2.7.2 X-Ray

A promising technique already far evolved is 1x X-ray proximity printing [26]. For soft X-rays with wavelengths ranging between 0.8-2 nm diffraction effects are negligible down to linewidths of 100 nm. The resolution is thus given by (2.5). For example, an exposing wavelength of $\lambda$ = 0.8 nm with a mask-to-wafer separation of d_g = 10 $\mu$m and a process-related constant of k = 1.6 yields a resolution of 0.14 $\mu$m. The variation in the mask-to-wafer spacing reduces the achievable resolution and effectively blurs the image. The following relationship can be derived from (2.5),

$$\frac{\Delta W}{W} = \frac{\Delta d_g}{2d_g},$$ (2.7)

whereby the technology parameter k₃ is kept constant. The tolerable positioning error $\Delta$d_g, i.e., the depth of focus thus equals the maximally allowable linewidth change $\Delta$W/W times twice the distance d_g. In the above example we have d_g = 10 $\mu$m, which yields a focal depth of 2 $\mu$m if 4% linewidth control is required. This is a significant improvement upon optical projection printing (cf. Figure 2.2). Another big advantage of X-ray exposure is its negligible sensitivity to small, low atomic mass particles that frequently degrade image quality in optical lithography. The great particle defect tolerance relieves clean room specifications and increases process latitude. Severe problems arise from the fragility and dimensional instability of the mask, and the high precision alignment required due to the 1x replication method. The starting material of the mask is usually a silicon wafer with a borosilicate film on top. Subjecting the wafer to elevated temperatures causes the boron to diffuse into the silicon. The resulting heavily-doped boron film acts as an etch-stop, when the structure is etched from the backside to form a thin silicon membrane. The membrane is bonded to a pyrex support ring, and patterned with a high atomic number material like gold to absorb the X-rays. The repair of the costly X-ray masks is very important and can be accomplished by photolytic or electron-beam induced deposition and ion beam sputter-erosion. Another challenge is the development of bright X-ray sources for high volume production. Two types of X-ray sources are available today, the synchrotron and the laser-induced plasma radiation. Due to the finite size of these sources they behave similarly to partially coherent optical sources. The arising phenomenon is usually called penumbral blurring since it has been considered to be problematic. Recent work indicates that some penumbra has the beneficial effect of washing out diffraction peaks at exposure boundaries caused by coherency. Modern sources have sufficient intensity to provide an optimum penumbral blur for a given design rule. Finally, special resists have to be developed further to minimize the effect of secondary electron processes. The usage of low atomic number resist materials like carbon and oxygen reduces the spreading of the exposure volume, because the energy of the generated photoelectrons is then lower, which prevents them to travel far away once they are released in the resist.