Patent Publication Number: US-6660649-B2

Title: In-situ balancing for phase-shifting mask

Description:
This is a divisional application of Ser. No. 09/957,571 filed Sep. 9, 2001, now U.S. Pat. No. 6,548,417. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to the field of semiconductor integrated circuit manufacturing, and more specifically, to a phase-shifting mask and a process for fabricating a phase-shifting mask. 
     2. Discussion of Related Art 
     Improvements in photolithography have increased the density and enhanced the performance of semiconductor devices by shrinking integrated circuits (ICs). As described by the Rayleigh criterion, the minimum critical dimension (CD) which can be resolved by a wafer stepper is directly proportional to the wavelength of the illumination source and inversely proportional to the numerical aperture (NA) of the projection lens. However, diffraction tends to degrade the aerial image when the CD becomes smaller than the actinic wavelength. The actinic wavelength is the wavelength of light at which a mask is used in a wafer stepper to selectively expose photoresist coated on a substrate, such as a Silicon wafer. As needed, a resolution enhancement technique (RET), such as a phase-shifting mask (PSM), may be used to achieve a wider process latitude. Unlike a binary mask that only uses Chrome to control the amplitude of light transmitted through a quartz substrate, a PSM further modulates the phase of light to take advantage of destructive interference to compensate for the effects of diffraction. 
     An alternating PSM (AltPSM) is a type of PSM that is particularly helpful in improving contrast when patterning very small CDs, such as the gate length of a transistor in a device. AltPSM introduces a phase shift of 180 degrees between the light transmitted through adjacent clear openings so destructive interference can force the amplitude between the two images to zero. A phase shift of 180 degrees is implemented by creating a difference in the optical path lengths through adjacent openings in an opaque layer, such as Chrome. A subtractive process may be used to etch a trench into the quartz substrate in alternate openings. However, incident light may scatter off the sidewalls and bottom corners of the etched trench and cause an imbalance in the aerial image that varies as a function of focus. Such a waveguide effect may be manifested as a CD error and a placement error. 
     The intensity and phase in the aerial image of an AltPSM may be balanced in various ways. A selective biasing approach enlarges the CD of the etched opening relative to the unetched opening to balance the aerial image. An etchback approach undercuts the edges of the chrome in both openings to balance the aerial image. A dual-trench approach etches a deep trench in the phase-shifted opening and a shallow trench in the non-phase-shifted opening to balance the aerial image. 
     Each approach for balancing the aerial image has drawbacks. The selective biasing approach may be limited to the discrete values available on the design grid unless a gray beam-writing scheme is used. The etchback approach may result in defects, such as chipping or delamination of the overhanging chrome between adjacent openings. The dual-trench approach adds complexity and cost by requiring additional processing. 
     Thus, what is needed is a phase-shifting mask (PSM) having in situ balancing of intensity and phase and a method of forming such a PSM. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS.  1 ( a )-( d ) is an illustration of a method of forming a mask blank with a balancing layer according to the present invention. 
     FIGS.  2 ( a )-( c ) is an illustration of a method of forming an alternating phase-shifting mask with a balancing layer located below the absorber layer and in the non-phase-shifted opening according to the present invention. 
     FIG. 3 is an illustration of an alternating phase-shifting mask with in situ balancing according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PRESENT INVENTION 
     In the following description, numerous details, such as specific materials, dimensions, and processes, are set forth in order to provide a thorough understanding of the present invention. However, one skilled in the art will realize that the invention may be practiced without these particular details. In other instances, well-known semiconductor equipment and processes have not been described in particular detail so as to avoid obscuring the present invention. 
     The present invention describes an alternating phase-shifting mask (AltPSM) having in situ balancing of intensity and phase and a method of forming such a phase-shifting mask (PSM). A method of fabricating an AltPSM, according to the present invention, will also be described. 
     A mask is built from a substrate  1100 , such as quartz, that is transparent at the actinic wavelength. The actinic wavelength is the wavelength of light used in a wafer stepper to selectively expose photoresist on a wafer. An embodiment is shown in FIG.  1 ( a ). 
     A balancing layer  1200  is formed over the substrate  1100 . An embodiment is shown in FIG.  1 ( b ). The thickness of the balancing layer  1200  is typically about 3.0-70.0 nm. Direct current (DC) magnetron sputtering may be used to form the balancing layer  1200 . 
     The balancing layer  1200  is formed from a material that is transmissive and may be capable of phase-shifting. Transmission is usually a function of wavelength and may be measured in a tool, such as the n&amp;k Analyzer 1280 from n&amp;k Technology, Inc. The material, structure, and thickness of the balancing layer  1200  are chosen so that the transmission through the non-phase-shifted opening  1410  is modified, such as reduced, just enough to match the transmission through the adjacent phase-shifted opening  1420 . The modification in transmission may be about 1.5-12.0 percent. This will achieve the goal of matching the intensity of light transmitted through the two adjacent openings. 
     In addition to having suitable optical properties, the balancing layer  1200  must possess chemical resistance, radiation durability, and compatibility with the underlying substrate  1100  and the overlying mask absorber layer  1300 . Phase and transmission should not be appreciably affected by the mask fabrication process, the mask inspection or repair process, or the mask cleaning process. In most cases, the balancing layer  1200  should have a low film stress. 
     The balancing layer  1200  may be formed from an elemental metal, such as Gold, or from suitable compounds, including Fluorides, Silicides, Oxides, or Nitrides of materials, such as Aluminum, Chromium, Hafnium, Molybdenum, Nickel, Niobium, Tantalum, Titanium, Tungsten, or Zirconium. Examples include Chromium Fluoride (CrO x F y ), Zirconium Silicon Oxide (ZrSi x O y ), Molybdenum Silicon Oxide (MoSi x O y ), Aluminum Nitride (Al x N y ), or Silicon Nitride (Si x N y ). 
     In some embodiments, one or more of the materials forming the balancing layer  1200  may be non-stoichiometric. For example, Oxygen may be present in small quantities or may be absent. In other embodiments, the balancing layer  1200  may be non-homogeneous. In still other embodiments, the balancing layer  1200  may be a multilayer. 
     An absorber layer  1300  is formed over the balancing layer  1200 . The result is a mask blank  1000 , with an embodiment being shown in FIG.  1 ( c ). The absorber layer  1300  is opaque at the actinic wavelength with an optical density (OD) of about 2.5 to 4.0. The absorber layer  1300  has a thickness of about 60.0-180.0 nm. The absorber layer  1300  may be thinner if it is formed out of a material with a larger extinction coefficient (k). 
     The absorber layer  1300  may be formed from a metal, such as Chromium. The absorber layer  1300  may also be formed from a refractory metal, such as Molybdenum, Tungsten, or a related alloy or compound. Other materials, such as amorphous Carbon or amorphous Silicon, may also be used. 
     The absorber layer  1300  may include certain materials to provide desired properties. One example is the inclusion of certain materials in the bulk of the absorber layer  1300  to reduce stress, or prevent pinholes, or minimize edge roughness. 
     A graded or multilayer structure may be desirable for the absorber layer  1300 . An example is the inclusion of other materials at the upper surface of the absorber layer  1300  to decrease reflectivity below 10% at the actinic wavelength so as to minimize flare, or scattered light, when the mask  1020  is used in a wafer stepper. Still another example is the inclusion of some materials at the lower surface of the absorber layer  1300  to improve adhesion. The lower surface of the absorber layer  1300  is located over the balancing layer  1200 . In one embodiment, the absorber layer  1300  is Chromium Oxynitride at the upper surface, Chromium in the middle, and Chromium oxide at the lower surface. 
     A first-level photoresist  1400  is applied over the absorber layer  1300  on the mask blank  1000 , with an embodiment being shown in FIG.  1 ( d ). The first-level photoresist  1400  should have high resolution and good CD linearity as well as good dry etch resistance. The first-level photoresist  1400  may have a thickness of about 160-640 nm. A chemically amplified resist (CAR) may be used if high sensitivity to radiation is desired to increase throughput. The first level photoresist  1400  is usually an e-beam photoresist so an e-beam writer may be used to pattern images in the photoresist. The system configuration may be chosen to achieve finer resolution and higher pattern fidelity. For example, vector scan of an e-beam with an acceleration voltage of 40-50 keV may be preferable over raster scan of an e-beam with an acceleration voltage of 10-20 keV. 
     Reactive Ion etch (RIE) may be performed in a parallel plate reactor with high plasma density and low gas pressure. The dry etch chemistry may be based on Chlorine, such as Cl 2 /O 2  or BCl 3 . In some cases, it may be advantageous to add an assist gas, such as H 2  or HCl, since Hydrogen radicals can scavenge excess Chlorine radicals. If appropriate, in-situ descum, such as with Oxygen, may be done to remove residue in the images in the photoresist prior to RIE. 
     Loading effects in the RIE may affect etch rates, etch uniformity, and etch selectivity. It is desirable to increase etch selectivity so as to minimize erosion of the first-level photoresist  1400 . Sidewall passivation can help control the etch profile and the etch bias. Helium may be added to increase plasma density, reduce DC bias, and increase the selectivity over the first-level photoresist  1400 . 
     The absorber layer  1300  is usually etched without appreciably etching the underlying balancing layer  1200 . An embodiment is shown in FIG.  2 ( a ). The etch is predominantly anisotropic to produce mostly vertical profiles in the sidewall of the openings  1310  and  1320 . After removal of the first-level photoresist  1400 , the CD of the etched features  1310  and  1320  are measured with a Scanning Electron Microscope (SEM). 
     The etched features are inspected for defects. Defect inspection may be based on a comparison of two nominally identical patterns printed in different portions of the mask (die-to-die) or based on a comparison of a pattern printed on the mask and the layout data corresponding to the pattern (die-to-database). A focused ion beam (FIB) tool, such as a Micrion-8000EX from the FEI Company, may be used to repair an opaque defect using physical ion sputtering or gas-assisted etch (GAE). 
     Further processing is performed to introduce a phase shift of 180 degrees between light transmitted through adjacent openings. An additive process (not shown) may be used. In an additive process, a transparent layer, such as spin-on-glass (SOG), is deposited through openings in the absorber layer onto the substrate, such as fused silica or quartz, followed by removal of the transparent layer in alternate openings. 
     An additive process is susceptible to optical mismatch of materials in the light path and accompanying internal losses at the interfaces so, in many cases, a subtractive process is preferred. After the first-level photoresist  1400  is stripped, a second-level photoresist  1500  is applied. An embodiment is shown in FIG.  2 ( b ). The second-level photoresist  1500  may have a thickness of about 300-600 nm. 
     The second-level photoresist  1500  is usually an optical photoresist which is patterned with a laser writer that uses raster scan with multiple beams of deep ultraviolet (DUV) light. Laser written features are susceptible to corner rounding and CD nonlinearity so laser proximity correction is usually incorporated to improve pattern fidelity. Laser writing is typically performed with multiple passes to average out subsystem errors that may otherwise adversely affect stripe butting, placement linearity, edge roughness, and CD uniformity. 
     The opening  1310  that will be non-phase-shifted is covered by the second-level photoresist  1500 . The opening  1320  that will be phase-shifted is uncovered by exposing and developing a larger opening  1525  in the second-level photoresist  1500 . An embodiment is shown in FIG.  2 ( b ). 
     The larger opening  1525  in the second-level photoresist  1500  is aligned to the etched opening  1320  in the absorber layer  1300 . The larger opening  1525  is biased larger than the etched opening  1320  so as to accommodate registration errors, CD errors, and overlay tolerances in photolithography and etch. Thus, the placement of the larger opening  1525  in the second-level photoresist  1500  in relation to the etched opening  1320  is not very critical since the edge of the etched opening  1320  in the absorber layer  1300 , and not the edge of the larger opening  1525  in the second-level photoresist  1500 , will determine the width  1155  of the trench  1125 . The width  1155  may be measured top-down in a SEM. 
     After the second-level photoresist  1500  is exposed and developed, RIE is performed with chemistry based on Fluorine, such as CHF 3 , CF 4 /He/O 2 , or SF 6 /He. The etch may be done in a high-pressure Inductively Coupled Plasma (ICP) system or a Magnetically Enhanced Reactive Ion Etching (MERIE) system. The etch is anisotropic and creates a trench in the etched opening. The depth of the trench corresponds to a phase shift of 180 degrees in the light that will be transmitted through the etched opening  1125  relative to the light that will be transmitted through the unetched opening  1310 . 
     It may be desirable to use a voting technique to create a phase shift of 180 degrees between adjacent openings. Voting involves performing a process in several smaller steps instead of in one large step so that any spatial or temporal non-uniformity in the process may be smoothed out. In particular, voting significantly improves control of the depth of the trench, and thus the phase shift, by reducing the impact of any extraneous material that may block the etch. 
     In one embodiment, a triple-voting technique may be used. In other words, the etch of the quartz substrate  1100  through the opening  1320  is separated into three shorter etches. Each of the shorter etches in the voting technique produces a phase shift, or phase angle, of about 60 degrees. Each of the shorter etches is preceded by application, exposure, and developing of an opening  1525  in a second-level photoresist  1500 . The first of the shorter etches will include removal of the balancing layer  1200  in the opening  1320  before etching into the quartz substrate  1100 . The second and third of the shorter etches will only include etching deeper into the quartz substrate  1100 . 
     The photoresist is stripped after each of the shorter etches so the phase angle and the intensity may be measured. The etch times may be adjusted as needed based on feedback from the measurements. It is desirable for the phase angle to have a range of less than 2.0 degrees and the intensity to have a range of less than 0.2%. The phase angle may be measured on a tool, such as the MPM-248 or the MPM-193 from Lasertech Corp. The intensity may be measured on a tool, such as the MSM100 Aerial Image Measurement System (AIMS) or MSM193 AIMS from Carl Zeiss. The values selected for the numerical aperture (NA) and the partial coherence should be similar to the values on the wafer stepper in which the mask will subsequently be used. 
     After stripping the second-level photoresist  1500  for the third time (when using a triple-voting technique), a plasma process or a wet process may be used to clean up the substrate  1100  and remove any undesirable film or defect that may be present. A shorter or less aggressive version of a conventional isotropic wet etch may also be used to remove defects. 
     FIG.  2 ( c ) shows an embodiment of an alternating PSM  1020  having in situ balancing of intensity and phase. The phase-shifted opening  1125  has an etch depth of  1145  in the substrate  1100 . Underetching or overetching the substrate  1100  will introduce errors in phase angle. Errors in the phase shift, or phase angle, can accumulate and significantly reduce the depth of focus (DOF) of isolated features. However, phase errors usually have a relatively minor effect on the DOF of dense features. 
     The etch depth  1145  in the substrate  1100  and the sidewall profile of the etched opening  1125  may be determined by measuring the overall step height with an Atomic Force Microscope (AFM) and taking into account the thickness of the absorber layer  1300  and the thickness of the balancing layer  1200 . Deviation of the sidewall profile from the vertical may also reduce the DOF for isolated features. 
     The sidewall of the trench  1125  should be smooth, with a slope of 85-90 degrees. The bottom surface of the etched trench  1125  should be flat and smooth, but, in some cases, the bottom surface may be concave or convex and rough (not shown). The bottom comers of the trench  1125  may be rounded (not shown). Deviations and non-uniformities in the size and shape of the trench  1125  will introduce errors in intensity and phase of the transmitted light. 
     In most cases, the balancing layer  1200  is left intact in the non-phase-shifted opening  1310 . However, if necessary, such as when the etch depth  1145  is slightly too shallow or slightly too deep, it may be desirable to compensate by adjusting, such as by reducing, the thickness of the balancing layer  1200  in the non-phase-shifted opening  1310  so as to balance the intensity and phase of the light transmitted through the non-phase-shifted opening  1310  relative to the light transmitted through the phase-shifted opening  1125 . 
     Other embodiments include embedding an intermediate layer (not shown) to serve as an etch stop for the etch into the quartz substrate. However, any additional layer may add complexity and affect the aerial image unless it is completely removed prior to completion of fabrication. 
     Another embodiment of the present invention includes a phase-shifting mask (PSM) having in situ balancing of transmission and phase. Such a mask may be transmissive or reflective. The mask may be used with deep ultraviolet (DUV) light or with extreme ultraviolet (EUV) light. 
     One embodiment of the present invention is a transmissive, alternating phase-shifting mask, as shown in FIG.  3 . The AltPSM  2020  has a substrate  2100  that is transparent to light at the actinic wavelength. The actinic wavelength is the wavelength of light used in a wafer stepper to selectively expose photoresist on a wafer. 
     Fused silica, or quartz, is commonly chosen for the substrate  2100  at an actinic wavelength in the range of about 193-436 nanometers (nm). Modified fused silica, such as low-hydroxyl-content fused silica or fluorine-doped silica, may be used at short actinic wavelengths, such as about 157 nm. 
     Alternatively, a crystalline fluoride, such as Calcium Fluoride, may be used at short actinic wavelengths, such as about 126 nm. However, a crystalline fluoride often possesses characteristic properties, such as birefringence, large coefficients of thermal expansion, and low resistance to chemical etches. Compensation for any undesirable property may be accomplished with appropriate design of the optics subsystem and the wafer stepper system. 
     A balancing layer  2200  is disposed over the substrate  2100 . The balancing layer  2200  may have a single-layer structure or a multilayer structure. The balancing layer  2200  should be thin, such as about 3.0-70.0 nm, to reduce absorption. The balancing layer  2200  is partially transmissive and may be capable of phase-shifting. Transmission is usually a function of wavelength and may be measured in a tool, such as the n&amp;k Analyzer 1280 from n&amp;k Technology, Inc. 
     In addition to having suitable optical properties, the balancing layer  2200  must possess sufficient chemical resistance, radiation durability, and compatibility with the underlying substrate  2100  and the overlying absorber layer  2300 . Phase and transmission should not be appreciably affected by the mask fabrication process, the mask inspection or repair process, or the mask cleaning process. The balancing layer  2200  may include materials, such as Chromium Fluoride (CrO x F y ), Zirconium Silicon Oxide (ZrSi x O y ), or Molybdenum Silicide (MoSi). Other materials include Fluorides, Silicides, and Nitrides of metals, such as Aluminum, Chromium, Hafnium, Molybdenum, Nickel, Niobium, Tantalum, Titanium, Tungsten, or Zirconium. 
     An absorber layer  2300  is disposed over the balancing layer  2200 . The absorber layer  2300  is opaque at the actinic wavelength with an optical density (OD) of about 2.5-4.0. The absorber layer  2300  has a thickness of about 60.0-180.0 nm. The absorber layer  2300  may be thinner if it has a larger extinction coefficient (k). 
     The absorber layer  2300  may be a metal, such as Chromium. The absorber layer  2300  may also be a refractory metal, such as Molybdenum or Tungsten, or a related alloy or compound. The absorber layer  2300  may also be a material such as amorphous Carbon or amorphous Silicon. 
     In one embodiment, the absorber layer  2300  is Chromium. A graded structure or a multilayer structure may be desirable. For example, inclusion of Oxygen towards the lower surface of the absorber layer  2300  will improve adhesion to the balancing layer  2200  below. Inclusion of Oxygen and Nitrogen towards the upper surface of the balancing layer  2200  can reduce reflectivity below 10% at the actinic wavelength, thus minimizing flare, or scattered light, when the mask  2020  is used in a wafer stepper. 
     The phase-shifted opening  2125  includes a trench in a first portion of the substrate. The trench in the first portion of the substrate has a width  2155  and a depth  2145 . The adjacent non-phase-shifted opening  2310  includes the balancing layer  2200  over a second portion of the substrate with a width  2350 . There is no trench in the second portion of the substrate in one embodiment. In another embodiment, there is a shallow trench (not shown) in the second portion of the substrate. 
     The phase-shifted opening  2125  and the non-phase-shifted opening  2310  are separated by a narrow strip  2355  of the absorber layer  2300 . In some cases, the narrow strip  2355  may be absent (not shown) between the two openings. Phase edge masks may be used, especially when the openings are very small. 
     The depth  2145  corresponds to a difference in the optical path length of light transmitted through the transparent substrate  2100  between the phase-shifted opening  2125  and the non-phase-shifted opening  2310 . A phase shift of 180 degrees is desired with a range of less than 2.0 degrees. The depth  2145  has about the same magnitude as the illumination wavelength when the transparent substrate  2100  is quartz and the ambient is air. 
     In a wafer stepper, Numerical Aperture (NA) of the projection lens and partial coherence of the exposure light will determine the diffraction around the absorber layer  2300  at the edges of the trench  2125 . A wafer stepper typically has an NA of 0.45 to 0.80 and a partial coherence of 0.30 to 0.90. Consequently, the optimal difference in the optical path length through the transparent substrate  2100  may vary, depending on the trench depth  2145  and the trench width  2155 . 
     The sidewall of the trench  2125  should be smooth, with a slope of 85-90 degrees. The bottom surface of the etched trench  2125  should be flat and smooth, but, in some cases, the bottom surface may be concave or convex and rough (not shown). The bottom corners of the trench  2125  may be rounded (not shown). Deviations and non-uniformities in the size and shape of the trench  2125  will introduce errors in intensity and phase of the transmitted light. 
     In still other embodiments, certain intermediate or embedded layers (not shown) may be disposed between the absorber layer  2300  and the balancing layer  2200 . Intermediate or embedded layers (not shown) may also be disposed between the balancing layer  2200  and the substrate  2100 . 
     Optical Proximity Correction (OPC) may be included in the mask  2020  claimed in the present invention. OPC involves the biasing of CDs to compensate for print bias and etch bias. It is often helpful to iteratively simulate the aerial image resulting from using PSM with OPC. It can also be helpful to combine the mask  2020  claimed in the present invention with Off-Axis Illumination (OAI) to enhance certain types of features and layouts. 
     Many embodiments and numerous details have been set forth above in order to provide a thorough understanding of the present invention. One skilled in the art will appreciate that many of the features in one embodiment are equally applicable to other embodiments. One skilled in the art will also appreciate the ability to make various equivalent substitutions for those specific materials, processes, dimensions, concentrations, etc. described herein. It is to be understood that the detailed description of the present invention should be taken as illustrative and not limiting, wherein the scope of the present invention should be determined by the claims that follow. 
     Thus, we have described a phase-shifting mask (PSM) having in situ balancing of intensity and phase and a method of forming such a PSM.