Publication
 No. 59033091 JP A1, which teaches the contact of a "plate-like" object
 into tight contact with a workpiece. Once tight contact is achieved, the
 workpiece can then be laser drilled to form holes therethrough. Similar to
 the above reference, however, the use of a plate-like object not only
 requires additional process steps for removal, but also fails to remove
 the debris generated by the drilling operation.
 Lastly, U.S. Pat. No. 4,857,698 to Perun, herein incorporated by reference,
 teaches the use of an adhesive tape as a backer material. In particular,
 Perun shows an adhesive tape that is adhered to an article which is then
 drilled to form holes therein. However, the use of adhesive tape can be
 costly, and its removal can result in structural damage to a workpiece,
 especially if the workpiece is thin and lightweight. In addition, the
 removal of residue from the tape adhesive requires additional processing
 steps.
 Therefore, there exists a need for a support material, hereinafter referred
 to as a "support," that can be used in the formation of vias or the like,
 such that the entry and exit portions of the vias are evenly formed (not
 distorted). In addition, there exists the need for a support that will
 remove any debris generated by the drilling operation from the workpiece
 surface upon removal of the support.
 SUMMARY OF THE INVENTION
 The present invention overcomes the deficiencies of the related art by
 providing a method of forming vias in a substrate or the like, such that
 the entry and exit portions of the via are evenly formed and any debris
 generated by the via formation is removed with the support.
 According to one aspect of the present invention, a method of forming a via
 in a substrate is provided. The method includes the steps of: (1)
 laminating a support to the substrate; (2) forming a via in the substrate;
 and (3) stripping the support from the substrate.
 According to a second aspect of the present invention, a method of forming
 a via in a substrate is provided. The method includes the steps of: (1)
 providing the substrate; laminating a photoresist support to the
 substrate; (2) forming at least one via through the substrate; and (3)
 stripping the support from the substrate.
 According to a third aspect of the present invention, a method of forming a
 via in a substrate is provided. The method includes the steps of: (1)
 providing a substrate; (2) laminating a photoresist support to the
 substrate; (3) exposing the photoresist support; (4) forming a via in the
 substrate; and (5) stripping the photoresist support from the substrate
 It is therefore an advantage of the present invention to provide a method
 of forming a via in a substrate or the like, such that the entry and exit
 portions of the via are evenly formed (not distorted) and any debris
 generated by the formation of the via is removed.

It is noted that the drawings of the invention are not to scale. The
 drawings are merely schematic representations, not intended to portray
 specific parameters of the invention. The drawings are intended to depict
 only typical embodiments of the invention, and therefore should not be
 considered as limiting the scope of the invention. In the drawings, like
 numbering represents like elements between the drawings.
 DETAILED DESCRIPTION OF THE INVENTION
 Referring now to the drawings, FIG. 1 shows a substrate 10 or the like,
 having vias formed therethrough in accordance with a known method. The
 substrate 10 generally includes dielectric or insulative layers 14, an
 electrically conductive layer 16 therebetween, and vias 18. It should be
 appreciated, however, that substrates, such as those shown throughout
 FIGS. 1-3, are well known to those of ordinary skill in the art, and
 accordingly, the form of the substrate 10 may vary. For example, the
 substrate 10 may have several additional electrically conductive layers 16
 and dielectric layers 14. In addition, electrically conductive layers 16
 may be etched to form a circuit pattern.
 The substrate 10 of FIG. 1 shows vias 18 that were formed in the substrate
 10 without the use of a support or with the use of an inadequate support.
 As can be seen, the entry 12 and exit 12A portions of the vias 18 were
 distorted during the via forming operation. Such a phenomena is referred
 to as volcanoing, due to the physical appearance of the entry 12 and exit
 12A portions. Volcanoing is caused by failure to reinforce the entry 12
 and exit 12A portions of the vias 18 during the drilling of the substrate
 10. In particular, as the drill (not shown) enters the substrate at entry
 portion 12 and exits at exit portion 12A, the substrate material displaced
 by the drill is forced towards the entry 12 and exit 12A portions of the
 vias 18. This leaves the substrate 10 not only with distorted vias, but
 also with an excess of debris disposed along the surface of the substrate
 10.
 However, by providing a support at the exit portion 12A, the entry portion
 12, or both, the substrate is reinforced and the problems associated with
 volcanoing and excess debris are eliminated. FIG. 2 shows a substrate 10
 with vias 18 drilled therethrough. As shown, the substrate was laminated
 with a support 20 prior to the drilling operation. In particular, the
 support 20 is laminated to the substrate 10 during the original formation
 of the substrate 10 or thereafter, but in all cases, support 20 is
 laminated prior to the formation of the vias 18. Preferably, support 20 is
 laminated to the substrate 10 using a hot roll laminator. Alternately,
 vacuum lamination or other suitable lamination technique may be employed
 to laminate the support 20 to the substrate 10.
 The support 20 is preferably a photoresist, such as Dupont Riston 9008 or
 other acrylate-based photoresist, however, it should be appreciated that
 many other photoresists may suffice. In addition, although support 20 is
 shown as having been laminated to both sides of the substrate 10, it
 should be appreciated that a user may selectively laminate the support 20
 to the underside or exit portion 12A of the of the substrate 10, or to the
 top or entry portion 12 of the substrate 10.
 Once the vias 18 have been drilled in a manner known in the art, the
 support 20 is stripped from the substrate 10 leaving clean, evenly formed
 vias 18 without debris, as shown in FIG. 3. As indicated above, the
 support 20 is preferably a photoresist material. As such, the photoresist
 support 20 can be removed according to any method generally known in the
 art. For example, a Dupont Riston 9008 resist can be stripped using a 25%
 solution of Dupont Riston S-100X in H.sub.2 O. The advantages of using a
 photoresist as a support 20 include its ease of removal after the vias 18
 have been drilled. Specifically, related art devices using plating,
 require additional mechanical processing steps to effect its removal. In
 contrast, the photoresist support 20 can be easily removed with chemical
 processes. In addition, any debris generated during the via formation is
 deposited onto the photoresist material. Accordingly, removal of the
 photoresist support 20 from the substrate 10 is accompanied with removal
 of all debris from the substrate 10.
 FIGS. 4-6 show three alternative methods of forming the via(s) 18 in the
 substrate 10. Referring first to FIG. 4, the first step 24 of the method
 22 is to laminate a support to the substrate. This is shown in FIG. 2, and
 is performed according to methods known in the art. The second step 26 of
 the method 22 is to form the via in the substrate. Via formation is also
 well known in the art and is preferably accomplished with a 355 nanometer
 Nd:YAG laser. However, it should be appreciated that many other methods of
 via formation exist. For example, the via could be formed with a
 mechanical drill. In addition, by forming a via in a substrate having the
 support of the present invention, a substrate having evenly formed vias
 will result. The final step 28 of the method 22 is to strip the support
 from the substrate. As indicated above, stripping of the support may
 accomplished by any suitable method known in the art. In addition, since
 the support collects any debris generated during the formation of the via,
 the subsequent removal of the support also serves to remove the debris
 from the substrate surface.
 FIG. 5 shows an alternative method 30 of forming a via in a substrate. In
 particular, the first step 32 of the method 30 to provide a substrate. As
 stated above, the form of the substrate may vary and one such example is
 shown throughout FIGS. 1-3. The second step 34 of the method 30 is the
 lamination of a photoresist support to the substrate. The third step 36 in
 the method 30 is to form at least one via through the substrate. Since the
 form of substrates may vary, so may the quantity of vias and the method
 used to form the vias. Accordingly, the number of vias formed will vary
 depending on the needs of the user. In addition, the via is preferably
 formed using a 355 nanometer Nd:YAG laser, however, it should be
 understood that other equivalent means for forming vias exist. By forming
 the via in a substrate having the support of the present invention, a
 substrate having an evenly formed via will result, as shown in FIG. 3. The
 last step 38 of the method 30 is to strip the support from the substrate.
 Again, the stripping may be accomplished by any means generally known in
 the art. Moreover, any debris generated during the via formation is
 deposited onto the photoresist support. Accordingly, the stripping of the
 support will also remove all debris from the substrate surface.
 Finally, FIG. 6 shows an additional alternative method 40 of forming a via
 in a substrate. Specifically, the first step 42 of the method 40 is to
 provide a substrate. As previously indicated, the substrate provided may
 take varying forms depending on the needs of the user. The second step 44
 is to laminate a photoresist support to the substrate. The next step 46 in
 the method 40 is to expose the photoresist support. Specifically, the
 photoresist support is exposed to a light source to stiffen and strengthen
 the support. This is so that the support may provided rigid reinforcement
 to the substrate during the drilling operation. Preferably the support is
 exposed to an ultraviolet light source having a dose of approximately 80
 milijoules/cm.sup.2. However, those in the art will understand that other
 means for exposing the support exist. The next step 48 in the method 40 is
 to form the via in the substrate. As indicated, the via may be formed by a
 variety of known techniques, however, the via is preferably formed by a
 355 nanometer Nd:YAG laser. Moreover, by forming the vias in a substrate
 having the support of the present invention, evenly formed vias will
 result. Finally, the last step 50 of the method 40 is to strip the
 photoresist support from the substrate. Once again, methods of stripping
 the photoresist support from the substrate are commonly known in the art.
 Moreover, because any debris generated during the formation of the via is
 deposited onto the photoresist support, the subsequent removal of the
 support from the substrate will also remove the debris from the substrate.
 EXAMPLE
 A substrate is provided having a dielectric material laminated to both
 sides of a metal foil. The dielectric material is a composite of
 poly(tetrafluoroethylene) filled with inorganic particles (e.g., fused
 silica). The particle dimensions are on the order of 5 micrometers or
 less. About 40% (by weight) of the dielectric material is composed of
 these particles. One example of such a dielectric material is provided by
 Rogers Corporation under the name RO-2800. The metal is typically copper,
 or copper-clad Invar. The substrate is laminated at a temperature of
 380.degree. C. and a pressure of 1700 PSI. Dupont Riston 9008 photoresist
 is laminated to both sides of the substrate using a hot roll laminator at
 a temperature of 105.degree. C. and a pressure of 100 PSI. Using a
 frequency-tripled Nd:YAG laser operating at a wavelength of 355 nm, vias
 are formed through the entire cross-section, i.e., through the top
 photoresist layer, the substrate, and the bottom photoresist layer. It is
 not required, however, that the vias extend completely through the bottom
 photoresist layer. Following via formation, the photoresist layers are
 stripped from both sides of the substrate using a 25% solution of Dupont
 Riston S-1100X in H.sub.2 O. The stripping solution is applied in a
 conveyorized spray system at a temperature of 140.degree. F. and a spray
 pressure of 30 PSI. Following the stripping step, the substrate is rinsed
 using DI water (room temperature, 20 PSI) and dried using an air shower at
 a temperature of 120.degree. F.
 The foregoing description of the preferred embodiments of this invention
 has been presented for purposes of illustration and description. It is not
 intended to be exhaustive or to limit the invention to the precise form
 disclosed, and obviously, many modifications and variations are possible.
 Such modifications and variations that may be apparent to a person skilled
 in the art are intended to be included within the scope of this invention
 as defined by the accompanying claims.