Semiconductor device with plate heat sink free from cracks due to thermal stress and process for assembling it with package

#This invention relates to a semiconductor device and, more particularly, to a semiconductor device coated with a plate heat sink and a process of assembling a plate heat sink with a package.

#While an active element such as a transistor is operating, the active element generates heat. If the active element is fabricated on a semiconductor pellet, the active element heats the semiconductor pellet, and the temperature rises. If the active element is a kind of power transistor, the power transistor generates a large amount of heat, and the temperature rise is serious.

#Metal usually has the heat transfer coefficient larger than that of the semiconductor material. For this reason, a metal plate is attached to the semiconductor pellet so as to increase the heat radiation capability of the semiconductor device. The metal plate attached to the semiconductor pellet is called as "Plate Heat Sink", and a semiconductor device with the plate heat sink is hereinbelow simply referred to as "plate heat sink device".

#The features and advantages of the plate heat sink device and the fabrication process will be more clearly understood from the following description taken in conjunction with the accompanying drawings in which:

#FIGS. 1A to 1C are cross sectional views showing a process for assembling a plate heat sink device with a package according to the present invention;

#FIG. 2 is a cross sectional view showing the structure of another plate heat sink device according to the present invention; and

#FIGS. 1A to 1C illustrate a process for assembling a plate heat sink device with a package embodying the present invention. The process starts with preparation of a plate heat sink device 1. The plate heat sink device 1 largely comprises a semiconductor pellet 1a and a metal plate heat sink 1b formed on a reverse surface of the semiconductor pellet 1a. Indium gallium arsenide is epitaxially grown on a gallium arsenide layer 1c, and forms a strained lattice layer. Though not shown in the drawings, electrodes are formed on the indium gallium arsenide layer 1d, and the gallium arsenide layer 1c and the indium gallium arsenide layer 1d serve as essential parts of a high electron mobility transistor. In this instance, the gallium arsenide layer 1c is 30 microns to 50 microns thick, and the indium gallium arsenide layer 1d is 10 nanometers to 15 nanometers thick.

#The metal plate heat sink 1b is bonded to the reverse surface of the gallium arsenide layer 1c, and is formed of gold. The metal plate heat sink 1b of gold imparts good heat radiation capability to the plate heat sink device 1. In this instance, the metal plate heat sink 1b is 10 microns to 30 microns thick. The internal stress of the metal plate heat sink 1b slightly warps the plate heat sink device 1, and the radius of curvature R1 ranges from 25 cm to 100 cm, and the center C1 is on the opposite side to the indium gallium arsenide layer 1d with respect to the metal plate heat sink 1b as shown in FIG. 1A. As a result, a small amount of tensile stress takes place in the indium gallium arsenide layer 1d.

#Subsequently, the metal plate heat sink 1b is subjected to an orientation. In detail, the plate heat sink device 1 is heated at 200 degrees to 300 degrees in centigrade for 30 minutes, and the orientation of the crystal grains of gold are changed from (111) plane to the combination of (11) plane and (200) plane. Prior to the heat treatment at least 200 degrees in centigrade for at least 30 minutes, although the gold is not single crystal, (111) plane is almost 100 percent. After the heat treatment, the gold contains (111) plane about 70 percent and (200) plane about 30 percent. Of course, the gold is not single crystal after the heat treatment. The metal plate heat sink 1b is elongated, and the radius of curvature is R1 is increased from 10 cm to 20 cm as shown in FIG. 1B. The center C2 becomes closer than the center C1, and the tensile stress in the indium gallium arsenide layer 1d is increased.