Lithography has a variety of useful applications, including the manufacture of semiconductor devices, flat-panel displays, and disk heads. Lithography is used to transmit a pattern on a mask or reticle to a resist layer on a substrate through spatially modulated light. The resist layer is then developed and the exposed pattern is either etched away (positive resist) or remains (negative resist) to form a three dimensional image pattern in the resist layer. The three dimensional image pattern can then be etched into the substrate or serve as a mask for implantation.
The quality of the image pattern that is developed in the resist layer is effected by exposure and focus. Exposure determines the average energy of the image per unit area and is set by the illumination time and intensity. One effect of changing the value of the exposure is changing the critical dimension (CD) of the lithographically produced lines. The CD is the smallest resolvable dimension of a line or space.
Focus determines the decrease in modulation relative to the in-focus image. Focus is set by the position of the surface of the resist layer relative to the focal plane of the imaging system.
Local variations of exposure and focus can be caused by variations in the resist layer thickness, substrate topography, as well as stepper focus drift. Because of possible variations in exposure and focus, image patterns generated through lithography require monitoring to determine if the patterns are within an acceptable tolerance range. Focus and exposure control are particularly important where the lithographic process is being used to generate sub-micron lines.
Test patterns are sometimes included on the production mask in the scribe line areas. The test patterns can then be transmitted onto the surface of the resist along with the image of the desired device, e.g., an integrated circuit. The image of the test pattern can then be monitored to determine if the image of the desired device is within specification.
FIG. 1 is an example of a conventional test pattern 10 including two tuning fork patterns 12 and 14 in the X and Y coordinate directions, respectively. As shown in FIG. 1, tuning fork pattern 14 is a line/space pattern with densely packed lines 14a and an isolated line 14b. Tuning fork patterns 12 and 14 are useful in measuring the proximity effect associated with the uniformity of the CD in the X and Y directions. The proximity effect alters the uniformity of the CD due to diffraction. Diffraction will reduce the CD of densely packed lines, i.e., lines 14a, and will increase the CD of an isolated line 14b. By measuring the CD of the densely packed lines 14a and isolated line 14b, it can be determined whether the line widths fall within an acceptable range, which is typically .+-.10 percent of the CD.
While test pattern 10 is useful in monitoring the critical dimension and the proximity effect, it does not provide information relating to whether or not the pattern is in focus. Consequently, a separate test pattern must be used to monitor the focus.
The test pattern features are typically measured using a scanning electron microscope (SEM), particularly where the feature size is sub-micron. Where several different test patterns require measurement, for example, where patterns useful in measuring the CD and the focus are located on different test patterns, the SEM must be adjusted to individually view each separate pattern. Repositioning the substrate relative to the SEM so that the SEM may view different test patterns requires time, which reduces the throughput of the system.
Accordingly, there is need for a lithographic test structure that provides information relating to focus and the uniformity of the critical dimension without requiring inspection of separate patterns or the use of a separate test masks.