The present invention relates to a method of bonding two wafers made out of materials selected from semiconductor materials, the method making use of plasma activation of the surface of at least one of the two layers that are to be bonded together.
Methods of this type are generally known. The term “bonding” is used herein to mean bonding by molecular bonding during which two perfectly plane surfaces are bonded together without applying any adhesive—and this being possible at ambient temperature. The quality of the bond is characterized in particular by its bonding energy, which represents the connection force between the two bonded-together wafers.
In order to consolidate bonding by molecular bonding between two wafers, the bonded-together wafers are subjected to heat treatment. Such heat treatment enables the bonding energy of the two wafers to be raised to a value of at least about 500 millijoules per square meter (mJ/m2), where this corresponds to the values that are typically desired. In conventional manner, such heat treatment is generally carried out at a temperature of at least about 900° C. (which in the context of the present specification defines the “high temperature” domain). When bonding between a wafer made of Si and a wafer made of Si or of SiO2, the bonding energy is maximized when heat treatment is carried out at temperatures of the order of 1100° C. to 1200° C.
“Plasma activation” of a surface for bonding is defined as exposing a surface to a plasma (which can be carried out in particular under a vacuum or at atmospheric pressure) prior to putting the surfaces for bonding together into contact. More precisely, in prior art activation techniques, the surface of a wafer for activation is exposed to a plasma during an exposure step in which the exposure parameters are controlled so that each is set at a given respective value, which values remain unchanged during plasma activation.
First order “exposure parameters” are as follows:
power density; this is the density of the power feeding the plasma, which represents a power density per unit area in watts per square centimeter (W/cm2), and is sometimes also referred to in the present specification by the term “power”, for simplicity;
pressure (pressure in the enclosure containing the plasma); and
the nature and the flow rate of the gas fed into the enclosure. Such activation serves in particular to perform bonding by molecular bonding, obtaining high bonding energies without requiring any recourse to heat treatment that would necessarily need to be carried out at high temperatures.
Plasma activation thus makes it possible to obtain high bonding energies between two wafers, at least one of which is activated prior to bonding, and after applying heat treatment over relatively short durations (e.g. about 2 hours (h)), and at temperatures that are relatively low (e.g. about 600° C., or less). Such activation is thus advantageous for stabilizing a structure comprising two bonded-together wafers, when it is desired to avoid subjecting the structure to temperatures that are too high (as applies in particular to heterostructures, defined as being structures comprising layers made of materials having coefficients of thermal expansion that are significantly different). Such activation can also be advantageous for obtaining high bonding forces at a given temperature.
Such activation is thus advantageous for making multilayer structures involving the bonding together of two wafers. Transfer methods (in particular methods of the SMART-CUT® type as described in general terms in the work “Silicon-on-insulator technology: materials to VLSI”, 2nd edition (Jean-Pierre Colinge)) constitute one example of an application that can benefit from plasma activation to enhance bonding. The present invention is therefore directed at providing improvements in such methods.