JPS614221A - Bonding of semiconductor substrate - Google Patents

Abstract

PURPOSE:To provide a bonding provided with an improved ohmic characteristic without using adhesive, etc., by a method in which, after oxide films on the bonding faces are removed using chemical process, etc., they are heat-treated while closely contacted with each other. CONSTITUTION:After both bonding faces of two semiconductor substrates are mirror polished preferably with a surface roughness below 500Angstrom , oxide films on the bonding faces are removed by immersing them into, for example, hydrofluoric acid, are washed and are dried. In a clean atmosphere below a dust floating number of 20 pieces /cm<2>, the substrates are heat-treated at a temperature above 200 deg.C, or preferably above 500 deg.C, with both the polished faces closely contacted with each other in order to perform bonding. This bonding has an inferior mechanical strength just after forming the bonding, as compared with the case where bonding of semiconductor substrates is formed with natural oxide films having a sufficient thickness left. The bonding of this invention, however, has the same bonding strength as the case having the natural oxide films left, when heat-treating the substrates at a temperature above 200 deg.C.

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates to a method for bonding semiconductor substrates by firmly bonding and integrating two semiconductor substrates without using an adhesive or the like.

[Technical background of the invention and its problems]

Conventionally, diffusion, ion implantation, epitaxial growth, and the like have been used to form layers having different conductivity types and impurity concentrations on a semiconductor substrate. If semiconductor substrates could be directly bonded to each other without using an adhesive, it would be very convenient because it would eliminate the high-temperature, long-time diffusion process and epitaxial growth required to obtain thick diffusion layers and growth layers.

Conventionally, when silicon wafers whose Li surfaces have been polished are brought into contact with each other while wet with water, alcohol, etc., it has often been observed that the two adhere to each other. However, this is due to surface tension due to liquid such as water, and is not observed in dried wafers.

In contrast, the present inventors performed hydrophilic treatment on the polished surface of a mirror-polished silicon wafer under oxidizing conditions, and then
It has been discovered that a strong bonded body can be obtained by bringing these polished surfaces into close contact with each other in a clean atmosphere substantially free of foreign matter, and this has been proposed previously. In this case, 2
It has been revealed that heat treatment at a temperature of 00° C. or higher results in a stronger bonded body. By this method, a high impurity concentration substrate and a low impurity concentration substrate can be bonded together and good electrical characteristics can be obtained. As a result of a detailed study of this bonding phenomenon, it has been found that the formation of a natural oxide film on the surface of the silicon substrate is a necessary condition for obtaining a strong bonded body. The presence of such a natural oxide film can be confirmed, for example, by a method such as ellipsometry.

However, it has been found that this direct bonding method has a problem in that the ohmic characteristics of the bond are not very good, especially when semiconductor substrates with a low impurity concentration are bonded together. This is thought to be because the natural oxide film left on the bonding surface is essentially an insulating film and forms a kind of electrical barrier.

[Purpose of the invention]

The present invention has been made in view of the above-mentioned points, and an object of the present invention is to provide a method for bonding semiconductor substrates in which a bond exhibiting good ohmic characteristics can be obtained by a direct bonding method.

(Summary of the Invention) The present invention preferably involves mirror polishing each joint surface of two semiconductor substrates to a surface roughness of 500 Å or less, and then removing the surface oxide film by, for example, immersing it in hydrofluoric acid. The polished surfaces are brought into close contact with each other by washing and drying in a clean atmosphere in which the amount of floating particles is 20 particles/cd or less, and then heat-treated at a temperature of 200° C. or higher, preferably 500° C. or higher to form a bonded body.

〔Effect of the invention〕

According to the present invention, a bonded body of semiconductor substrates can be obtained, and by adding a step 7 of removing a natural oxide film, good ohmic characteristics of the bond can be obtained. Compared to bonding semiconductor substrates with a sufficiently thick natural oxide film remaining,
Although the mechanical bonding strength immediately after is inferior, it has been confirmed that by performing heat treatment at 200° C. or higher, the same bonding strength as when a thick natural oxide film is left can be obtained.

If the present invention is used as an alternative technology for forming deep diffusion layers or thick epitaxial layers, it will not only simplify the process, but also make it easier to control impurity concentration and thickness, making it possible to improve the performance of many semiconductors. Great effects can be expected when applied to devices.

(Examples of the Invention) The present invention will be described in detail below. 20Ω・α n-type (1
00) Two silicon substrates were prepared, and the surfaces to be bonded were mirror-polished to a surface roughness of 500 Å or less. Next, it was immersed in concentrated hydrofluoric acid for 1 minute, and then washed with running water for 2 minutes. Thereafter, the substrates were dried, and the polished surfaces of both substrates were brought into close contact with each other in a clean room with substantially no foreign matter present. This was placed in an oven and heat treated at 200°C for 1 hour.

The thus obtained bonded substrate exhibited sufficiently strong bonding strength. As a result of measuring the relationship between heat treatment temperature and bonding strength,
For samples heat-treated at 500°C or higher, the strength matched the strength of silicon itself within a range of error.

Regarding the sample heat treated at 1000℃ for 2 hours,
When electrodes of gold-1% antimony were formed and voltage-current characteristics were measured using a laser from Curve 1, it was confirmed that the electrodes were linear over a wide voltage range and exhibited good ohmic characteristics. As a reference example, a similar silicon substrate was mirror-polished, then boiled in a hydrogen peroxide-sulfuric acid mixture for 3 hours to form an oxide film, and then washed and dried to form a bonded body in the same manner as in the example. Formed. Although the bonding strength was sufficient with this method, the voltage-current characteristics were not linear and the bonding characteristics were non-ohmic.

Claims (1)

[Claims] Each bonding surface of two semiconductor substrates is mirror-polished, the oxide film on each polished surface is removed, and then washed with water, dried, and the bonded surfaces are directly brought into close contact under conditions with virtually no foreign matter present. 1. A method for bonding semiconductor substrates, characterized in that heat treatment is performed at a temperature of 200° C. or higher.