Electric heating – Metal heating – By arc
Reexamination Certificate
1998-12-18
2003-06-03
Heinrich, Samuel M. (Department: 1725)
Electric heating
Metal heating
By arc
Reexamination Certificate
active
06573471
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a welding method for a semiconductor material such as single crystal or polycrystal silicon (hereinafter referred to as “Si”), gallium (hereinafter referred to as “Ga”) or arsenic (hereinafter referred to as “As”), wherein the semiconductor materials are welded by means such as electron beam, laser beam, or arc.
2. Description of the Related Art,
The physical and electrical properties of Si, when used as a semiconductor, have made it very popular. Si is especially utilized as a bulk material for substrates of DRAMs (Direct Random Access Memories) or MPUs (Micro Processor Units). The demand therefor is growing mostly for use as wafers. The diameters of wafers are becoming larger in order to produce more circuits in a single operation. Si ingots, of which wafers are made, currently exceed 200 kg in weight. As a consequence of this weight, it is necessary to consider how to hold and transport such ingots during subsequent manufacturing processes.
Besides the use of Si in semiconductors, it is often used also for manufacturing jigs for handling products. This use arises from the need to avoid contamination of metal in the products. The difficulty of manufacturing such jigs increases with increases in the weight and diameter of the ingots.
Though one may consider processing the Si ingots themselves to enable transport thereof, such a measure is attended by further problems such as higher costs and complication of manufacturing processes that need to be solved, since Si is a material that is hard to process and also requires certain treatments such as post-process washing.
For obtaining high-performance silicon wafers, thermal treatment at high temperature or film forming is performed by batch processing. Metallurgical tools for holding wafers during such batch processing have conventionally been made of quartz. However, single-crystal silicon boards are increasingly used to satisfy the need for wafers of larger diameter. The use of such boards is advantageous because the characteristics of the boards are identical to those of the wafers. As a result, use of single-crystal silicon boards are effective in decreasing the contamination level of the wafers.
However, it is presented a drawback in that these boards need to be manufactured through cutting from silicon ingots of single crystal whereby mechanical processing of these becomes quite expensive. Thus, it is desirable to find a way to freely connect these components in order to improve the degree of freedom for processing them.
Japanese Patent Application No. 3-107853 (1991) discloses a connection technique for wafers in which SOI wafers are manufactured by pasting wafers. Such a connection technique is employed with the aim of improving electric characteristics. This technique differs from the purpose of the present invention which is directed to structural characteristics.
Semiconductor materials such as Si are very brittle and are destroyed when subjected to high thermal gradients applied thereto. Accordingly, it was believed impossible to join semiconductor materials such as Si by welding.
OBJECTS AND SUMMARY OF THE INVENTION
It is an object of the invention to provide a welding method for semiconductor materials which overcomes the drawbacks of the prior art.
The inventors have achieved the present invention based on the fact that melt welding of semiconductor materials such as Si is possible while limiting metallic contamination or other contamination, such as oxidation, by employing a heat source of high energy density. The present invention enables welding of brittle semiconductor materials such as Si which were considered to be impossible to weld due to their very brittle characteristics. Utilizing a heat source of high energy density makes it possible to weld members made of semiconductor materials such as Si which could only be manufactured using mechanical processing techniques in the prior art. Besides joining members of Si, the present invention permits the manufacture of members having shapes which would be impossible to product using mechanical processing. Thus, the present invention improves the degree of freedom for processing semiconductor materials.
According to the welding method for semiconductor materials of the present invention, a heat source of high energy density is employed for performing melt welding of semiconductor materials. The semiconductor materials may include Si, Ga or As. The heat source of high energy density may includes electron beam, laser beam, plasma-arc or arc welding techniques.
Adequate control during partial melting of members to be welded is difficult using ordinary type heat sources of low energy density. A substantial amount of heat must be applied to achieve suitable melting. This produces such a large heat shock in semiconductor materials such as Si, before sufficient partial melting occurs, that at least partial destruction of the members to be joined results. Utilizing a heat source of high energy density as in the present invention, partial heating and melting at very small energy occurs over a limited area. The portion subjected to melting can be gradually expanded while restricting the total amount of heat input. This reduces the heat shock imparted to the semiconductor materials such as Si to realize a desired melting portion. It should be noted that, while it is preferable that the heat source have a high energy density of not less than 10 kw/cm2, the present invention is not limited to this value.
In the welding method for semiconductor materials according to the invention the application of energy progresses outward from the initial point at the time of starting welding. The energy output is decreased toward the end of the welding operation.
Since the energy output can be progressed or decreased to achieve desired output values, cracks due to thermal stress resulting from abrupt temperature distribution changes are reduced. Since output control is easy especially with electron beams or laser beams, partial heating is enabled while freely controlling the temperature increase/decrease rate.
The welding method for semiconductor materials according to the invention includes preheating the workpiece before welding. The preheating temperature is preferably not less than 300° C., and more preferable, not less than 600° C.
Preheating increases the area in which satisfactory conditions for welding exists. While Si is very brittle at room temperature, it has a ductility similar to steel when it is heated to not less than 600° C. When preheating is performed, the cooling speed of the welding portion decreases as a matter of course. Thus, thermal stress owing from partial and abrupt shrinkage is reduced. Further, the region in which the temperature exceeds 600° C. is expanded beyond the proximity of the welding portion, whereby not only relief of thermal stress is achieved but also ductility is improved in portions at which the thermal stress is large, so that probability of cracking is decreased.
The welding method for semiconductor materials according to the invention includes performing the welding while adding a filler material which is identical to the semiconductor material.
Referring now to
FIG. 1
, an effective means for relieving thermal shocks is a method in which a filler material
1
, which is a material identical to the base material such as Si in a form of a stick, wire or powder, is added to a welding portion
2
. Utilizing Si materials or the like as filler elements
1
, most of the energy of the heat source is consumed for melting the filler element
1
whereby the amount of heat input to the members
3
,
3
being welded, is decreased. Thus, thermal shock applied to the members
3
,
3
is decreased and favorable welding without destruction can be achieved.
For electron beam welding, electricity must pass through the Si member. However, resistivity values of Si vary greatly depending on the amount of impurities included therein, and may reach up to 10,000 ohm-cm at room temper
Hanada Yoichiro
Kuriyama Kazuya
Darby & Darby
Heinrich Samuel M.
Komatsu Ltd.
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