CORRELATION OF INPLANE AND OUT-OF-PLANE DISTORTION IN X-RAY-LITHOGRAPHY MASKS

The implementation of x-ray lithography requires that in-plane distortion caused by absorber stress be eliminated (i.e., be made less than approximately 10% of minimum feature sizes). As a rule of thumb, for large-area membranes (i.e., greater-than-or-equal-to 30 mm diam) stress-induced distortion is negligible if absorber stress is kept below 10(8) dynes/cm2 (10 MPa). The most common way of determining absorber stress on membranes is to measure the out-of-plane deflection in a Linnik or Mireau interferometer. Out-of-plane deflection is caused by absorber bending moment. It is possible, in principle, for an absorber to have a stress that varies with depth in such a way that bending moment is zero but net in-plane stress is not. To determine if there is a one-to-one correspondence between out-of-plane and in-plane distortion, and in order to check the validity of Yanof's model, we have developed two methods of measuring in-plane distortion: a moire method and a holographic interferometry (HI) method. Both require that one etch or lift-off a grid on one surface of the x-ray mask membrane using holographic lithography and reactive-ion etching. On the opposite surface an x-ray absorber is deposited and patterned. In the moire method another holographic exposure is performed, at the same spatial period as the grid, but at a small azimuthal rotation. After development, a clear moire pattern is formed, from which one can calculate the in-plane distortion and corresponding absorber stress. In the HI method, the mask is placed in the standing wave produced by two intersecting laser beams. A fringe pattern can then be seen on a charge coupled device camera. The HI method provides a higher sensitivity to in-plane distortion than the moire method and should enable one to measure in-plane strain at a level corresponding to in-plane distortion below 1 nm for typical pattern geometries.