LASER-BASED SYSTEM FOR THE TOTAL REPAIR OF PHOTOMASKS

Abstract: A low power laser-based system for repairing clear and opaque defects in a photomask. The system includes a laser source for providing a beam having a wavelength between 0.40 µm and 2.0 µm. The beam is coupled to a computer controlled scanning device which scans the beam across a defective area of the photomask to repair the defect by thermal deposition of micron-size films on the defect or by laser-induced degradation of a polymer coating the defect. Excess metal is removed from the photomask surface via laser vaporization.

Description: The present invention relates to a system for repairing both clear and opaque defects in photomasks. The process of fabricating photomasks (usually chrome or iron oxide on soda, lime, glass or quartz) typically leaves behind several defects. The defects can be classified as opaque or clear; the former relating to the case where excess chrome exists in an unwanted area, the latter relating to the case where chrome is missing from a desired area. These defects can also arise during use of the mask in standard lithographic processes. Currently, it is possible to repair opaque defects (i.e., remove excess chrome) by laser vaporization. An apparatus and method for processing photomasks is disclosed in US Patent 4,190,759, in which an Ar laser beam of a pulse width 10 to 20 nanoseconds is irradiated and scanned relatively and two dimensionally on a metal thin film formed on a transparent substrate from the transparent substrate side and through the transparent substrate with a superposition number of laser beam spots set up to at most 15 and to thereby remove a predetermined region of the metal thin film. The irradiated metal film begins to melt from the substrate side and is removed away explosively from the substrate, leaving the substrate substantially unaffected. The repair of clear defects is much more complex. The conventional method of repairing clear defect relies on a lift-off process, i.e. spinning on photoresist, sputtering on a metal, followed by lift-off. This process is not only time consuming and expensive, but also runs the risk of introducing new defects since the entire photomask is affected. In addition, high resolution, e.g. the repair of a 2µm diameter pinhole, may be difficult to achieve. Another more site-specific method exists. Here, a microdrop of ink is dispensed on the clear defect, and then the entire photomask is baked in an oven to promote adhesion. This process is of limited resolution. Defects less that 10µm square usually cannot be repaired. Furthermore, the repair is not durable -- it is easily removed during standard photomask cleaning procedures. Another site-specific method for defect repair employs an ultraviolet laser (typically at 257nm) and is disclosed in D.J. Ehrlach et al., IEEE Electron Device Letters, Vol. EDL-1 No. 6 (1980), pp. 101-103. Here, the laser is used in conjunction with a metal-bearing gas. The laser light photo-dissociates (i.e., breaks the molecular bonds) the molecules in the vapour phase. The metal fragments subsequently impinge on the surface eventually forming a thin film. This procedure is difficult to employ in the semiconductor industry. Ultraviolet lasers generally occupy an inordinate amount of space. Since clean room space, where the repair takes place, is limited and costly, such a repair system is undesirable. In addition, ultraviolet laser light particularly that generated at 257 nm, is unreliable, thus making such a system undesirable in a production environment. The required ultraviolet optics is also difficult to obtain and costly. In terms of laser-based systems for opaque defect repair, several problems are also encountered. For example, such systems depend on mechanical scanning stages for the positioning of the defect area under the laser irradiation zone. To avoid the large costs involved, manufacturers typically employ low-resolution stages -- typically 10µm for positioning. Finer adjustments must be made manually. This slows down the repair process and makes the accurate repair of small defects on the order of 1µm very difficult. Further, such systems do not have the capability of scanning over small areas say 10µm square. Thus, to remove excess chrome in such an area would required several individual laser repairs, i.e. the area in which chrome is to be removed would have to be brought under the laser irradiation zone in several discrete steps. In accordance with the present invention, the disadvantages of prior photomask systems have been overcome. The system of the present invention is a low-power laser based system for repairing both clear and opaque defects in a photomask. The present invention provides an instrument for repairing clear and opaque defects in photomasks comprising: a laser source for providing a laser light beam; optical means for viewing the photomask within the optical field of view; means for locating defect areas on the photomask within the optical field of view; means for focusing said laser light beam onto the photomask; means for precisely positioning the focused laser beam at a predetermined area on the photomask; means for continuously scanning the focused laser beam over a predetermined area; characterised in that said laser beam has a wavelength between 0.4µm and 2.0µm, and in that the instrument further includes a cell for enclosing a photomask and containing a metal-bearing gas; and means for causing said laser source to emit laser light pulses of selected power and duration and at selectable frequency including an single pulse. An acousto-optical scanning device is provided to position and/or scan the laser beam across a predetermined area of the photomask so that small defects may be accurately repaired. Imaging/focusing optics are coupled to the output of the scanning/positioning device to allow simultaneous viewing of the photomask and focusing of the laser beam on a predetermined area of the photomask. In addition, the system includes an acousto-optic modulator interfaced to a computer for the automatic control of the intensity and pulse duration of the laser beam in response to the power of the laser transmitted through the repair area as sensed by a detector. The computer is compatible with photomask inspection systems such as those manufactured by a commercially available photomask inspection system. Thus, information regarding the location of a photomask defect can be fed in from the commercially available photomask inspection system into the computer. The computer then drives a low resolution X-Y stage which will bring the defect into the optical field of view. The computer is then used to control the scanning/positioning device to accurately position the laser beam with respect to the defect on the photomask and/or to scan a predetermined area on the photomask. The computer-controlled positioning of the laser beam with respect to the defect is aided by a numbered grid which is superimposed on the monitor's image of the photomask. The system may be employed to repair clear defects on a photomask by a method utilizing laser-induced degradation of a novolak-based polymer coating the photomask. In carrying out this method, the scanning device of the system scans the laser beam across the defective area of the photomask for localized heating thereof to darken the polymer coating the photomask. The surface power of the laser as sensed by the detector provides an indication of the darkening level of the polymer and in response thereto, the computer controls the power level of the laser beam to heat the polymer coating the photomask to a temperature between 200 o C and 500 o C so as to brown the polymer. When the detector indicates that the polymer has been adquately browned, the computer, in response thereto increases the power level of the laser beam to heat the polymer to a temperature which is greater than 500 o C so as to char the polymer. The system may also be employed to repair clear defects on a photomask by thermal deposition of micron-size metallic films. In order to carry out this method, a photomask is positioned in a gas cell containing a metal-bearing gaseous compound. A nucleation layer is formed on a surface of the photomask by shining ultraviolet light thereon. After the nucleation layer is formed, the system of the present invention is employed to focus the lower power laser beam on a defective surface of the photomask for localized heating of the area of the photomask to which the beam is incident to thermally decompose molecules of the metal-bearing gaseous compound on the heated area to deposit a metal film thereon. These and other objects and advantages of the invention, as well as the details of an illustrated embodiment, will be more fully understood from the following description and the drawings. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram of the photomask repair system of the present invention; FIG. 2 is a cross-sectional view of a gas cell which may be used with the system of FIG. 1; and FIG. 3 is a representation of the path that the laser follows when scanning a clear or opaque defect. The system of the present invention repairs clear and opaque defect in a photomask 10 which is securely attached to a low-resolution X, Y scanning table 12. The table 12 also contains a low-resolution Z travel. The system employs a laser source 14, the power level of which is less than or equal to 2 watts, the beam having a wavelength between 0.40µm and 2.0µm. The laser source 14 may be a gas laser such as a HeNe or argon ion laser. A solid state laser source such as a GaAs of a GaAlAs laser can also be used having the property of being extremely compact. The laser beam from the source 14 is directed into a modulator/attenuator 16. The modulator may be an acousto-optical modulator which is used to adjust the pulse duration of the laser beam in response to a control signal from a computer 18. The modulator is also used as a shutter capable of providing a single laser pulse of a predetermined duration. In the preferred embodiment, the shutter speed is adjusted between 1µsec and 1sec. The attenuator adjusts the power level or intensity of the laser beam. The power level of the laser beam at the surface of the photomask can also be controlled by the attenuator, the attenuator varying the surface laser power between 0.01 milliwatts and 100 milliwatts. The modulator/attenuator 16 may be such as manufactured by Newport Electro-Optics and Intra Action, although other types of modulator/attenuators such as electro-optical may be suitable for use in the system of the present invention. The beam output from the modulator/attenuator 16 is coupled to an X, Y scanning/positioning device 20 which may be an acousto-optical, electro-optical or mechanical scanning device capable of scanning and/or positioning the laser beam across a very small area such as a 128µm x 128µm area continuously or positioning the beam anywhere within this area. A suitable scanning/positioning device is also manufactured by Newport Electro-Optics and Intra Action. The laser beam output from the scanning device 20 is reflected into a high power focusing objective 52 via reflector 55. The objective focuses the beam to its diffraction-limited spot size. In the preferred embodiment, the objective has a numerical aperature of 0.2 or higher and a magnification of 20x or higher. The objective also facilitates the simultaneous viewing of the photomask by the operator. The photomask may be illuminated either by a set of reflected-light optics 44 or transmitted-light optics 46 which is directed through the photomask and objective via reflector 54. The sample and laser beam are simultaneously brought into focus by adjusting the Z (i.e., distance between photomask and objective) position of the mechanical stage 12. In addition, fine focusing is accomplished by the computer control of the piezoelectric driver 50 onto which the objective 52 is mounted. Sample viewing and laser beam positioning is facilitated by directing the light from the photomask via reflector 58 into video camera 22. The image of the photomask is then formed on monitor 24. The monitor 24 also has a numbered grid superimposed on it. This grid corresponds to an area of 128µm x 128µm on the photomask. Depending on the particular computer system used, a variety of grid sizes may be used. The grid size is chosen to be compatible with the bit-size of the computer. The table 12 on which the photomask 10 is positioned may be transparent or may include an aperture 26 which allows the laser beam, transmitted through the photomask, to pass to a detector 28. The detector 28 is responsive to the beam transmitted thereto for sensing the amount of power at the surface of the photomask 10. The detector 28 provides a signal representing the surface laser power to the computer control 18 which in response thereto provides a control signal to the modulator/attenuator 16 to vary the intensity of the laser beam as described below. The computer control 18 also controls the positioning of the laser beam with respect to the photomask 10. The computer control 18 may be compatible with the commercially available photomask inspection system which stores the location of the photomask defects. This data can then be used by computer 18 to control the mechanical movement of the table 12 in the X and Y directions in order to locate the defect approximately under the beam. Thus, data obtained independently by a photomask inspection system can be used to control the table 12. The mechanical movement of table 12 provides coarse positioning of the photomask with respect to the laser beam. The computer control 18 also controls the scanning/positioning device 20 to precisely locate the beam with respect to the photomask. The determination of the precise location on the photomask is aided by the numeric grid superimposed on the television monitor 24. The coordinates of the defect as obtained from the grid on the monitor 24 are then fed into the computer 18 which in turn controls the scanning/positioninng device 20 which precisely locates the focused laser beam onto the defect area. The scanning device 20 under the control of the computer 18 thus provides fine adjustments in the positioning of the beam with respect to the photomask 10. The system shown in FIG. 1 may be employed to carry out a method wherein clear defects on the photomask 10 are repaired by laser-induced degradation of a polymer coating the photomask. In order to carry out this method, the surface 30 of the photomask is coated with a novolak-based polymer, preferably a positive photoresist, to provide a coating layer having a thickness between 2,000Å and 20,000Å. Other polymers may be used provided that they char or darken in response to heating; the polymer must also adhere satisfactorily when coated on the photomask. In addition, the polymer coating must be pinhole free. Furthermore, the uncharred region of the polymer must be capable of being removed easily without damaging the photomask. Another requirement of the particular polymer used by the subject method is that the charred region must have an optical density of at least 2.5 and be able to withstand the rigorous cleaning method normally encountered in photomask cleaning. The coating procedures used are standard procedures and are well-known to those skilled in the art. Very basically, the polymer is sprayed on the photomask as the photomask is spun so that a relatively uniform coating results across the surface of the photomask. After coating the photomask 10 with the polymer, the photomask is positioned on the table 12, and the laser beam is scanned by the device 20 across the defective area of the photomask to heat the polymer coating to a temperature between 200 o C and 500 o C so as to brown the polymer. The detector 28 senses the power of the laser at the surface of the photomask to provide a signal representative thereof and of the power absorption of the photomask to provide an indication of the darkening level of the polymer. The computer 18 in response to the signal output from the detector 28 determines when the polymer coating has been adequately browned by the laser beam and in response to such a determination varies the control signal applied to the modulator/attenuator 16 to increase the intensity of the laser beam so as to heat the polymer to a temperature which is greater than 500 o C to further darken or char the polymer. After the polymer is charred, that portion of the polymer which is not exposed to the laser is removed by immersing the photomask 10 in an organic solvent such as a keytone alcohol or inorganic solvent such as an alkali, and in the preferred embodiment, acetone is used as the solvent. The charred portion of the polymer has undergone a significant change in chemistry so that it is relatively insoluble with respect to the uncharred portion of the polymer. Although the mechanism by which the charred portion of the polymer becomes relatively insoluble is not completely understood, it is believed that the charred polymer has a much higher carbon ratio which, in turn, reduces its solubility. Further details of the method for providing laser-induced polymer degradation for photomask repair may be found in our European patent No. 165, 686 (claiming priority of the US patent application Serial Number 622 367), entitled "Method for repairing a Photomask by laser-induced polymer degradation" filed concurrently herewith, which is incorporated herein by reference. The system shown in Figure 1 may also be employed to carry out a method of repairing clear defects in a photomask by thermal deposition of micron-size metallic films on the photomask. In order to carry out this method, the photomask 10 is positioned in a gas cell 32 which contains a metal-bearing gaseous compound 34. The gas cell 32 includes metal end plates 36 and 38 and a transparent window 40. The window 40 may, for example, be made of quartz. The photomask 10 may also form a window of the gas cell 32 with the surface 42 on which a metal deposit is to be made forming an inner surface of the cell. After positioning the photomask 10 in the gas cell 32, a nucleation layer is formed on the photomask to seed the surface 42 thereof. The nucleation layer is a random deposit of parts of the gaseous compound 34 on the surface 42 of the photomask forming one of a few monolayers on the substrate surface which are substantially invisible and removable. The nucleation layer may be formed on the photomask 10 by shining ultraviolet light through the window 40 onto the surface 42 of the photomask 10 to randomly deposit parts of the metal-bearing compound 34 on the photomask. After the nucleation layer is formed, the gas cell 32 is positioned on the table 12. The laser beam is then focused onto the defective surface on the photomask for localized heating of an area of the photomask to which the laser beam is incident to thermally decompose molecules of the gaseous compound 34 on the heated area to deposit a metal film thereon. Metal films are thermally deposited on the desired areas of the photomask by scanning the beams across these areas under the control of the scanning device 20. Details of the method for thermal deposition of micron-size metallic films on a photomask may be found in our co-pending application, EP-A-172604 (claiming priority of US patent application Serial Number 622,366, issued as US Patent 4,543,270), entitled "Method for Depositing a Micron-Size Metallic Film on a Transparent Substrate Utilizing a Visible Laser", filed concurrently herewith, which is incorporated herein by reference. The system of Figure 1 may also be employed to remove opaque defects from the photomask 10. In order to remove opaque defects (i.e. excess chrome), the computer 18 controls the modulator/attenuator 16 to adjust the power of the laser beam to a level which will remove the metal patterns from the surface of the photomask 10 as well known in the art. The scanning/positioning device 20 then scans the beam across the defective area of the photomask to remove any unwanted metal deposits. The scanning/positioning device 20 can also be used to select a single 1µm area of the photomask for removal of chrome. The system of FIG. 1 represents an improvement over prior-art opaque-defect repair systems for the following reasons. First, the system described herein causes the laser to be scanned in response to commands by computer across the opaque defect in a continuous-path rather than normally moved across the defect in discreet steps. Second, in prior art systems, the photomask is moved in discreet steps with respect to the laser rather than moving the laser in a continuous path with respect to the photomask as taught by the subject invention. By scanning the laser with respect to the photomask, the speed of the process can be greatly increased and the system taught by the present invention the opaque defect area is divided into rectangular sections, and the coordinates of each corner of each rectangle is determined from the grid on the monitor and then fed into the computer. The computer then causes the laser to be automatically scanned in a boustrophedonic path until the entire area of the rectangle has been covered. This removes all of the metal within that rectangle. The same process is then repeated for each rectangular area until the entire opaque defect is repaired. The path that the laser follows is illustrated in FIG. 3. The scanning process used by the laser is similar when the laser is used to repair clear defects. Both opaque and clear defects can be repaired in one processing sequence. This can be accomplished in one of two ways. One way to accomplish both clear and opaque defect repair in a single process is to coat the photomask with the novolak-based polymer. Next, darken the polymer over the clear defects. Then, ablate the polymer over the opaque defects, followed by vaporizing the metalization forming the opaque defect. Another way to repair both clear and opaque defects in a single process is to place the photomask in a cell containing a metal-bearing gaseous compound. Next, the laser beam is focused on the clear defect to deposit metal over the clear defect area. Then, the cell is vented to removed the metal-bearing gaseous compound. The laser is then focused on the opaque defect to vaporize the metal forming the opaque defect.

Claims: An instrument for repairing clear and opaque defects in photomasks comprising: a laser source for providing a laser light beam; optical means for viewing the photomask within the optical field of view; means for locating defect areas on the photomask within the optical field of view; means for focusing said laser light beam onto the photomask; means for precisely positioning the focused laser beam at a predetermined area on the photomask; means for continuously scanning the focused laser beam over a predetermined area; characterised in that said laser beam has a wavelength between 0.4µm and 2.0µm, and in that the instrument further includes a cell for enclosing a photomask and containing a metal-bearing gas; and means for causing said laser source to emit laser light pulses of selected power and duration and at selectable frequency including an single pulse. An instrument according to claim 1 wherein said laser source is a solid state laser. An instrument according to claim 1 wherein said laser source is a gas laser. An instrument according to claim 1 wherein said optical means includes: a high-power focusing objective for creating an image of the photomask; a camera for receiving the image from the objective; and a monitor to display the image from the camera. An instrument according to claim 4 wherein said monitor provides a visual display of the defective areas of the photomask and the focused laser beam. An instrument according to claim 4 wherein said monitor includes a screen and further includes a numbered grid superimposed on the monitor screen to provide a means for assigning an absolute coordinate of any area of the photomask within the objectives filed of view. An instrument according to claim 6, wherein said numbered grid is 128 x 128. An instrument according to claim 6 wherein said numbered grid is 512 x 512 or less. An instruments according to claim 1 further including a set of reflected and transmitted light optics for the illumination of the photomask. An instrument according to claim 1 further including a computer controlled low resolution X, y table for bringing the defective areas of the photomask into the optical field of view. An instrument according to claim 10, wherein said computer controls movement of said table using data obtained from an independent photomask inspection system. An instrument according to claim 11, wherein said data specifies the location of the defects on the photomask and is used by the computer to drive the X, Y stage. An instrument according to claim 1 wherein said means for focusing includes means for direction the beam through a high power objective. An instrument according to claim 13 wherein said objective has a numeral aperture of 0.2 or higher. An instrument according to claim 13 wherein said objective has a magnification of 20X or higher. An instrument according to claim 13 wherein said means for focusing further includes a table adjustable in the X-Y and Z plans for facilitating focusing of the laser beam onto the photomask. An instrument according to claim 13 where in said means for focusing further includes a piezoelectric driver on which the objective is mounted for the fine control of the distance between the objective and photomask. An instrument according to claim 17 wherein said means for focusing further includes a computer for controlling the piezoelectric driver. An instrument according to claim 1 wherein said means for positioning includes a computer-controlled scanning/positioning device. An instrument according to claim 19 wherein said scanning/positioning device is an acousto-optic device. An instrument according to claim 19 wherein said scanning/positioning device is an electro-optic device. An instrument according to claim 19 wherein said scanning positioning device is a mechanical device. An instrument according to claim 14 wherein said means for positioning further includes a television monitor for determining the coordinates of the defect to produce data to be fed into the computer to control the scanning/positioning device to direct the focused laser beam onto the defect. An instrument according to claim 1 wherein said means for continuously scanning the laser beam includes a computer controlled laser scanning device. An instrument according to claim 24 wherein said laser scanning device is acoustic-optic. An instrument according to claim 24 wherein said laser scanning device is electro-optic. An instrument according to claim 24 wherein said scanning device moves said laser in a boustrophedonic path across the surface of the defect. An instrument according to claim 24 further including means for dividing the defect area into one or more predetermined areas and for determining the coordinates for each area, said means including a television monitor and wherein said coordinates are fed into said computer for controlling the scanning device and continuously scans the focused laser beam over the defect. An instrument according to claim 28 wherein the predetermined area is less than or equal to a 128µm x 128µm area. An instrument according to claim 28 wherein the predetermined area is less than or equal to a 512µm x 512µm area. An instrument according to claim 1 wherein said means for causing includes a computer controlled modulator. An instrument according to claim 31 wherein said modulator is acoustic-optic. An instrument according to claim 31 wherein said modulator is electro-optic. An instrument according to claim 31 wherein said computer control can adjust the laser power at the surface of the photomask between 0.01 milliwatt and 100 milliwatts. An instrument according to claim 1 wherein said means for causing includes a computer controlled modulator for generating laser pulses of adjustable number and duration. An instrument according to claim 35 wherein the modulator is acousto-optic. An instrument according to claim 36 wherein the laser pulse duration is selected within the range of 1µsec to 1sec. An instrument according to claim 1 wherein said focused laser light beam results in localized heating of an area of the substrate to thermally decompose molecules of said gas on the heated area to deposit a metal film thereon. An instrument according to claim 1 further including means for venting said cell to remove said gaseous compound from said cell. Use of the instrument according to any of claims 1 to 40 for repairing clear and opaque defects in a photomask.

Inventors: OPRYSKO, MODEST M., YOUNG, PETER L.
Applicants: GOULD INC.
Application year: 1985
Main classification: G03F1
IPC codes: G03F1, H01L21