This invention relates generally to making of masks for semiconductor use and more particularly to calibration of an electron beam tool used in writing to a substrate.
In today""s fabrication of Integrated Circuits (IC) and other semiconductor devices, lithographic delineation procedures are used to yield positive or negative images to bring about selective processing, e.g. etching, implantation, diffusion, deposition, etc. This is especially true in fabrications of masks where the fabrication tool provides Blocking regions and Transparent regions which when illuminated by electron radiation yields an image defined by relatively low and high electron intensities, respectively. A Blocking region is usually defined as the mask region resulting in a degree of electron attenuation in the image which is of consequence in device fabrication. By contrast, a Transparent region is the mask region resulting in a degree of electron attenuation in the image which is small relative to blocking regions in terms of device fabrication.
In the semiconductor industry, there is a continuing trend toward an increased device density. To achieve this, there is a continued effort towards the scaling down of device dimensions on semiconductor wafers. As smaller feature sizes become the new requirements (i.e. decreased width and spacing of interconnecting lines, etc.), new ways have to be utilized to achieve their manufacturing. High resolution lithographic processes are used as one of these manufacturing techniques to yield small component features.
In general, lithography refers to processes for pattern transfer between various media. In lithography for integrated circuit fabrication, a silicon slice, the wafer, is coated uniformly with a radiation-sensitive film, the resist. The film is then selectively exposed to radiation, such as optical light, x-rays, or an electron beam, through an intervening master template or the mask, forming a particular pattern. (In a mask, this leads to the creation of Blocking and Transparent regions which when illuminated by electron radiation yields an image defined by relatively low and high electron intensities, respectively.)
Most often exposed areas of the coating become either more or less soluble than the unexposed areas (depending on the type of coating) in a particular solvent developer. The more soluble areas are removed with the developer in a developing step, the less soluble areas remain on the silicon wafer forming a patterned coating. The pattern corresponds to the image of the mask or its negative. The patterned resist is used in further processing of the silicon wafer.
At various stages in forming the patterned resist coating and processing the silicon wafer, it is desirable to measure critical dimensions resulting from the lithographic process. Critical dimensions can include the size of features in the wafer or patterned resist such as line widths, line spacing, and contact dimensions. Several calibration methods are developed and can be used such as scanning electron microscopy (SEM) systems.
In such calibration system, because of the super fine structures to be calibrated, an electron beam is often scanned across the sample. The beam interacts with the sample to produce measurable responses that vary with position over the course of a scan. There are several other ways to check for accuracy as well to ensure image precision at a later time.
One of the methods used historically by the E beam lithography tools is one that involves checking for stitch errors in determining more important underlying errors. This is often done by comparing the measurement of 4 corners of a target taken in a grid like subfield and comparing it with the measurement of the same corners taken from adjacent subfields. The values are usually then compared against a narrow and wide range for functional purposes. There are several problems with this approach, however. For one, the narrow range is easily exhaustible (exceeded), which makes continuous testing difficult. Second, this approach leaves much information uncovered which may mean that certain electromechanical system errors can go undetected. In order to make this approach more reliable, sometimes an operator has to be dispatched to visually and continuously monitor the testing which adds both inconvenience and cost to the functioning of the test. Therefore, an improved method and structure is needed to detect electromechanical problems in an automated manner and with more reliability
These and other objects are provided by the present invention for an apparatus and method for detection of electromechanical and mechanical errors in an electron beam device. First, a gridlike subfield is provided where each grid can be considered a target subfield. Then, a plurality of target points on each target subfield is indicated and the combined variance of displacement of target points on each target subfield is calculated to nominal position of the target subfield. Later, the combined target subfields stitching standard deviation or variance is also calculated by determining separation of each target point in each subfield with that of overlapping target points on the adjacent subfields. Variance statistics based on the sample variance of all measurements done in the field, variance statistics based on of measurements on a particular row and variance statistics based on differences between horizontally and vertically stitched measurements are made.
A stitched measurement is a measurement made on a particular target two different ways: in this case, the same measurement made two times on two different raster scans. Ratios of these variances can be used to form significance tests related to analysis of variance (ANOVA) and in the form of F statistics, and are well known in statistical literature. A significance test is then conducted using data from the variances derived from adjacent sets of horizontal and vertical values of said target points resulting in calculation of separated vertical and horizontal FSTITCH values. When an F statistic, such as FSTITCH is greater than some threshold, it indicates statistical significance where significance tables for the F statistics are readily available in the literature for threshold selection depending on an acceptable overkill rate known in the literature as alpha significance. By comparing FSTITCH values for horizontal and vertical values to some threshold greater than one, an error alert can be provided when FSTITCH values exceed that threshold. The horizontal statistic and vertical statistics may be used to indicated isotropy or anisotropy. Lastly, a general F statistic based on average row variance and the variance over the entire field variance may be used as one indication of an electromechanical disturbance in raster scanned data, particularly in an E beam system.