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The photolithography has its resolution limited by diffraction effects. To improve the resolution, therefore, the diffraction effects are reduced by reducing the wavelength. However, if the wavelength is reduced further, all optical materials become opaque because of the fundamental absorption, but transmission increases again in the X-ray region. This led to the requirement of X-rays for lithography purpose.
In X-ray lithography an X-ray source illuminates a mask, which casts shadows on to a resist-covered wafer. The mask and resist material for X-ray lithography are mainly determined by the absorption spectra of these materials in the X-ray region.
An electron resist can also be referred to as an X-ray resist, since an X-ray resist is exposed largely by the photoelectrons produced during X-ray absorption. The energies of these photoelectrons are much smaller than the 10 keV to 50 keV energies used in electron lithography, making proximity effects negligible in the case of X-ray and promising higher ultimate resolution.
Most of the polymer resists containing only H, C, and 0, absorb very small X-ray flux. This small absorption has the advantage of providing uniform exposure throughout the resist thickness and the disadvantage of reduced sensitivity. .
As in optical and election lithography, the negative resists are limited in resolution by swelling during development. Thus minimum features of only 0.75 micro meters can be resolved in a commercial resist.
Since the wavelength of X-ray is small, diffraction effects can be ignored and simple geometrical considerations can be used in relating the image to the pattern on the mask. The opaque parts of the mask cast shadows on to the wafer below. The edge of the shadow is not absolutely sharp because of the finite diameter of the focal spot of the electrons on the anode [X-ray source] at a finite distance from the mask. The blurring of shadow can be evaded by the following equation.
∂ = Sg/D
Where ∂ = blur
g = gap between mask and the wafer
D = Distance of source from the mask
The various factors contributing to total registration error in X-ray lithography are machine imprecision and mask stacking errors due to placement errors of pattern generator] and mask distortion. Wafer process related contributions also have a role in the total registration error.
In earlier years of development X-ray sources was often an electron beam evaporator with its chamber modified to accept a mask and wafer. The target metal could be changed easily to modify the X-ray spectrum. X-ray generation by electron bombardment is a very inefficient process. Most of the input power is converted into heat in the target. The X-ray flux is generally limited by the heat dissipation in the target. Much high X-ray fluxes are available from generators which have high speed targets. Another type of source, which provides still greater amount of flux, is the plasma discharge source in which the plasma is heated to a temperature high enough to produce X-radiation. The plasma chamber has problems such as reliability and contaminations.
The mask for X-ray lithography consists of an absorber on a transmissive membrane substrate. The absorber is usually gold which is a heavy metal. Also it can be easily patterned. The transmissive membrane substrate is a polymer such as polymide and polyethylene terephthalate.