Multi-layer articles, such as multi-layer devices or multi-layer circuits, are typically made as a plurality of layers that are formed in sequence. Multi-layer articles can be made by successive forming of patterned layers on a single substrate, either on one side or on two opposite sides of the substrate. Alternatively, the patterned layers can be formed on a plurality of substrates, and the plurality of substrates can subsequently be assembled together.
Whether a single substrate or a plurality of substrates is used, the patterns in a multi-layer article generally need to be registered with respect to each other. Herein, the terms “registration” and “alignment” refer synonymously to providing a desired geometrical relationship between patterns formed on a substrate. Typically a multi-layer device or circuit has degraded performance if the patterned layers are not registered to within a given set of tolerances.
Conventionally, alignment structures are formed as part of at least one of the patterns of a multi-layer article and the locations of the alignment structures are determined optically. Positioning of a subsequent pattern formed on a substrate, or positioning of a second patterned substrate, is done with reference to the optically determined location of the alignment structures.
Formation of the patterns can be done by additive processing or by subtractive processing. In additive processing the pattern is formed as material is deposited on the substrate. Printing of a pattern is an example of additive processing. Printing can be done in analog fashion, as material is transferred from a master pattern or printing plate to the substrate. Alternatively, printing can be done digitally as a controller controls a printhead, for example, to deposit dots of material at specified locations in order to form the pattern. A familiar example of multi-layer printing is the color printing of images, where successive layers of cyan, magenta, yellow (and optionally black or other color) inks are deposited in registration with each other. In many types of printing systems alignment marks are printed in a first layer near a plurality of edges (typically opposite edges) of the substrate. Cameras or other optical sensing devices are used to monitor the locations of the alignment marks. The analog or digital printing of one or more subsequently printed layers can be controlled using spatial adjustment of the printing device or the substrate so that the subsequent printed layer is registered with reference to the alignment marks. For printing systems where the printhead or the substrate are moved in a nominally linear fashion with respect to each other, the timing of the printing of the subsequent layer(s) can also be controlled on the basis of identified locations of the alignment marks to help provide registration of the patterns. Location of the alignment marks at the outside margins of the substrate can be advantageous both from the standpoints of a) improved angular registration by locating the alignment marks far apart, and b) ability to subsequently remove the margins and the alignment marks in the finished multi-layer article.
Formation of patterns can also be done using subtractive processing. In subtractive processing a blanket layer is typically formed on the substrate. Then material is selectively removed to form the pattern. A familiar example is the processing of semiconductor devices as schematically shown in FIGS. 1A and 1B using a mask aligner 10. A first layer of material can be deposited on the substrate 50. Photoresist 58 can be deposited on the first layer. The photoresist is then exposed with radiation 25 from an exposure station 20 of the mask aligner 10, typically through a first mask 30 having both the desired first pattern 32 and alignment marks 35 and 36 to be associated with the first layer. In the example of FIG. 1A, the pattern 32 is a four by four array of boxes 34 and the alignment marks 35 and 36 are cross-hairs. The photoresist 58 is developed so that the negatives of pattern 32 and the alignment marks 35 and 36 are no longer covered by photoresist. (For simplicity, the positive image rather than the negative image of four by four pattern 52 of boxes 54 and alignment marks 55 and 56 in photoresist layer 58 is shown in FIG. 1A.) The corresponding exposed areas of the first layer of material are subsequently etched away, resulting in the substrate 50 shown in FIG. 1B with the four by four pattern 62 of boxes 64 and cross-hair alignment marks is formed on the surface of the substrate 50. The remaining photoresist 58 is also removed. A second layer (not shown) can then be deposited on the first patterned layer. Photoresist (not shown) can be deposited on the second layer. A second mask 40 is then used to delineate a second pattern 42 in photoresist in registration with the first patterned layer. In particular, cameras 15 of the mask aligner are used to view substrate 50 through second mask 40 as shown in FIG. 1B. The second pattern 42 in this example is a four by four array of circles 44. The desired registration of the second pattern 42 on second mask 40 to the first pattern in this example is when each of the circles 44 is located at the center of a corresponding one of the boxes 64. Mask aligner 10 is used to move second mask 40 or substrate 50 in the X, Y and θ directions until alignment marks 45 and 46 on second mask 40 are registered with corresponding alignment marks 65 and 66 formed in the first layer of material on wafer 50. In the example shown in FIG. 1B, alignment marks 45 and 46 are clear cross-hairs in an opaque field. The clear cross-hairs 45 and 46 are typically designed to be slightly larger than the cross-hairs 65 and 66, so that it is easier to detect when cross-hairs 65 and 66 are centered within clear cross-hairs 45 and 46.
Optical alignment of a sequence of patterned layers works very well when the reference alignment structures can be readily detected optically. For example, if the alignment features have a significantly different reflectance or color than the substrate on which they are formed, then there is effective optical contrast between the alignment features and the substrate.
In some types of multi-layer articles, patterns are formed using materials that do not have effective optical contrast relative to the underlying substrate. For example, it can be difficult to align with reference to an alignment mark that is formed of a substantially transparent material (that is, substantially transparent for the thickness of the patterned layer). Transparent materials can be used in displays, in optical devices, in touch screen sensor films, in photovoltaic devices, and in transparent electromagnetic shielding, for example.