# Patent

## Title
Method of stiffening coreless package substrate

## Abstract

Embodiments of the present invention relate to a method of stiffening a semiconductor coreless package substrate to improve rigidity and resistance against warpage. An embodiment of the method comprises disposing a sacrificial mask on a plurality of contact pads on a second level interconnect (package-to-board interconnect) side of a coreless package substrate, forming a molded stiffener around the sacrificial mask without increasing the effective thickness of the substrate, and removing the sacrificial mask to form a plurality of cavities in the molded stiffener corresponding to the contact pads. Embodiments also include plating the surface of the contact pads and the sidewalls of the cavities in the molded cavities with an electrically conductive material.

## Background
FIELD
Embodiments of the present invention relate to semiconductor substrate technology, and in particular, a method of stiffening a coreless package substrate.
BACKGROUND
Conventional semiconductor package substrates typically include at least one core layer impregnated in a dielectric material to provide mechanical rigidity to the substrate. Latest trends of electronic devices such as mobile phones, mobile internet devices (MIDs), multimedia devices and computer notebooks demand for slimmer and lighter designs. Coreless substrates are adopted for fabrication of components in such electronic devices to enable a thinner profile of the components. The thickness of coreless substrates can be, for example, as little as approximately 25% of the thickness of cored substrates.
However, coreless substrates are more susceptible to warpage problem during Surface Mount Technology (SMT) processes compared to conventional substrates with core layers. SMT processes typically involve subjecting package substrates to heating and cooling which in turn create expansion and contraction of the substrate. The difference in coefficient of thermal expansion (CTE) of the various materials forming the substrate results in different rates of expansion and contraction and hence stress in the substrate. The resulting stress warps the substrate and causes manufacturing problems during component package assembly as well as during surface mount process at Original Equipment Manufacturers (OEMs). Unlike conventional substrates, coreless substrates do not have a core layer to provide flexural rigidity against package warpage. It is known that materials such as glass cloth and glass fibers are impregnated in the dielectric material of coreless substrates to provide flexural rigidity.



BRIEF DESCRIPTION OF THE DRAWINGS
Embodiments of the invention are illustrated by way of example and are not limited in the figures of the accompanying drawings, in which like references indicate similar elements.
FIG. 1 is a cross-sectional view of a semiconductor substrate stiffener according to an embodiment.
FIG. 2 is a flowchart of a method to fabricate a semiconductor substrate stiffener according to an embodiment.
FIG. 3 is a cross-sectional view of an initial build-up of a semiconductor substrate having a sacrificial mask disposed according to an embodiment.
FIG. 4 is a cross-sectional view of a semiconductor substrate after a sacrificial mask being exposed and developed according to an embodiment.
FIG. 5 is a cross-sectional view of a semiconductor substrate having a stiffening layer formed around the remaining sacrificial mask according to an embodiment.
FIG. 6 is a cross-sectional view of a semiconductor substrate having the remaining sacrificial mask removed and cavities in molded stiffener formed according to an embodiment.
FIG. 7 is a cross-sectional view of a semiconductor substrate having a photoimagable material formed on molded stiffener and on regions around cavities in a molded stiffener according to an embodiment.
FIG. 8 is a cross-sectional view of a semiconductor substrate having the surface of contact pads and the sidewalls of cavities in molded stiffener being plated according to an embodiment.



DETAILED DESCRIPTION
Embodiments of the present invention relate to a method of stiffening a semiconductor coreless package substrate to improve rigidity and resistance against warpage. An embodiment of the method comprises disposing a sacrificial mask on a plurality of contact pads on a second level interconnect (package-to-board interconnect) side of a coreless package substrate, forming a molded stiffener around the sacrificial mask without increasing the effective thickness of the substrate, and removing the sacrificial mask to form a plurality of cavities in the molded stiffener corresponding to the contact pads. Embodiments also include plating the surface of the contact pads and the sidewalls of the cavities in the molded stiffener with an electrically conductive material.
FIG. 1 is a cross-sectional view of a semiconductor substrate stiffener according to an embodiment. Coreless package substrate 100 includes (i) conductive vias 130 embedded in dielectric material 120 and (ii) solder resist layer 170 formed on the top side of substrate 100 with openings corresponding to first level interconnect (FLI) contact pads 140. Top (FLI) side 190 of substrate 100 is adapted to chip-to-package interconnection. FLI contact pads 140 is connected to semiconductor die (not shown) via die bumps 150. Substrate 100 also includes a plurality of second level interconnect (SLI) contact pads 160 formed on bottom (SLI) side 195 of substrate 100. SLI side 195 of substrate 100 is adapted to package-to-board interconnection. SLI contact pads 160 are interconnected with FLI contact pads 140 via conductive vias 130. Molded stiffener 110 of thickness D is formed on SLI side 195 with cavities 170 corresponding to SLI contact pads 160 and exposing a surface of SLI contact pads 160. Other embodiments of the invention include the surface of SLI contact pads 160 and the sidewalls of cavities 170 being plated with electrically conductive surface finish 180.
FIG. 2 is a flowchart of a method to fabricate a semiconductor substrate stiffener according to an embodiment. Operation 200 includes disposing a sacrificial mask on a plurality of contact pads on the second level interconnect side of a coreless package substrate. Next, in operation 210, a layer of molded stiffener is formed around the sacrificial mask without increasing the effective thickness of the substrate. In operation 220, the sacrificial mask is then removed to form a plurality of cavities in the molded stiffener exposing the surface of the contact pads on the second level interconnect side of the substrate. For an embodiment, the surface of the contact pads and the sidewalls of the cavities are plated with an electrically conductive material. Details of the method are presented in the following paragraphs.
FIG. 3 is a cross-sectional view of an initial build-up of a semiconductor substrate having a sacrificial mask disposed according to an embodiment. Coreless package substrate 300 is fabricated using known processes through post-panel separation. FLI side 190 includes solder resist 170 formed with openings exposing FLI contact pads 140. SLI side 195 is not covered by solder resist. The surface of SLI contact pads 160 may be planar with or extending above the surface of SLI side 195 of substrate 300.
First, a layer of sacrificial mask 310 is disposed on SLI side 195 to mask the surface of SLI contact pads 160. Sacrificial mask 310 includes any chemical that is photoimagable and thermally decomposable. A variety of sacrificial mask 310 is available from the market traded under brand names such as Unity series (from Hitachi). For an embodiment, sacrificial mask 310 is a polycarbonate-based polymer thermally decomposable at temperature higher than the mold cure temperature of molded stiffener 110. For an embodiment, sacrificial mask 310 is thermally decomposable at around 180° C. and higher. Sacrificial mask 310 may include chemicals such as polynorbornene or poly (alkyl carbonates), which upon thermal decomposition, are broken down as volatile organic compounds or gases, and leave behind minimal residues to be removed.
Sacrificial mask 310 can be applied to SLI side 195 in various known methods such as lamination, spraying, needle or jet dispensation. The thickness of sacrificial mask 310 disposed on SLI side 195 depends on, among others, the design rules and considerations for substrate 300 and the type of SLI interconnect substrate 300 is adapted for. For example, where substrate 300 is designed to have solder spheres (not shown) as the type of interconnect between substrate 300 and board, the thickness of sacrificial mask 310 may be about half the height of the solder spheres.
After disposing sacrificial mask 310 on SLI side 195, sacrificial mask 310 is patterned to define portions 410 sacrificial mask 310 remaining only on SLI contact pads 160. FIG. 4 is a cross-sectional view of a semiconductor substrate after a sacrificial mask being exposed and developed according to an embodiment. Portions 410 of sacrificial mask 310 corresponding to SLI contact pads 160 are retained while portions of sacrificial mask 310 around SLI contact pads 160 are removed. Where sacrificial mask 310 is a polymer, patterning of sacrificial mask 310 yields a plurality of polymer islands remaining on the SLI contact pads 160. Known photolithography methods such as exposure to ultra-violet (UV), I-line, G-line and etching may be employed to define and pattern portions 410 of sacrificial mask 310 remaining on SLI contact pads 160. The type and steps of patterning and developing sacrificial mask 310 may be influenced by the type and chemical composition of sacrificial mask 310 used in the process as determined by a person of ordinary skill in the art. Cleaning is performed to remove any residue left behind by sacrificial mask 310.
Next, a stiffening layer is formed on SLI side 195. Embodiments of the invention include forming a stiffening layer by way of molding. FIG. 5 is a cross-sectional view of a semiconductor substrate having a molded stiffener formed around remaining portions 410 of sacrificial mask 310 according to an embodiment. Molding of stiffening layer 510 can be achieved using known molding techniques. Substrate 300 is loaded in a molding chassis. Molding compound is introduced in the molding chassis and flows into regions of SLI side 195 not masked with portions 410 sacrificial mask 310. Substrate 300 is clamped under pressure to allow the molding compound to stabilize and partially cure to form stiffening layer 510.
The height of stiffening layer 510 is planar with remaining portions 410 of sacrificial mask 310 masking SLI contact pads 160. Height D of stiffening layer 510 (as shown in FIG. 1) is also such that stiffening layer 510 insignificantly increase or does not increase the effective thickness of substrate 300. For example, by forming stiffening layer 510 of height D on SLI side 195 of substrate 300, the standoff or clearance between SLI side 195 and an adjacent surface of a circuit board (not shown) to which substrate 300 is connected via an interconnect (not shown) would not be increased. For an embodiment, height D of stiffening layer 510 is about half the height of solder spheres connecting substrate 300 to a circuit board. For another embodiment, height D of stiffening layer 510 is at least approximately 50 μm. Stiffening layer 510 has a lower coefficient of thermal expansion (CTE) and a higher flexural modulus relative to substrate 300. For an embodiment, stiffening layer 510 has coefficient of thermal expansion at room temperature between 3 ppm/° C. and 15 ppm/° C. For an embodiment, stiffening layer 510 has flexural modules between 10 kN/mm2 and 50 kN/mm2 
After forming stiffening layer 510, cavities 170 in stiffening layer 510 are formed to house interconnects adapted for interconnection between substrate 300 and circuit board. Remaining portions 410 of sacrificial mask 310 are removed by thermally decomposing the same. FIG. 6 is a cross-sectional view of a semiconductor substrate having the remaining sacrificial mask removed and cavities in a molded stiffener formed according to an embodiment. For an embodiment, substrate 300 is baked to thermally decompose portions 410 of sacrificial mask 310 remaining SLI contact pads 160, followed by post-mold cure to completely cure stiffening layer 510. Substrate 300 is baked at around 180° C. and higher. Post-mold cure is carried out at around 125° C. and higher. For another embodiment, substrate 300 is simultaneously having stiffening layer 510 cured and portions 410 of sacrificial mask 310 thermally decomposed at around 180° C. and higher to form molded stiffener 110 and cavities 170 in molded stiffener 110. Substrate 300 is subsequently cleaned to remove any chemicals or residue left behind by thermal decomposition of portions 410 of sacrificial mask 310. Various known methods of cleaning such as plasma etching may be employed. Removal of residues left in cavities 170 ensures that the surface of SLI contact pads 160 is free from any foreign material prior to subsequent surface treatment process.
For an embodiment, cavities 170 defined by molded stiffener 110 have a tapering profile. The tapering profile is a result of interactive effects between portions 410 of sacrificial mask 310 and curing mechanism of molding compound forming stiffening layer 510. The tapering profile is also affected by the process parameters during the thermal decomposition of sacrificial mask 310 and molding of stiffening layer 510. The tapering profile of cavities 170 in molded stiffener 110 may be positive or negative as a result of the aforementioned interactive effects and process parameters. However, embodiments of the invention do not preclude cavities 170 from having a non-tapering profile. Interconnects, such as solder spheres and contact pins, when disposed in cavities 170, contact with the surface of SLI contact pads 160.
An embodiment of the method to stiffen substrate 300 further comprises plating the surface of SLI contact pads 160 and the sidewalls of cavities 170 with an electrically conductive material. For an embodiment, plating comprises first depositing and patterning a photoimagable material on molded stiffener 110 followed by depositing surface finish 180 on cavities 170 and SLI contact pads 160. FIG. 7 is a cross-sectional view of a semiconductor substrate having a photoimagable material formed on molded stiffener and on regions around cavities in a molded stiffener according to an embodiment. Photoimagable material 710 may include a composition of reactive monomeric and/or polymeric substances along with photoactivated initiators. Photoimageable material 710, when exposed to light of proper wavelength, will activate cross-linking of catalysts, initiators or activate free radical polymerization of monomers (or polymers) in the composition. Photoactivation will transform a positive-acting composition from an insoluble state to a soluble state while a negative-acting composition will be transformed from a soluble state to an insoluble state. Readily-available photoimagable material 710 products are available in the market under various brand names suited for specific application and process needs. Photoimageable material 710 is patterned on the surface of molded stiffener 110 to mask regions where plating is not intended. A mask is employed to mask the openings of cavities 170 during the patterning of photoimagable material on molded stiffener 110. Known methods of lithography may be used for patterning of photoimagable material 710.
Next, electrically conductive surface finish 180 is deposited on the surface of SLI contact pads 160 and the sidewalls of cavities 170. FIG. 8 is a cross-sectional view of a semiconductor substrate having the surface of contact pads and the sidewalls of cavities in molded stiffener being plated according to an embodiment. For an embodiment, the plating process is electroless plating. Electroless plating of surface finish 180 on the surface of SLI contact pads 160 and the sidewalls of cavities 170 may include electroless copper, electroless Nickel/Immersion Gold (ENIG) plating and electroless gold plating. For another embodiment, the plating process is an electrolytic plating process. After plating surface finish 180 on the surface of SLI contact pads 160 and the sidewalls of cavities 170, photoimagable material 710 is removed. Photoimagable material 710 is removed by develop methods known in the art. Substrate 300 may then be subjected to known substrate fabrication steps.
Embodiments of the invention provide a method to stiffen coreless package substrates without increasing the effective thickness of the substrates. The stiffening provides coreless package substrates with improved rigidity and resistance against package warpage.
In the foregoing specification, reference has been made to specific embodiments of the invention. It will, however be evident that various modifications and changes may be made thereto without departing from the broader spirit and scope of the invention. The specification and drawings are, accordingly, to be regarded in an illustrative rather than restrictive sense.

## Claims

1. A method, comprising:
disposing a sacrificial mask on a plurality of contact pads on a second level interconnect side of a coreless package substrate, the side being adapted to package-to-board interconnection;
forming a molded stiffener around the sacrificial mask without increasing the effective thickness of the substrate; and
removing the sacrificial mask to form a plurality of cavities in the molded stiffener corresponding to the contact pads.

2. The method of claim 1, wherein the sacrificial mask is a thermally decomposable and photoimagable polymer.

3. The method of claim 2, wherein the sacrificial mask is a polycarbonate-based polymer decomposable at around 180° C. and higher.

4. The method of claim 1, wherein the molded stiffener has a lower coefficient of thermal expansion and a higher flexural modulus than the coreless substrate.

5. The method of claim 4, wherein the molded stiffener has a coefficient of thermal expansion value at room temperature between 3 ppm/° C. and 15 ppm/° C. and flexural modulus value between 10 kN/mm2 and 50 kN/mm2.

6. The method of claim 1, wherein forming the molded stiffener comprises molding portions of the second level interconnect side not masked by the sacrificial mask with a molding compound.

7. The method of claim 6, further comprising curing the molding compound at around 125° C. and above.

8. The method of claim 7, wherein removing the sacrificial mask comprises thermally decomposing the sacrificial mask.

9. The method of claim 8, further comprising simultaneously curing the molding compound and thermally decomposing the sacrificial mask at around 180° C. and higher.

10. The method of claim 1, further comprising plating the surface of the contact pads and the sidewalls of the cavities with an electrically conductive surface finish.

11. The method of claim 10, further comprising:
patterning a photoimagable material on the molded stiffener other than on the surface of the contact pads and the sidewalls of the cavities; and
depositing the surface finish on the surface of the contact pads and the sidewalls of the cavities.

12. A method of forming a semiconductor substrate, comprising:
disposing sacrificial polymer layer on a second level interconnect side of a coreless package substrate, the side having a plurality of contact pads and being adapted to package-to-board interconnection;
patterning the sacrificial polymer layer to yield a plurality of polymer islands remaining on the contact pads;
molding the second level interconnect side to yield a stiffening layer planar with the polymer islands;
thermally decomposing the polymer islands to form a plurality of cavities in the stiffening layer; and
plating the surface of the contact pads and the sidewalls of the cavities with an electrically conductive surface finish.

13. The method of claim 12, wherein the sacrificial polymer is thermally decomposable at around 180° C. and higher.

14. The method of claim 13, wherein the stiffening layer contributes insignificant increase in the effective thickness of the coreless substrate.

15. The method of claim 14, wherein the stiffening layer has a lower coefficient of thermal expansion and a higher flexural modulus than the coreless substrate.

16. The method of claim 15, wherein the stiffening layer has a thickness about half the height of solder balls configured for connection with the contact pads.

17. The method of claim 16, further comprising removing residual polymer in the cavities.