Patent Number: 055704050
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENT In describing the preferred embodiment of the present invention, reference will be made herein to FIGS. 1-7 of the drawings in which like numerals refer to like features of the invention. Referring to FIGS. 1 and 2, the exposure mask includes a mask substrate 10 which is optionally covered by a two layer plating base composed of a first layer 12 of chromium having a thickness of approximately 50 Angstroms (5 nanometers) and second layer 14 of gold having a thickness of 300 Angstroms (30 nanometers). Centrally located on the mask substrate 10 is an X-ray transparent pattern window 16 and surrounding the pattern window 16 is an a X-ray absorptive frame 18. Four mask-to-wafer alignment mark windows 20 are positioned around the pattern window 16. They are used to hold alignment marks for aligning the mask relative to the wafer before the exposure is made. These windows may be completely etched through for optical transparency and covered with a polyamide film to hold the mask-to-wafer alignment marks. As can be seen in FIG. 2, the pattern window 16 is constructed by etching away the silicon mask substrate 10 from the second side (back side) until the silicon becomes X-ray transparent in area 22. The plating base materials 12 and 14 are also X-ray transparent, although a slight reduction in the magnitude of the X-ray beam may occur as it passes through the plating base layers and the remaining silicon. The four mask-to-wafer alignment mark windows 20 may also be constructed as is seen in the cross section of FIG. 2, or they may be completely etched through and covered by a polyamide film. The mask substrate 10 also includes the two cross-shaped pattern-to-mask alignment marks 24 which are an important element of this invention and are described more fully below. Referring to FIG. 2, the sidewall 26 of the mask window can be seen to form an angle of 52.degree. degrees with the first and second surfaces 28, 30 of the substrate 10. This is the result of the anisotropic etching process which results in a larger opening on the side from which the etching is initiated as the etchant attacks the sidewall as well as the thinning second surface during the etching process. As can be seen in FIG. 2, the etching of the mask windows 16, 20 was begun from the second surface 30 of the mask substrate 10. As will be seen in connection with the description of the pattern to mask alignment marks 24, they were created at the same time as the mask windows and share this angular sidewall characteristic. When the mask is used, X-rays that impact the corner 32 between the angled sidewall 26 and the pattern window area 22 may be scattered into the region below the circuit pattern window. To prevent this, the frame 18 is formed of an X-ray absorptive material and covers the corner 32 around the perimeter of pattern window 16. An X-ray opaque molybdenum mask 34 is also provided below the second surface 30 to limit the size of the exposure region. Generally, a circuit pattern (not shown) is constructed of an X-ray absorptive material and is made at the same time as the frame 18 within its confines. Mask-to-wafer alignment marks (also not shown) are formed above the mask-to-wafer alignment mark windows 20. FIG. 3 shows an expanded view of a pattern-to-mask alignment mark 24 as seen from the second surface 30. FIG. 4 is a cross-sectional view along the line 4--4 in FIG. 1 which also corresponds to the dotted cross-sectional line 4--4 in FIG. 3. FIG. 4 illustrates the angled side walls 36, 38 formed by the anisotropic etching process. The angled side walls 36, 38 produce an approximately V-shaped valley for the pattern-to-mask alignment mark which has a depth just slightly less than the thickness of the mask substrate 10. This produces a thin region 40 of silicon at the base of the valley close to the first surface 28. The region 40 will pass electrons during scanning of the first surface by an electron beam lithography system, whereas the adjacent thicker silicon regions on either side of region 40 will backscatter the electrons signal. Thus, by monitoring the return signal, the absence of backscattered electrons indicates the presence of the alignment mark region 40. Referring to FIG. 3, the alignment mark preferably consist of a horizontal alignment mark leg 42 and a vertical alignment mark leg 44 which form a cross shaped alignment mark. Other configurations for the alignment mark may also be used, for example alignment mark leg 42 can be placed at an entirely different, non-intersecting location than the alignment mark leg 44, or the point of intersection may be moved up or down or left or right. A T-shaped mark and other intersection points and shapes are equally suitable provided that defined locations on the mask substrate can be identified relative to the mask windows. When the mask is constructed, the mask substrate 10 is initially etched from the second surface 30 to produce the desired mask windows 16, 20 and the pattern-to-mask alignment marks 24. Any conventional method of etching a silicon substrate may be used to form the etched mask windows and the pattern-to-mask alignment marks. Because the pattern-to-mask alignment marks are made at the same time as the desired mask windows 16, 20, their relative positions are maintained quite accurately. The plating base layers 12 and 14 are added after etching. An electron beam sensitive layer is added next. The frame 18 and the circuit pattern (not shown) to be positioned above the pattern window 16 are preferably produced by an electron beam lithography system in the electron beam sensitive layer. The electron beam lithography system scans the surface of the mask to accurately determine the location of the pattern-to-mask alignment marks 24. It then produces the desired circuit pattern by writing the pattern into the electron beam sensitive layer. The electron beam lithography system determines the location of the pattern-to-mask alignment marks 24 by scanning through a collection of electron beam scan subfields 46, 48 as shown in FIG. 5. Subfields in one column of the entire field that do not overlap the pattern-to-mask alignment mark 24 have been marked 46. Clearly, many other subfields exist that do not overlap the pattern-to-mask alignment mark 24. One subfield that does overlap the pattern-to-mask alignment mark 24 has been marked 48, and this subfield is shown in greater detail in FIG. 6. Each subfield 46, 48 includes numerous scans of the electron beam as illustrated by scan lines 50 in FIG. 6. FIG. 7 illustrates the return signal detected in the electron beam lithography system during scan line 50 in FIG. 6 as it crosses the region 40. When the electron beam is scanning on either side of region 40, the return signal of backscattered electrons is high as indicated in regions 52, 54 in the graph of FIG. 7. However, as the electron beam enters region 40, the electrons stop being backscattered and begin to pass through the alignment mark, into the valley and out the second surface of the mask substrate. In this region 56 of FIG. 7, which corresponds to region 40 in the pattern-to-mask alignment mark, the return signal drops close to zero. The return signal over the entire field of FIG. 5 is most preferably analyzed in a computer within the electron beam lithography system. The return signal is digitized allowing accurate identification of the exact location of the alignment mark. This location is then used in combination with the corresponding location of any additional alignment marks to determine the starting location for the electron beam to write the frame 18 and/or desired circuit pattern into the electron beam sensitive layer. Subsequent conventional processing of the electron beam sensitive layer produces the final X-ray absorptive circuit pattern and frame. While the present invention has been particularly described, in conjunction with a specific preferred embodiment, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art in light of the foregoing description. It is therefore contemplated that the appended claims will embrace any such alternatives, modifications and variations as falling within the true scope and spirit of the present invention.