Semiconductor device fabrication including a non-destructive method for examining lithographically defined features

Abstract: A non-destructive double exposure method of examining photoresist features in section by, e.g., scanning electron microscopy, is described. The resist is exposed twice with one exposure defining integrated circuit features (5, 7, 9) and the other exposure defining an edge type feature (11) which overlaps an integrated circuit feature (5). Resist development produces a sectioned integrated circuit feature (17) which can be examined.

Description: Background of the Invention As semiconductor integrated circuits (ICs) are made with smaller features, control over processing parameters becomes both more difficult and critical. For example, the permissible variations in device feature sizes become smaller as the feature sizes are reduced. Integrated circuits are typically manufactured by exposing selected portions of a resist, which covers an underlying substrate, to radiation. The resist is then developed, and depending upon whether the resist is positive or negative, the exposed or unexposed portions of the resist are removed. The resist patterns are transferred into the substrate material using such processes as dry etching or ion implantation to form the IC device features. The term "substrate" is used by us to mean the material underlying the resist. In general, the dimensional control over IC device features is dependent on control of both the lateral dimensions and the profiles of the resist features. While the reason for the dependence on the lateral resist dimensions is obvious, the reason for the dependence on resist profiles is subtle and depends on the precise pattern transfer process used. Thus, it is important to be able to examine not only the lateral dimensions of resist features, but also their profiles, especially at or near the resist-substrate interface. Unfortunately, examination of the resist profiles of features which are either enclosed or near other features, is difficult and may require the destruction of the wafer. The examination of holes in resists, used to form electrical contacts within the IC, is especially difficult when such holes have lateral dimensions less than 1.0 µm and are defined in resist with a typical thickness greater than 1.0 µm. In order to ensure that the holes are etched into the substrate material immediately under the resist, typically a oxide, with the proper dimensions, it is usually necessary to determine that the profiles forming the holes in resist are close to vertical, that there is no resist remaining at the bottom of the holes, and that the dimensions at the bottoms of the holes are within prescribed limits. If these conditions on the resist features are not met, the pattern transfer will be imperfect and serious loss in device yield will occur, resulting in unwanted expenses. There are two examination techniques generally used within the IC fabrication industry at the present time. Optical techniques are generally satisfactory for contact windows with diameters greater than 2.0 µm. However, windows smaller than 2.0 µm in diameter generally have an aspect ratio, i.e., ratio of window diameter to resist thickness, comparable to the numerical aperture, typically approximately 0.9, of the microscope objective lens used to observe the holes. Consequently, it is difficult to interpret the optical image when focusing below the top of the hole ad examination of the bottom of the hole is impractical. Techniques using scanning electron microscopes (SEMs) offer better performance than do optical techniques because the use of shorter wavelength radiation allows the use of much smaller numerical apertures. Both high and low voltage SEMs are presently used for IC examination. High voltage, approximately 20 KeV, SEMs give excellent micrograph images of resist features but generally require deposition of a conductive coating on the wafer to avoid the deleterious effects of charging. Optimized micrographs can be used to determine the dimensions of the contact windows in resist. However, if, as is frequently the case, the aspect ratio of such windows exceeds unity, it is necessary to mechanically cleave the substrates ad examine window cross-sections for unambiguous results. Low voltage SEMs can be used to examine resist samples without a conductive coating, but with a loss of image contrast. The best results are obtained with the wafer tipped almost 40 degrees from the incident electron beam. The tilting, which minimizes charging, is not possible when examining high aspect ratio contact windows because the bottom of the contact window is obscured. Mechanical cleavage of the wafer to reveal window cross-sections is the standard procedure if unambiguous results are desired. However, mechanical cleaving has several major disadvantages in practice. Some disadvantages relate to cost. The procedure destroys the wafers and it is thereby costly. Preparation of cleaved wafers can be especially lengthy and, therefore, costly, if the IC does not have a regular array of contact windows to cleave through. Most logic array chips are of this type. The lengthy cleaving procedure can delay the manufacturing process until results are obtained, with a cost increment Additionally, the procedures discussed do not automatically provide for a suitable metric, within the field of view of the examination system, for calibration purposes. This reduces the accuracy of measurement, since it relies on prior calibration of the SEM under observation conditions that are frequently not precisely duplicated during the examination. Precise measurement is therefore difficult because surface charging conditions and details of the beam focus can alter the calibration. U.K. Patent Application 2,000,606 discloses a method of manufacturing semiconductor integrated circuits. The method comprises coating a substrate surface of at least one wafer of a lot having a plurality of wafers with a positive resist and exposing the resist to radiation at least twice. One of the exposures defines circuit features which are developed and the second is a non selective exposure. According to the present invention there is provided a method as defined in claim 1. We have found that integrated circuit features may be nondestructively examined in resist by first exposing the resist in the normal manner through a mask and then exposing the resist a second time to an advantageously smaller exposure energy. The first exposure uses a mask which defines features identical to those features present in the integrated circuit and associated test pattern, if any. The second exposure uses a mask having an edge type feature which is aligned so that it intercepts the integrated circuit resist feature whose cross section will be examined. In a preferred embodiment, the second exposure is done without any prior processing of the resist. After completion of the second exposure, the wafer is developed thereby generating the desired cross section of the integrated circuit resist feature. The generic process will be referred to as the photocleave process and the resulting resist features as having been photocleaved. If the resist feature definition is deemed adequate upon examination of the photocleave, further processing of the wafer examined, as well as other wafers in the lot, may be done. Otherwise, the processing sequence may be terminated or the wafers may be subjected to reworking. Brief Description of the Drawing FIGs. 1-2 are useful in explaining our invention prior to resist development; FIGs. 3-4 illustrate photocleaved features; and FIGs, 5-7 are useful in explaining the desirability of multiple exposures. Detailed Description An illustrative embodiment of our invention will be described in schematic detail. A single wafer from a lot having a plurality of wafers is examined. Is is to be understood that the term "examine" is used broadly and may include, for example, measurement of feature sizes. FIG. 1 shows a substrate 1 onto which a positive resist 3 has been spun in conventional manner. The substrate 1 may be patterned, and therefore not flat. The resist 3 has been exposed with radiation through a mask in a manner well known to those in the semiconductor industry. The mask defines features which are identical to features present in active integrated circuit devices. In FIG. 1, the boundaries of the exposed features 5,7, and 9, commonly referred to as latent images and, in this case, holes in a positive resist, are shown. It should be understood that the features are latent images and thus, not evident upon visual examination. In a preferred embodiment, the resist is exposed again through the same, or a different, mask containing a simple edge-type feature. The latent image 11 of the edge-type features is shown superimposed on the latent image 5 in FIG.2. The aligmnent of the edge-type feature is such that its latent image 11 intersects the latent image 5 of a hole-feature. In may applications, it is advantageous to arrange for the the latent image 5 to have as sharp a gradient as possible. The doubly exposed photoresist is then developed in conventional and well known manner. The resulting resist features remaining on the substrate 1 after development are shown schematically in FIG. 3. It is seen that the latent images 7 and 9 have developed into contact holes 13 and 15 in resist. Latent image 5 has developed into a cleaved contact hole 17 in resist with all resist forward of the surface 11 removed. As mentioned previously, we refer to the contact window as having been photocleaved and the process for achieving this as the photocleave process. The photocleaved window can be easily examined with a SEM, advantageously with the wafer appropriately tipped, for incomplete removal of resist at the bottom of the window, and for the resist profile at the resist-substrate interface. The diameter of the window can also be measured if the cleaving is done across an appropriate chord of the cleaved contact window. If the information derived from the examination is adequate, i.e., the feature is adequately defined, device fabrication may proceed. That is, processing of the wafers in the lot continues. The schematic diagrams do not show the effects of standing wave interference effects which generally cause the walls of the resist forming the windows and the photocleaved surface to be corrugated. The presence of such standing waves will not reduce the value of the photocleave, although they can complicate the interpretation of the photocleaved section. The deposited energy chosen for the photocleave exposure is not critical although it is desirably less than the energy used for the other exposure. If it is desired to remove all the resist in front of the surface 11, the photocleave exposure energy must be larger than the threshold energy, (E th ), for complete resist removal. Generally a photocleave exposure of about 1.5 E th is advantageous. The location of the photocleave surface 11 relative to the center of the contact window can be chosen to reveal those aspects of the contact window in resist which are of most interest with respect to device fabrication. In order to avoid absolute calibration of the instrument used to measure the photocleaved features, the edge-type feature used for photocleaving can incorporate a metric for calibration purposes. An example of a developed contact window cleaved with a linear edge containing a metric 19 for pitch calibration of the SEM measuring tool is shown in FIG. 4. The photocleaved hole has a diameter 18 between corners 21 and 23. Depending upon the instrumentation used to perform the photocleave exposure, it may be advantageous to perform the second photocleave exposure after the first exposure has been developed. In the case of resist contact window measurement, prior development of the contact window images in positive resist can facilitate the correct alignment of the photocleave mask to the windows. As will be readily appreciated, the methods described for practicing the photocleave process are nondestructive with respect to the wafer. After the photocleave process, the resist can be stripped off, another resist coating applied and the wafer returned to the mother lot for reexposure and continuation of the device fabrication sequence for the entire lot. One skilled in the art might ask why one could not expose the resist once with a mask containing features already sectioned. Referring to the pattern configuration shown in FIG. 4, such a mask would contain a sectioned contact window and metric features. After exposure and development the printed geometries in resist would appear as shown in FIG. 5 rather than as in FIG. 4. Comparing FIG.5 with FIG. 4, it will be noticed that there are important differences in detail when using a printer whose resolution is limited. In FIG. 5, the intersections of the linear features with the device feature are not sharp, but rather are rounded, as indicated by the large radii of curvature of the corners 25, compared to the very small radii of curvature of the corners 21 and 23 in FIG. 4 produced by the same printer. Furthermore, the hole diameter 27 measured in FIG.5 may not be correct because of the large curvature of the corners 25; although it will be correct when measuring the dimension 18 in FIG. 4. In other words, when using a printing system of limited resolution, two separate exposures are required because one of the exposures must image the device features exactly as they will be printed in the device to avoid subtle feature distortions caused by using a composite mask. Still further insight into the advantage of a multiple versus single exposure can be gained by discussing the fundamental reason why the corner of a printed feature is rounded. In FIG. 6, a sharp corner of a feature on the photomask is shown. As perceived by the imaging system within the printer, the corner is comprised of an infinite number of line segments dl separated by a distance w which is a function of the distance, x , as shown in FIG. 6. When w is larger than the resolution limit, r , of the imaging system, the line segments comprising the corner can be imaged into the resist without distortions. Consequently, referring to FIG. 7, the resist edges comprising the corner are linear in the regions 28-29 and 30-31. When w is less than r , the imaging system cannot project an aerial image of the edge segment with sufficient modulation for the resist to be accurately defined. Since the modulation drops quickly for w less than r , there is insufficient intensity in the aerial image of the segments closer to the nominal location of the corner 32, and a radius appears on the printed corner in resist as shown in FIG. 7. On the other hand, when such a corner is defined by the intersection of two linear features during separate exposures, the limited resolution of the printer is of no consequence, except for controlling the line edge profiles, and the corner rounding is much reduced.

Claims: A method of semiconductor integrated circuit fabrication comprising the steps of: coating a substrate surface of at least one wafer of a lot having a plurality of wafers with a positive resist; exposing selectively said resist to radiation first and second times, said first time defining integrated circuit features, said second time defining edge-type features, at least one of said edge-type features overlapping at least one of said integrated circuit features; developing said resist, said developing generating at least one cross-section of said at least one of said integrated circuit features; and examining said at least one cross-section of said intergrated circuit features with an SEM. A method as recited in claim 1, CHARACTERIZED IN THAT said integrated circuit feature comprises a hole. A method as recited in claim 1, CHARACTERIZED IN THAT said integrated circuit feature comprises a line segment.

Inventors: CUTHBERT JOHN DAVID, SCHROPE DENNIS EARL, YANG TUNGSHENG, CUTHBERT, JOHN DAVID, SCHROPE, DENNIS EARL, YANG, TUNGSHENG
Applicants: AMERICAN TELEPHONE & TELEGRAPH, AMERICAN TELEPHONE AND TELEGRAPH COMPANY
Application year: 1988
Main classification: H01L21
IPC codes: H01L21, G03F1, G01N21, G03F7, H01L27