Process for assembling substrates with low-temperature heat treatments

The invention relates to techniques for assembling substrates.

In general, a bond between two substrates or surfaces can be obtained after a preparation of the surfaces giving them a hydrophilic or hydrophobic character.

The use of heat treatments to reinforce the direct bonding can cause, for a certain number of bonded structures, the appearance of defects at the bonding interface. These defects are due to the degassing of by-products of the molecular bonding reaction: for example, water, hydrogen or hydrocarbon molecules.

For a certain number of bonded structures, it is known that these defects can be resorbed by heat treatments performed at very high temperatures. These temperatures are, for example, between 900° C. and 1300° C. and are based on the preparation of surfaces before bonding. Unfortunately, for other bonded structures, this solution cannot be used.

The limitation of the surface oxide thickness or the presence of various materials facilitates the appearance of defects at the bonding interface.

In the case of thin films (with a thickness below around ten μm or several dozen nm), heat treatments, at temperatures below 1000° C., for example between 600° C. and 800° C., cause the formation of bonding defects in the form of blisters or zones without adherent film. These defects cannot be suppressed by higher-temperature heat treatments. For example, the bursting of bubbles is promoted by the fineness of the layers. These defects make the structures produced unusable. Currently, this phenomenon limits the production of oxide film structures, embedded at the bonding interface, that are fine (thickness below 50 nm) or ultra-fine or even Si layers directly bonded to Si plates.

Similarly, for heterostructures (for example P-doped Si bonded to N-doped Si), heat treatments cause the formation of bonding defects under certain conditions. A high-temperature heat treatment (1000° C.) would cause interdiffusion of the doping agents.

For certain heterostructures, if the damage is excessive in heat treatments within the temperature range below 800° C., this damage can no longer be repaired by a treatment between 1100° C. and 1300° C.

When the heat treatments at higher temperatures cannot be used (incompatibility with the component production process in progress, for example), the bonding defects are then prohibitive.

This therefore raises the problem of reducing or even eliminating, in the case of (direct) molecular bonding, the defects due to degassing at the bonding interface.

The solutions currently used to overcome the formation of defects involve primarily the removal of water at the bonding interface by using, in particular, ultra high vacuum (UHV) bonding techniques. However, these techniques are not suitable for industrial use. There are also techniques that consist of forming channels at the bonding interface in order to evacuate the by-products of the molecular bonding reaction. Unfortunately, such techniques are destructive and present usage problems.

A problem is therefore to find a treatment solution for reducing defects that enables industrial implementation while using the entire surface.

According to the invention, a process for producing a bond between a first and a second substrate comprises:

a) a step of preparing surfaces to be assembled,

b) an assembly of these two surfaces, by direct molecular bonding,

c) a heat treatment step involving at least maintaining the temperature of the surface or of the bonding interface within the range of 50° C. to 100° C. for at least one hour.

Step c) also comprises, after the step of maintaining the temperature within the range [50 to 100° C.] for at least one hour, a step of maintaining the temperature within the range strictly above 100° C., and below 500° C. (i.e. within the range ]100° C. to 500° C.]) for at least one hour.

The term “substrate” refers to a massive substrate or a substrate consisting of a stack of a plurality of layers of different types.

This heat treatment according to the invention enables good preparation of the conditions for degassing the surfaces in contact by molecular adhesion.

It makes it possible to minimise the defect density at the bonding interface. At lower temperatures, such a heat treatment makes it possible to more easily eliminate the by-products of the degassing of the interface, by diffusion at the bonding interface.

The standard heat treatments, at higher temperatures, make it possible to increase the bonding energies of structures, and/or to create a fracture in a zone implanted by one (or more) species, for example gaseous, prior to the bonding. A treatment according to the invention can therefore be a complement to the standard heat treatments, at higher temperatures, which reinforce the bonding.