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(i9) United States
(12) Patent Application Publication
Furman et al.
(io) Pub. No.: US 2008/0265404 Al (43) Pub. Date: Oct. 30, 2008
(54) STRUCTURE AND METHODS OF
PROCESSING FOR SOLDER THERMAL
INTERFACE MATERIALS FOR CHIP
(75) Inventors: Bruce Furman, Poughquag, NY
(US); Madhusudan K. Iyengar,
Woodstock, NY (US); Paul A.
Lauro, Brewster, NY (US); Yves
Martin, Ossining, NY (US); Roger
R. Schmidt, Poughkeepsie, NY
(US); Da-Yuan Shih,
Poughkeepsie, NY (US); Theodore
G. Van Kessel, Dutchess County,
NY (US); Wei Zou, Wappingers
Falls, NY (US)
Connolly Bove Lodge & Hutz LLP
Suite 800,1990 M Street, N.W.
Washington, DC 20036 (US)
(73) Assignee: International Business Machines Corporation, Armonk, NY (US)
(21) Appl.No.: 11/742,161
(22) Filed: Apr. 30, 2007
(51) Int. CI.
H01L 23/34 (2006.01)
H01L 21/00 (2006.01)
(52) U.S. CI 257/706; 438/122; 257/E23.08;
Assemblies for dissipating heat from integrated circuits and circuit chips are disclosed. The assemblies include a low melt solder as a thermal interface material (TIM) for the transfer of heat from a chip to a heat sink (HS), wherein the low melt solder has a melting point below the maximum operating temperature of the chip. Methods for making the assemblies are also disclosed.
Patent Application Publication Oct. 30, 2008 Sheet 1 of 2 US 2008/0265404 Al
(Deg. C/W) o.04
—o—-ln-Bi 88C/171W □ ln-Bi 50C/103W
—a ln-Bi 95C/172W x—-ln-Bi 87C/172W
STRUCTURE AND METHODS OF
PROCESSING FOR SOLDER THERMAL
INTERFACE MATERIALS FOR CHIP
FIELD OF THE INVENTION
 The present invention relates to structures that provide heat dissipation for chip cooling using low melting temperature solders. The present invention further relates to methods of processing such structures.
BACKGROUND OF THE INVENTION
 Integrated circuits (IC) device and circuit chips generate heat during operation. If a sufficient quantity of such heat is not transferred away, the functional integrity of an integrated circuit or circuit chip can be compromised.  One method of removing heat from integrated circuit chips involves conducting heat away from microprocessor chips to a heat exchanger, or heat sink (HS), via a thermal interface material. The HS, which is usually located above the chip circuitry to be cooled, is thermally coupled to the chip circuitry by means of at least one thermal interface material (TIM). For example U.S. Pat. Nos. 6,724,078 and 6,504,723 each disclose using an indium solder, having a melting temperature of 157° C, as a TIM material. However, the structures and processes disclosed in those patents require metallization on both the back of the chip as well as on the Cu lid or heat sink. Such metallization creates additional manufacturing cost and complexity.
SUMMARY OF THE INVENTION
 The present invention provides an assembly including at least one heat sink, at least one chip, and at least one low melt solder positioned between the chip and the heat sink, wherein the low melt solder has a melting point that is below the maximum operating temperature of the chip. In addition, the present invention provides a method for making an assembly for chip cooling which method includes providing at least one heat sink, providing at least one chip, providing a solder wetting area on the heat sink, providing at least one low melt solder on the solder wetting area, wherein the low melt solder has a melting point that is below the maximum operating temperature of the chip, placing the chip in thermal contact with the low melt solder to form the assembly, and heating the assembly to melt the low melt solder.
BRIEF DESCRIPTION OF THE DRAWINGS
 FIG. 1 is an illustrative representation of a side view of an assembly having a low melting temperature solder ("low melt solder"), wherein the low melt solder is in physical contact with the back of a chip prior to heating the assembly to melt the solder.
 FIG. 2 is a chart showing the measured thermal resistance of an InBi low melt solder used as a thermal interface between a chip and a Cu lid.
DETAILED DESCRIPTION OF THE INVENTION
 The present invention, in at least one embodiment, provides an assembly having a low melting temperature solder ("low melt solder"), which is used as a thermal interface material (TIM) to transfer heat from an integrated circuit chip
to a heat sink (HS), such as a Cu lid. The low melt solder has a melting temperature that is lower than the maximum operating temperature of the chip.
 As used herein, the term "chip" can encompass any type of integrated circuitry, and includes, for example, a die that is sawed or cut from a semiconductor wafer.  As used herein, the term "heat sink" refers to any thermally conductive material or device used to transfer heat from integrated circuitry. An example of a heat sink is a copper lid (Cu lid) that is positioned above the chip circuitry to be cooled. The geometric configuration of the heat sink is not limited and can include, for example, a configuration having fins or heat pipes. The heat sink may further have liquid/vapor heat/mass transfer capability, such as a heat sink having a vapor chamber.
 The heat sink should include a contact or wetting area at its interface with the low melt solder. The solder wetting capability of the heat sink may be enhanced by metallizing or flashing on the heat sink at least one material such as chromium (Cr), nickel (Ni) or gold (Au) and alloys or mixtures of the same. The low melt solder is wetted by the Au layer and will form a thin layer with the Ni layer below. The thickness of this layer is typically very thin (for example, about 10 to about 100 nm) if samples are processed at low temperature (such as less than about 100° C.) but can increase significantly (for example about 100 to about 1000 nm) at higher temperatures (such as greater than about 150° C).  As used herein, the term "maximum operating temperature of the chip" refers to the maximum temperature at which a chip, such as an integrated circuit chip, can operate while still maintaining its functional integrity. While this parameter will vary depending on the type of chip or chip application, for many applications this temperature should not exceed about 125° C, and can often be expected to fall within the range of about 65° C. to about 85° C.  The low melt solder should have a melting point or range that is below the maximum operating temperature of the chip. Examples of suitable low mold solder materials include alloys such as InBi and InSnBi having liquid temperatures ranging from about 60° C. to about 72° C. For example an alloy containing In 66 wt % Bi 34 wt. % has a melting point of about 60° C. and an alloy containing In 51 wt %, Sn 16.5 wt% and Bi 32.5 wt % has a melting point of about 72° C.
 Sandwiched solder performs can also be used. An example of a sandwiched solder perform is a layer of indium (In) sandwiched between two layers of low melt solder.  The low melt solder may be attached or joined to the heat sink at temperatures well below its melting point or melting range. In at least one embodiment, the low melt solder may be attached or joined to the heat sink at ambient room temperature.
 The low melt solder can then be placed in thermal contact with a chip. For example, the low melt solder can be in direct physical contact with the back of a chip (i.e., without the need of metallization on the back of the chip). The area of the low melt solder closest to the chip should preferably be greater than the area of the back of the chip such that the periphery of the low melt solder extends beyond the periphery of the chip. The extent should preferably not approach any surrounding components and in some cases a seal can be provided prevent excess low melt solder from being released to other areas. This seal can also provide a moisture barrier for some applications. This allows the solder to not only achieve