Patent Number: 051503921
Section: description

DESCRIPTION OF A PREFERRED EMBODIMENT X-ray lithography is an important technology for the manufacture of deep submicron integrated circuits. The implementation of this technology is proximity printing, where an 1:1 X-ray mask is positioned 10 to 40 .mu.m above the surface of the wafer. The mask and wafer are then exposed with a broad beam of X-rays, leaving an image of the mask in a radiation-sensitive film on the surface of the wafer. Such a process requires an alignment accuracy between the mask and wafer of a few tens of nanometers in order that multiple lithography levels precisely register. In addition, it requires careful control of the gap between the mask and the wafer because too large a gap will degrade the image due to diffraction effects and penumbral blurring, and too small of a gap increases the change of accidental touching and subsequent damage to the very delicate mask and/or wafer. In the prior art, alignment is achieved by shining light through the mask, which is usually made on a very thin, semitransparent membrane. The wafer is adjusted relative to the mask until corresponding marks on the mask and wafer properly coincide. However, due to limitations arising from the wavelength of visible light, it is difficult to achieve the required accuracy. Gap control is achieved by precise mechanical fixturing of the mask and wafer, which is difficult to control better than several micrometers; currently a gap of 40 .mu.m is typically used. The scanning tunneling microscope and the atomic force microscope are two closely related instruments that are both capable of achieving atomic resolution in the X, Y and Z dimensions. In the present invention, combinations of elements of these microscopes with X-ray masks are effectively used for mask to wafer alignment. The atomic force microscope is disclosed in U.S. Pat. No. Re.33,387 and the scanning tunneling microscope is disclosed in U.S. Pat. No. 4,343,993, both of which are cited in the Prior Art section herein. The atomic force microscope also described by G. Binning, C. F. Quate and Ch. Gerber, Phys. Rev. Letters, Vol. 56, No. 9, March 1986, pp. 930-933, employs a sharply pointed tip attached to a spring-like cantilever beam to scan the profile of a surface to be investigated. At the distances involved, attractive or repulsive forces occur between the atoms at the apex of the tip and those at the surface, resulting in tinyl deflections of the cantilever beam. This deflection is measured by means of a tunneling microscope, i.e., an electrically conductive tunnel tip is placed within tunnel distance from the back of the cantilever beam, and the variations of the tunneling current are used to measure the deflection. With known characteristics of the cantilever beam, the forces occurring between the AFM tip and the surface under investigation can be determined. The present invention proposes an atomic force microscope or scanning tunneling microscope comprising a pointed tip provided for interaction with a wafer and means for approaching said tip to said surface to within a distance on the order of one tenth of a nanometer, and for scanning said tip across said surface in a matrix fashion. This atomic force microscope is characterized in that said tip is attached to one surface of an oscillating body carrying, on opposite sides thereof, a pair of electrodes permitting an electrical potential to be applied, that, in operation and with said tip remote from said surface, said body is excited to oscillate at its resonance frequency, and that, with said tip maintained at said working distance from said surface, said body oscillates at a frequency deviating in a characteristic manner from said resonance frequency, that said deviation is compared with a reference signal, and that the resulting differential signal is passed through a feedback loop to control said means for approaching the tip to said surface. The basic concept of a scanning tunneling microscope is to place a very sharp, conducting tip having tip dimensions on the order of the size of 1 atom in diameter close to a conductive surface. If the tip is brought very close to a conductive surface, i.e., within the space of the diameters of several atoms, (approximately within 5 angstroms), a tunneling current flows between the tip and the surface. That is, the probability density function of electrons for atoms in the tip overlaps in space the probability density function of electrons for atoms on the surface. As a result, tunneling occurs in the form of electron current flow between the tip and the surface if a suitable bias voltage between these two conductors is applied. The feature of the present invention is that the capabilities of the atomic force microscope are used for X-ray mask gap control and alignment by making the cantilever element as an integral part of the X-ray mask being controlled and aligned. One or more apertures are formed in the mask, in the form of U-shaped slots, leaving a portion of the mask material which functions as the cantilever element. The operation of the cantilever element formed from the X-ray mask is in accordance with standard atomic force microscope or scanning tunneling microscope techniques as described in the prior art. Referring to FIG. 1, a cross section of a perspective view of a wafer 10 and an X-ray mask substrate 11 with membrane 12 which contains a pattern to be transferred to the wafer 10 is illustrated in schematic form wherein a cantilever 14 and tip 16 portion such as used on an atomic force or scanning tunneling microscope are fabricated directly as part of the mask 11. The vertical (z) motion of the tip 16 with respect to the wafer 10 is achieved with a piezoelectric device 18 which is mounted on a movable support 22. Such a device could be a tube having an electrode divided into quadrants so that the end of the tube 20 could be positioned in three dimensions to allow for alignment of the end of the tube to the cantilever tip 16. X and Y motion of the tip 16 and the mask membrane 12 relative to the wafer 10 could be achieved by mounting the wafer 10 on an x-y stage driven by piezoelectric or other transducers. Moving wafers in lithography systems in the x, y and z directions in response to control signals is a technique well known to those skilled in the art. U.S. Pat. Nos. 4,560,880 and 4,870,668 discussed in the description of the prior art teach such systems. Thus, in FIG. 1, means 30 for translating device 18 in the x, y and z dimensions and means 32 for translating wafer 10 in the x, y and z dimensions are illustrated in very schematic form. Fabricating the cantilever element 16 requires only one or two additional processing steps since membrane technology is already used to fabricate the mask. Thus it is possible to fabricate the entire instrument directly onto the mask substrate 11 using multiple lithographic steps and piezoelectric thin film technology. The wafer 10 includes an alignment mark 24 as illustrated in FIG. 1. In operation, the wafer 10, mask membrane 12, and z piezoelectric tube 18 are held rigidly but adjustably with respect to each other by a mechanical fixture (not shown). The z piezoelectric tube 18 is lowered until tip 20 touches cantilever 14; it is then lowered further by the designed gap spacing, deflecting the cantilever 14 downward. The wafer 10 is then raised until it is detected by the tip 16 on the cantilever 14, either by sensing a tunneling current (STM) or a force (ATM). The wafer 10 is now at the correct z gap setting, and is scanned back and forth in the x and y directions until the location of the alignment mark 24 is determined by the cantilever tip 16 following the contours of the alignment mark 24, thus setting the proper alignment between the wafer and the mask in the x, y direction. The wafer 10 is first exposed by a mask which has alignment marks in place of the cantilevers. The pattern is transferred as a relief on the wafer surface for subsequent mark detection. The remaining masks would have three cantilevers located around the edge of the exposure field in order to align the wafer in all six degrees of freedom, as shown in FIG. 2. Spare cantilever locations could be provided in case any were damaged or broken. Deviations of the ends of the cantilevers from their design locations could be mapped out by measuring the alignment accuracy of test exposures.