(1) Field of the Invention
The present invention relates generally to aligning wafers, and in particular, to aligning wafers independent of the layers formed therein in the manufacture of integrated circuits.
(2) Description of the Related Art
Alignment marks and aligning wafers with respect to those marks are an important part of the process of manufacturing semiconductor devices and integrated circuits. As is known in the art, integrated circuits are fabricated by patterning a sequence of masking layers, and the features on successive layers bear a spatial relationship to one another. Thus, as a part of the fabrication process each level must be aligned to the previous levels. Alignment of one pattern layer to previous layers is done with the assistance of special alignment patterns designed on to each mask level. When these special patterns are aligned, it is assumed that the remainder of the circuit patterns are also correctly aligned. Since each layer must have alignment marks for proper registration with respect to the next layer, each alignment or registration then becomes layer dependent.
The tools that are used to pattern the various layers in a wafer are known as photomasks or masks and reticles. The patterns on the mask or the reticle are defined by a combination of opaque and translucent areas. A light source through the mask or the reticle projects the patterns onto the surface of a wafer, and depending upon the material that is being exposed to the light, the pattern is transferred on to the surface where the light arrives or not. A mask contains patterns that can be transferred to an entire wafer in one exposure. A reticle, on the other hand, contains a pattern image which must be stepped and repeated in order to expose an entire substrate.
The adjustment of the image of the mask being exposed to the previously produced patterns was originally performed by human operators, who compared the image locations under a microscope and adjusted the position of the mask to bring it into alignment with wafer patterns. Decreasing feature sizes, and the increasing number of alignments per wafer with step-and-repeat projection aligners, have been the impetus for developing automatic alignment systems. The principle of one type of automatic alignment procedure is illustrated on page 476 of S. Wolf's book, "Silicon Processing for the VLSI Era," vol. 2, Lattice Press, Sunset Beach, Calif., 1990. Alignment marks consisting of two rectangular patterns, each set at a 45.degree. angle to the directions of the motion an xy-stage table are fabricated on a wafer. Two corresponding rectangular patterns are located on the reticle of an optical aligner, and their image is projected onto the wafer. The superimposed alignment target and the reticle image are reflected back into the main optical element of the aligner, and then into an on-axis microscope. The image from the microscope is focused onto the face of a TV camera, and is subsequently digitized into a form that can be analyzed by a computer. When alignment is achieved, a signal is generated. The 45.degree. orientation of the alignment marks makes it possible to obtain both x an y registration information from the horizontal scan-lines of the video camera. The relative position of the wafer marks with respect to the reticle windows determines the registration of the two images.
Registration and alignment in wafer steppers are performed globally and locally. Global alignment performs rotational and translational alignment of the entire wafer, while local alignment provides alignment to a target within a particular region of a pattern image on the wafer. Global alignment is usually accomplished at a remote alignment station before a wafer is moved under the projection lens for exposure. An apparatus for global aligning is disclosed in U.S. Pat. No. 4,046,985, though for a different purpose other than for aligning masks. The aforementioned apparatus aligns the wafer to a fixed reference position, inverts the aligned wafer to expose its backside and transfers it with controlled motion to a set position under a laser apparatus. The laser beam scans the backside of the wafer to create in a kerf area between chip sites on the wafer an easily breakable cut for the purpose of subsequently separating and removing the chips. This method of aligning differs from the method that will be disclosed in the present invention.
U.S. Pat. No. 3,752,589 discloses a method of aligning the pattern of a photomask on one side of a wafer to the patterns placed on the underside of the wafer. The alignment of the mask with respect to the wafer is achieved by optically superimposing the images present on the mask and on the underside of the wafer and adjusting the mask relative to the wafer until the relative positions of the combined images are corrected to a predefined set of conditions. This method requires two viewing apparatus: one for the mask facing the wafer and the other on the opposing side facing the underside of the wafer, which is superfluous in the present invention as disclosed later in a preferred embodiment.
Cade, in U.S. Pat. No. 4,534,804, on the other hand, teaches a method of forming a laser alignment mark such that same mark now extending, after heat treatment, to both the front and the back of the wafer can be used to align photoresist photomasks to both the front and the back side of a silicon wafer. The alignment mark that is formed is actually a defect planted into the wafer by means of a laser beam. The wavelength of the laser beam is chosen such that it passes through the lightly doped wafer without absorption but is absorbed by a following heavily doped semiconductor layer to generate therein heat and resulting defect. The semiconductor wafer is then heated to cause the defects to migrate through a lightly doped epitaxial layer to the front surface thereof in which there is formed an identically positioned image of the mark scribed on the back side. It will be appreciated, however, that this method is limited to contiguous semiconductor materials without any intervening other types of layers.
Generally, the process of aligning masks in fabricating semiconductor devices in a substrate, and, subsequently, in "metallizing" or wiring the devices together to form integrated circuits on the substrate require different considerations. Up to the level where devices are fabricated, the alignment of masks is accomplished by projecting infrared rays from the underside of the wafer while observing the patterns by an infrared microscope. But this method is not applicable when metallized layers are present because metallic films are opaque to infrared rays. Similarly, the method of U.S. Pat. No. 4,534,804 cannot applied since the metallized layer would preclude the migration of a laser defect through it.
The process of metallization, or "personalization" requires the patterning of the metal layers to form the desired circuit pattern. This is accomplished by masking the metal layer with a photosensitive emulsion and then positioning a photographic mask thereon. The emulsion is next exposed to ultraviolet rays though the photographic mask and then the emulsion is developed and the unexposed emulsions is washed away with solvent. The exposed metal areas are then etched to form wiring patterns corresponding to those on the mask. Certain predetermined areas on the metal layer do also contain alignment marks for registration with the next level of metal layer to be deposited.
Before the next metal layer is deposited, however, an interlevel layer of a dielectric insulator is first blanket deposited over the wiring layer. An interlevel layer usually will form a relatively rough topography conforming to the geometrical features of the underlying layer. Since the depth-of-field limitations of submicron optical-lithography tools require surfaces to be planar within .+-.0.5 micrometers (See S. Wolf and R. N. Tauber, "Silicon Processing for the VLSI Era," vol. 2, Lattice Press, Sunset Beach, Calif., 1990, p. 203), planarization, as is well known in the field, of the dielectric layer will also be required if optical lithography is to be usable for fabricating integrated circuits with submicron feature sizes of today. Conventionally, planarization is performed in any number of ways including mechanical polishing, or a combination of mechanical and chemical polishing, called CMP. It will be appreciated that as more layers of metal and interlevel dielectric are deposited, the surface topography of each layer will vary because of the cumulative effect of the number of underlying features that are disposed on top of each other. Hence, the photographic alignment of masks of each layer to the preceding layer will depend upon the nature of the planarized surfaces of each layer, and therefore will vary accordingly. A method is disclosed in this invention where the layer dependency of alignment is eliminated.
To appreciate the layer dependency of alignment and its effects thereof, FIGS. 1a and 1b show a conventional system where wafer (20) is disposed between a movable xy-stage (30) and an alignment source (10). The rays emanating from source (10) are shown by numeral (15) in both FIGS. 1a and 1b. In FIG 1a, wafer (20) is provided with one level of metal (40), or wiring layer, and in FIG. 2a, with two levels of metal, where the second wiring layer is denoted by numeral (50). First and second metal layers, (40) and (50), respectively, are separated by an interlevel dielectric layer (45), and second layer (50) has on it dielectric layer (55). As is commonly used in the manufacture of semiconductor wafers, the interlevel dielectric layers (45) and (55) shown in FIGS. 1a and 1b are spin-on-glass, or SOG. For the reasons given earlier, both of the dielectric layers are subjected to planarization by means of chemical-mechanical polishing before the respective metals layers are deposited thereon. However, it will be noted that the roughness or non-planarity of second dielectric layer (55) is more pronounced than that of the first layer (45). This is because, as the number of metal interconnect levels are increased, the stacking of additional layers on top of one another produces a more and more rugged topography as mentioned earlier. Although polishing creates more planarity, nonetheless, the remaining non-planarity varies from one layer to another depending upon the cumulative effect on the topography of the underlying layer. As a result, the patterns on layer (50) visible through the second interlevel layer (55), for example, are more diffused and not as clear as the patterns on layer (40) are through layer (45). Furthermore, the depth-of-field varies from layer to layer on the optical-lithography tools that are used for aligning masks over different layers. Consequently, the signals generated by the edges of the patterns on metal layers vary depending upon the planarity of the layer over which a mask is placed. In other words, the signals for aligning masks over patterns and alignment marks on a wafer are layer dependent, and are sometimes weak, and at other times not distinct so as to cause misalignment and therefore product defects. What is needed, therefore, is a method whereby the aligning and alignment signals are independent of a particular layer over which a mask is positioned and that the alignment signal strength is invariant throughout the manufacture of a semiconductor substrate.