Background

This invention relates generally to the conversion of amorphous or polycrystalline semiconductor materials to substantially single crystal semiconductor material by a process known as zone-melting-recrystallization (ZMR) .

The development of silicon-on-insulator (SOI) technology has been complemented by the use of ZMR processing to produce single crystal silicon for solid state devices exhibiting reduced parasitic capacitance, simplified device isolation and design, and radiation hard circuits for space applications. Present ZMR processes require a well controlled mechanical system to translate a hot zone created by a moving strip heater across the surface of a heated silicon wafer. This system is elaborate, expensive, and has a number of mechanical parts that could degrade in time. U.S. Patent No. 4,371,421 entitled "Lateral Epitaxial Growth by Seeded Solidification" describes such a system.

A sample to be recrystallized is placed on a heater which raises the temperature of the sample close to its melting point. A strip heater positioned above the sample is then energized to induce melting of a zone on the sample directly beneath the strip heater element. The strip heater is then translated past the surface of the sample,

causing the melting zone to move in unison with the heater to induce melting then solidification of the sample to achieve lateral epitaxial growth thereby transforming the ^" sample into a single crystal material.

Summary of the Invention

The present invention comprises a new heating system that accomplished the same task with no moving parts. A moving heat zone is electrically provided using a heater block fabricated from

Alumina, Zirconia, or some other refractory material in such a way as to support a large number of small heating elements. In order to keep these heating elements separated during the process and prevent them from shorting out, they are placed in small grooves machined into the refractory block. Each of these wire elements is supplied with electrical current through a control circuit. With such a circuit, it is possible to provide any combination of heated elements at any desired temperature. When sufficient current is provided to a heating element, it will become hot due to its resistivity. The refractory block is machined in such a way as to provide support of a silicon wafer. The wafer is centered over the hot zone. The heating element lengths could be adjusted so that they do not extend beyond the edges of the wafer. This provides a significant advantage to current ZMR processes by limiting edge heating.

The above, and other features of the invention, including various novel details of construction and combination of parts, will now be more particularly described with reference to the accompanying drawings and pointed out in the claims. It will be understood that the particular zone-melt recrystallization method and apparatus embodying the invention is shown by way of illustration only and not as a limitation of the invention. The principal features of this invention may be employed in various embodiments without departing from the scope of the invention.

Brief Description of the Drawings

Figure 1 is a perspective view of the zone melt recrystallization apparatus of the present invention; and

Figure 2 is a schematic diagram of the control circuit for the apparatus of Figure 1.

Detailed Description of the Invention A preferred embodiment of the invention is illustrated in the perspective view of Figure 1. In operation, the entire block 10 would be raised to the temperature for ZMR operation just below the melting point of a semiconductor material 11. Then individual elements 13 are heated to a temperature required to melt the semiconductor 11. To create a hot zone 80 mils in width for example requires four heating elements with a 25/1000 inch spacing between

each element. These individual heating elements can be provided with enough additional current above their bias current to melt the silicon material. To move the hot zone ^" , the power would be provided to an adjacent heating element, to one side of the four presently being heated, while the element on the opposite side of the four hot elements would be provided only its bias current. In this way, the hot zone would be shifted over by one heating element. This process could be continued at any desired rate to move the zone across the wafer.

In a preferred embodiment, it is possible to provide varying degrees of current to individual wires. This permits gradual heating at the edge of the moving zone.

Through proper control, the heating elements could be heated in a more analog or continuous way in order to produce a much smoother transition as the heating zone is translated. Through proper design, this heater concept provides a way of significantly reducing the mechanical strains in the ZMR processing system. The moving zone could be made to move more uniformly and more smoothly than any mechanical system and at a significant reduction in overall system complexity and cost. In the configuration of Figure 1, a silicon wafer 11 is placed top side down on the plate 12 which is in thermal contact with elements 13.

Instead of picking up a wafer with pins as is done in current systems, it would be much more desirable to use a vacuum in this system.

A further advantage is that in order to view the molten zone in the present system, we use a video camera which must be placed at exactly the right angle with respect to the upper heater, which limits the field of view as it permits viewing of only a fraction of the molten zone. With the new system, the camera 14, which is sensitive to infrared light, would view the entire melt zone through the backside of the wafer 11. The infrared image can be used to provide a feedback signal to the control circuit to insure that heating rates are within predetermined tolerance.

Figure 2 shows a schematic diagram of the control elements of a preferred embodiment of the invention. The resistors R , R , and R_ represent individual heating elements. There are about 300 of these elements in the present embodiment. Only three are shown for purposes of illustration. A first DC current source I_ provides power to bring the heater close to the melting temperature of the wafer. A second DC current source I _p supplies power to bring each heater element to the melting temperature of the wafer when commanded by computer. Each element has a pair of transistors, one to connect the positive side of the I _p source, and the second to connect the negative terminal of the I _p source, to the desired element or elements. This

allows both icnreasing and decreasing the current of the selected elements around the I _β value.

The computer tells the multiplexer which elements will be effected by the Ip source. The computer also establishes the set points for the controlled elements which in combination with the video cameral provide the control of the pulse width modulator.

Another preferred embodiment utilizes a heater element wherein the elements are portions of a single wire wound about the block such that each portion is controlled by the circuit as shown in Figure 2.

Yet another embodiment uses carbon or graphite elements deposited on the plate, which may be made from alumina, zirconia, or some other refractory material. These elements can be formed into a sequence of parallel lines, each individually controlled. The elements can also be configured in a dot matrix type configuration.