Ion beam lithography has become an important tool in microelectronics when such circuits are required. In our previous Israeli Patent Number 88837, a method for the manufacture of an improved X-ray lithography mask was disclosed. According to this method a sequence of the following steps is involved:
(a) protection of the edges of a substrate;
(b) deposition of a continuous compact oxide layer over the non-protected portion of the front side of said surface;
(c) etching the backside of the substrate, obtaining an oxide membrane supported on the substrate, and
(d) obtaining a pattern delineation of said membrane.
The use of ion charge particles is an alternative advanced microlithography approach. The ion beam imaging combines several inherent advantages: high resolution beams, a possibility to influence its path by a magnetic field, a very low energy scatter and high energy levels of up to 150 keV. A main advantage of using ion beams over electron beams is the lack of backscattered and hence minimum proximity effect.
A problem which exists in masked ion beam lithography, is a result of the pattern-dependent thermally induced distortions. The energy of the beam being absorbed in the mask will result in its heating. For high-resolution lithography, this will effectively decrease the throughput by limiting the beam current density.
In a review by D. J. Elliot (Integrated Circuit fabrication technology, p. 276-281, McGraw-Hill Publishing Company, 1989), a membrane mask is used, having a thin membrane to self-support the pattern. Either a thin amorphous membrane, sufficiently transparent for the ions, or a channelling film of crystalline silicon are suggested for this use. In both cases, the membrane supports an ion-absorbing layer of gold. The ions are scattered in the amorphous membrane and this will degrade the overall performance.
The main problem encountered in the method for the manufacture of an ion beam mask, relates to the production of a thin and strong membrane capable to withstand dimensional stability and providing accuracy of the absorber pattern. This problem is solved by using a silicon membrane and exposing the resist in the channeling direction. However, residual scattering of ions in the mask membrane still exists even in the channeling of silicon membrane. This disadvantage is completely eliminated in an open-stencil mask, wherein a thin membrane with transmission holes is etched through the entire membrane thickness. A grid is provided as a support, consisting of a matrix of small transmission holes instead of completely open transmission areas. In this manner, most of the pattern restrictions associated with the stencil masks are eliminated. However, this grid increases significantly the mask area from which scattered ions may escape; furthermore, this type of mask will require a higher fluence to accomplish an exposure than simple stencil masks.
It is an object of the present invention to provide an improved ion beam lithography mask and a simple method for the manufacture thereof. It is another object of the present invention to provide an improved ion beam lithography mask which is highly stable, being substantially free of any stresses encountered in such known masks.