In many technical domains (e.g. microelectronic or photovoltaic technology) materials, such as e.g. silicon, germanium or sapphire, are often needed in the form of thin discs and plates (so-called wafers). As standard, such wafers are currently produced by sawing from an ingot, relatively large material losses (“kerf loss”) occurring. Since the source material used is often very expensive, great efforts are being made to produce such wafers with less material consumption and so more efficiently and inexpensively.
For example, with the currently normal methods almost 50% of the material used is lost as “kerf loss” when producing silicon wafers for solar cells alone. Considered globally, this corresponds to an annual loss of more than 2 billion euros. Since the cost of the wafer makes up the greatest percentage of the cost of the finished solar cell (over 40%), the cost of solar cells could be significantly reduced by making appropriate improvements to the wafer production.
Methods which dispense with the conventional sawing and can separate thin wafers directly from a thicker workpiece, e.g. by using temperature-induced stresses, appear to be particularly attractive for this type of wafer production without kerf loss (“kerf-free wafering”). These include in particular methods as described e.g. in PCT/US2008/012140 and PCT/EP2009/067539 where a polymer layer applied to the workpiece is used in order to produce these stresses.
In the aforementioned methods the polymer layer has a thermal expansion coefficient that is higher by approximately two orders of magnitude in comparison to the workpiece. Moreover, by utilising a glass transition a relatively high elasticity modulus can be achieved in the polymer layer so that sufficiently large stresses can be induced in the polymer layer/workpiece layer system by cooling in order to enable the separation of the wafer from the workpiece.
In the method according to the current prior art, the breaking process, which leads to separation of the wafer from the workpiece, is initiated randomly, neither the precise time of initiating the break nor the location being able to be specified. Often the break starts at a random weakpoint of the wafer, mostly at the edge or at the periphery, at a moment where the stress exceeds a critical value locally. As a result of this uncertainty regarding the location and time of the break being initiated, it is difficult to guarantee a stress field that is optimal for the breaking process at the time of initiating the break and at the place of initiating the break. This may lead to an unfavourable course of the break front and in particular to significant thickness variations of the separated wafer. Often, for example, the break runs in a number of break fronts along different directions, large and often abrupt thickness variations in the separated wafer possibly occurring if they converge again subsequently or disadvantageous overlapping of these break fronts taking place.