With anisotropic etching, the dissolution rate distribution in the resist is still the same, but there is a directional preference in cell removal, removing more cells in one direction than the other. Because of the absence of an imaging lens, it is relatively easy to change the wavelength of the imaging system. Multiple identical chips make possible the use of chip-to-chip inspection, rather than needing to resort to die-to-database inspection. With a positive resist, higher exposure leads to a faster dissolution rate, and vice versa for a negative resist. In proximity printing, the field size is limited only by the size of the mask substrate and the illumination system. The disadvantage of a reflective system, with the exception of two concentric mirrors, is the difficulty in keeping all of the optical elements on one straight optical axis. After development, this resist image is used as an etch, implant, plating, or lift-off mask for pattern transfer to a thinfilm layer on the wafer.
Combining feature sizes and types still requires one cut per type and per size. Papoulis, The Fourier Integral and Its Applications, McGraw-Hill, New York, Sec. However, besides the undesirable overcut profile, the exposure time can increase manyfold, lowering wafer throughput significantly. Mask damage issues and edge acuity requirements drove chrome masks to replace emulsion masks. There is no fundamental limit otherwise.
Even though the mask is four times larger than its image, the solid angle sustained by the imaging lens at the mask side is only 25% as much as that at the wafer side. This topic will be further covered in Chapter 6. Third, systems can be compared with identical criteria. These are expensive to implement in commercial systems. At a wavelength of 248 nm and shorter, the practice is strictly to use one single type of material while placing the burden entirely on a very narrow bandwidth.
The mask and wafer in these systems are placed horizontally. Furthermore, in the vicinity of 250 nm there are very few optical materials with sufficient transmissivity and mechanical working properties to incorporate into achromatic imaging lenses. The step-and-scan system was developed to meet these requirements. The contributions from magnification error, lens distortion, and laser table error in reduction mask making are added to the components shown in Table 2. The location of any given edge of a feature on the chip must be within a given tolerance from a nominal position. The E-D values of the new E-D windows are shown in Table 4. As will be discussed in Sec.
For most other purposes, the following approximation for E xo,yo is usually sufficient. In addition, alignment viewing must accommodate two focal positions; thus, it becomes more complicated. Though electron beam lithography has been long used for mask writing, it is yet very slow and typically takes almost a day to complete a high-end mask. The former is usually on the order of a few micrometers; the latter is in millimeters. In terms of a lack of generalization, it is clear that k1 and k3 are dependent on many variables.
The optical pulse is about 10 to 20 nsec long. In addition, a true isolated feature must be approximated with a periodic multiplicity of itself, separated by a large distance between them. One can fit another rectangle that has a width i. Practitioners of optical lithography may only have a vague idea of its theory, and likewise, theoreticians may not have the opportunity to practice the technology on the manufacturing floor. This outlook shows that the issues of minimum feature size, throughput and cost of nanolithography require an optimisation at the system level from product definition through restrictive design and topology rules to wafer-layer rules in order to control the cost of manufacturing and to achieve the best product.
Therefore, the spherical wavefront plays a key role in understanding diffraction-limited image formation. This takes care of the change of intensity as a function of α along the path of the light being absorbed. As wafer size continued to increase, the original 1X all-reflective projection-printing system was expanded in 1984 to include two sets of concentric mirrors15 and some refractive elements, making it a catadioptric system. We have drawn 30 deg for illustration purposes. Due to scanning, the field is not limited by the lens in the scan direction. The metrics will be fully described and extensively used in this book. In reality, even though the deviation is suppressed to tenths of wavelength, aberrations induced from the deviation cannot be ignored when optical lithography is pushed to its ultimate limit.
G , is the pupil function, which is zero outside the lens aperture and is unity inside when there is no aberration. Similarly, level 19 contains all intensity levels equal to and higher than 58 Chapter 3 0 1 a 2 3 0 1 b 2 3 Figure 3. The E-D trees of the edges are no longer identical to each other in terms of their relative position to the ideal edge positions, resulting in two separate trees whose common region is much less than before. Optical lithography galaxy for logic gates; 8. However, the actual size is 0.
A common E-D window becomes meaningless because no matter which individual window is centered, there can only be one feature that centers on the nominal exposure. The curve based on elliptical E-D windows has a sharper bend than the one based on rectangular windows. This leads to elliptical E-D windows, as shown in Fig. The minimum feature size is chosen to be 0. In most cases, once the aerial image is obtained, the resist image can be estimated with the lumped parameters, which consist of the resist thickness D, the absorption coefficient of the resist , and its development contrast. In microscopy, two points are considered resolved if their images are separable.