Method and system for optimizing intra-field critical dimension uniformity using a sacrificial twin mask

Disclosed is a method and a system for optimizing intra-field critical dimension of an integrated circuit. A first mask for an integrated circuit is provided comprising at least one device region. A second mask is provided by copying the first mask and a lithography operation is provided to the integrated circuit using the first and second masks, wherein the critical dimension of the integrated circuit is optimized using the second mask. The second mask comprises a plurality of sacrificial patterns, which may be a plurality of flat patterns or a plurality of grating patterns.

BACKGROUND

The present disclosure relates in general to integrated circuit manufacturing, and more particularly to a system and method to optimize critical dimension uniformity in manufacturing of integrated circuits by using a sacrificial twin mask.

In integrated circuit manufacturing technology, a resist layer is typically applied to a semiconductor wafer surface, followed by an exposure of the resist through a mask (e.g., a reticle or photomask). A post-exposure baking process is then performed to alter physical properties of the resist for subsequent processing. An after-development inspection (ADI) is then performed to inspect the critical dimension (CD) and profile of the exposed resist using a scanning electron microscope (SEM) to determine whether it conforms to a specification. If the resist is within specification, a pattern is etched or transferred and the resist is stripped. An after-etching inspection (AEI) is then performed on the wafer.

Traditional SEM inspection, however, becomes a bottleneck for providing accurate and repeatable CD and profile analysis due to electron charging effects that not only limit the accuracy and repeatability of CD metrology, but also cause damage at the measurement area. In response, an optical critical dimension (OCD) method is often used instead of SEM inspection. OCD can detect CD information including CD profile and wafer film thickness. OCD also has much less noise than SEM and the sampling ratio of OCD is more accurate than the sampling ratio of SEM. Thus, OCD provides more consistent and comprehensive CD information than SEM.

Both SEM and OCD may be used in after-development inspection and after-etching inspection to optimize CD uniformity. With existing SEM and OCD tools, inter-field critical dimension uniformity may be optimized. Inter-field CD uniformity optimization may be obtained by examining the die-to-die CD difference between a plurality of dies on a wafer. For example, inter-field CD uniformity optimization may be performed over 80 die to improve the quality of selected measurement points of a wafer surface area.

In addition to inter-field optimization, intra-field CD uniformity optimization may be performed with existing SEM and OCD tools. Intra-field CD uniformity optimization may be performed by examining CD differences within a die or field of the wafer. However, due to the large grating size of an OCD pattern, such as 60×60 um, the OCD pattern may not distribute uniformly in the chip and the sampling size is limited. In addition, the OCD pattern may not be used on some devices, such as a static random access memory (SRAM) cell. Thus, intra-field CD uniformity optimization is limited by the location of the OCD pattern and the number of OCD samplings that can be performed by a scanner.

Therefore, a need exists for a method and system for optimizing intra-field CD uniformity, such that intra-field CD uniformity optimization is not limited by the grating size or the location of device regions.

DETAILED DESCRIPTION

For the purposes of promoting an understanding of the principles of the invention, reference will now be made to the embodiments, or examples, illustrated in the drawings and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended. Any alterations and further modifications in the described embodiments, and any further applications of the principles of the invention as described herein are contemplated as would normally occur to one skilled in the art to which the invention relates. Furthermore, the depiction of one or more elements in close proximity to each other does not otherwise preclude the existence of intervening elements. Also, reference numbers may be repeated throughout the embodiments, and this does not by itself indicate a requirement that features of one embodiment apply to another embodiment, even if they share the same reference number.

Aspects of the present disclosure provide a method and system for optimizing intra-field CD uniformity by using a sacrificial twin mask. In an illustrative embodiment, a sacrificial twin mask is provided by making a copy of the original mask provided by the customer. The sacrificial twin mask comprises a mask that includes a plurality of sacrificial patterns. In one embodiment, the plurality of sacrificial patterns is a plurality of regular or repeating grating-like patterns, such as OCD grating patterns. In an alternative embodiment, the plurality of sacrificial patterns is a plurality of flat dummy patterns. The plurality of flat sacrificial patterns may be positioned over device regions of an integrated circuit. By providing a sacrificial twin mask having a plurality of sacrificial patterns, intra-field CD uniformity may be optimized without the limitations that normally result from OCD grating size and sampling size.

Referring toFIG. 1, an exemplary lithography process track1includes wafer supply racks2, a resist spin-on station3, a soft bake station4, an exposure station5, a post exposure bake station6, a development station7and a rinse/dry station8. A controller9automates the lithography process track1by communication with wafer supply racks2, soft bake station4, post exposure bake station5and optical metrology station10.

Process wafers are first supplied by wafer supply racks2to the resist spin-on station3to coat the resist on a wafer surface. The wafer is then soft-baked at the soft-bake station4and transferred to the exposure station5to expose the wafer. Afterwards, a post-exposure bake is performed on the wafer at the post-exposure baking station6and the wafer is transferred to the development station7. After development, the wafer may either be immediately transferred to an optical metrology station10or subjected to a rinse/dry at station8prior to being transferred to the optical metrology station10. The optical metrology station10includes a spectrometer for collecting spectra of scattered light from the resist in a digital format. The controller9processes the collected spectra of scattered light and performs a diffraction analysis. Aspects of the present disclosure may be implemented within the controller9or optical metrology station10or other parts of the lithography process track1without departing the spirit and scope of the present disclosure.

Referring toFIG. 2, a resist grating11is formed when a test process wafer is passed through the lithography track process1to form a resist pattern. For example, resist lines12and14are formed having a predetermined line width and pitch. Area16refers to an exemplary probe spot size for incident light from which scattered light spectra is collected using an optical critical dimension (OCD) based scatterometry. OCD based scatterometry collects one or more scattered spectra from the resist grating11and performs diffraction analysis to provide uniformity measurements and additional information. The additional information includes sidewall angle, resist thickness, ARC layer thickness, and under-layer film thickness. In this example, area16is about 50 um×50 um. Area18refers to an exemplary probe spot size by SEM for determining CD variation. In this example, area18has a field of view (FOV) of about 150K magnification and is about 1 um×1 um. Thus, the measurement box size of OCD using OCD scatterometry is about 50 um×50 um while the measurement box size of the SEM is about 1 um×1 um. The sampling ratio of OCD to SEM is greater than 2500 times.

Referring toFIG. 3A, a standard scan single line measurement of resist grating11with 150K magnification is shown. In this example, the sampling size is one line. With such a small sampling size, CD uniformity may not be fully represented. InFIG. 3B, an average line width (ALW) measurement of resist grating11with 35K magnification is shown. ALW measurement enlarges the field of view (FOV) and allows for multiple line measurements. Multiple line measurements increase sampling number and reduce CD error without sacrificing throughput. In this example, the sampling size is seven lines. Greater sampling size means more sampling points. With more sampling points, CD or CDU values would converge to a statistical true value. In this example, a six sampling point CD average is used as the SEM CD standard, because more than 30 sampling points as required by the standard scan for each measurement location is not practical.

Referring toFIG. 4, a diagram illustrating a graph of sampling effect on CD correlations between ALW measurement with 80K×35K x-y magnification and OCD is depicted. CD correlation is a function of sampling size and sampling methodology. The Y-axis of graph22represents CD measurements of ALW measurement with 80K×35K x-y magnification. The X-axis of graph22represents CD measurements of OCD. In this example, the CD correction (R2) is about 0.9496. Thus, CD measurements of ALW measurement with 80K×35K x-y magnification are very close to OCD measurements, which means that the six sampling point average SEM CD standard is almost as CD sensitive as OCD. In comparison to standard CD SEM measurement 150K×150K x-y magnification, low magnification, such as 80K×35K x-y magnification, CD SEM measurement may reduce the line edge roughness effect on the CD mean.

As discussed above, good CD measurement data or metrology and large sampling size provide better CD uniformity. However, the large grating size of OCD pattern becomes an obstacle both in terms of sampling limit and sampling location.

Referring toFIG. 5, a diagram illustrating an exemplary OCD grating is depicted. In this example, a mask24is provided with OCD grating pattern26. Due to the large grating pattern size of 60 um×60 um, the OCD grating pattern26does not distribute uniformly in the mask24and the sampling size is also limited. For example, only ten OCD samplings may be performed. In addition, the OCD grating pattern26may not be inserted onto devices such as SRAM cell28.

Since an ALW measurement with 80K×35K x-y -magnification provides CD uniformity that is close to OCD, a dummy grating pattern for an ALW measurement by SEM may also be used to optimize CD uniformity.

Referring toFIG. 6, a diagram illustrating an exemplary dummy grating pattern is depicted. In this example, mask24is provided with a dummy grating pattern32for ALW measurement by SEM. The size of the dummy grating pattern32is about 5 um×5 um. Therefore, more sampling may be performed. For example, 80-100 samplings may be performed. However, similar to OCD grating pattern26, dummy grating pattern32also may not be inserted onto devices such as SRAM cell34.

For any shortcomings of the above patterns, a sacrificial twin mask provides both larger sampling size and advantages of OCD grating or ALW dummy patterns. Referring toFIG. 7, a diagram illustrating an exemplary sacrificial twin mask is depicted. Customer mask38comprises a layout of a plurality of device regions. Sacrificial twin mask40is a copy of the customer mask38. The quality of sacrificial twin mask40is substantially the same as the customer mask38. In this example, customer mask38comprises a layout of device regions42and44. Device regions42and44may be regions for devices to be patterned on the substrate, for example, a SRAM cell.

Sacrificial twin mask40comprises a plurality of sacrificial patterns46. In an illustrative embodiment, the plurality of sacrificial patterns46may be implemented as a plurality of grating like or repeating pattern, such as OCD grating pattern26. Alternatively, the plurality of sacrificial patterns46may be implemented as a plurality of dummy grating patterns, such as dummy grating pattern32. As shown inFIG. 7, the plurality of sacrificial patterns46are substantially symmetrical to one another and are uniformly distributed over the sacrificial twin mask40. In one example, each of the plurality of sacrificial patterns46has a dimension of 3 um ×3 um with an area of about 9 um2. In another example, each of the plurality of sacrificial patterns46has a dimension of 70 um×70 um with an area of about 4900 um2.

In addition, a spacing47is present between the plurality of sacrificial patterns46. Spacing47is independent of OCD grating size. In this example, the spacing47in the sacrificial mask40is 1000 um. In order to reduce the interference between the plurality of sacrificial patterns46due to optical or etch loading effect, the dimension of spacing47is large in comparison to the dimension of the plurality of sacrificial pattern46. Since the plurality of sacrificial patterns46are OCD or dummy grating like, the plurality of sacrificial patterns46have some advantages of the OCD or dummy patterns. For example, global CD uniformity may be optimized using the plurality of sacrificial patterns46that are dummy like, which allows for larger sampling size due to the small grating size of dummy patterns. In addition, more consistent and better CD measurement data may be obtained from the plurality of sacrificial patterns46that are like OCD gratings. Furthermore, the limitation of OCD pattern location may be eliminated, because the plurality of sacrificial patterns46in the sacrificial twin mask40may now overlap device regions42and44.

In addition to global optimization of CD uniformity, local selective CD uniformity optimization may be performed with the plurality of sacrificial patterns46. Referring toFIGS. 8A-8C, diagrams illustrating exemplary local selective CD uniformity optimizations are depicted. InFIG. 8A, a plurality of sacrificial patterns are selected from sacrificial twin mask40for sampling. In this example, the plurality of selected sacrificial patterns48are located at four corners of the sacrificial twin mask40, the center of the sacrificial twin mask40, a plurality of selected sacrificial patterns48overlapping device region42and a plurality of selected sacrificial patterns48overlapping device region44.

InFIG. 8B, a plurality of sacrificial patterns50are selected from sacrificial twin mask40for sampling. In this example, the plurality of selected sacrificial patterns50are located at four corners of the sacrificial twin mask40, the center of the sacrificial twin mask40, and a plurality of sacrificial patterns50overlapping device region44. InFIG. 8C, a plurality of sacrificial patterns52are selected from sacrificial twin mask40for sampling. In this example, the plurality of selected sacrificial patterns52are located at four corners of the sacrificial twin mask40, a plurality of sacrificial patterns outside of device regions42and44. By allowing the selection of local sacrificial patterns48,50, and52, CD measurements may be obtained for the local selective patterns and CD uniformity optimization may be performed on only those local selective patterns. In this way, the performance of intra-field CD uniformity optimization may be improved.

As discussed above, OCD based scatterometry is used to collect one or more scattered spectra from the resist grating11and perform diffraction analysis such that uniformity measurements and additional information may be gathered. Referring toFIG. 9, a diagram illustrating an exemplary process wafer subjected to optical critical dimension (OCD) based scatterometry is depicted. Wafer60comprises a first layer62and a second layer64. The first layer62may comprise a substrate made of silicon or polysilicon. The first layer is also referred to as an OD layer. The second layer64may comprise a poly layer65, an antireflective layer67, and a patterned resist layer66. The poly layer65may include silicon dioxide. The patterned resist layer66may include material such as Si3N4.

Incident light68from a probing light source of a spectrometer may be directed to a probe area of the resist layer66forming an incident angle Θ of between 30 to 90 degrees with respect to the resist surface. A portion70of the incident light68is scattered from the surface of resist layer66after passing through resist portion72to produce detectable scattered light74, Scattered light74is collected by a conventional detector, such as a diode array detector, at different wavelengths, A diffraction analysis is then performed on scattered light74to obtain three dimensional information and other additional information of the resist layer66.

In order for OCD scatterometry to collect scattered light from resist layer66, the OCD scatterometry uses regular or repeating patterns on the first layer62. Any irregular pattern on the first layer62will disturb the OCD reflecting signal and OCD modeling. Thus, a second layer repeating-pattern grating overlying any irregular patterns of the first layer62may not be measured by the detector. In some customer-provided masks, however, the patterns of the first layer62are irregular.

Referring toFIG. 10, a diagram illustrating a customer provided mask comprising irregular or non-repeating patterns is depicted. Customer provided mask80comprises resist grating11in the second layer64and a plurality of irregular patterns82in first layer62. The plurality of irregular patterns82in the first layer62prevent the second layer repeating-pattern grating from being measured, which leads to the failure of the second layer intra-field CD uniformity metrology or data requirement.

The sacrificial twin mask of the present disclosure may be used to correct the problem of irregular patterns in the material layer. Referring toFIG. 11, a diagram illustrating one embodiment of the sacrificial twin mask for CDU optimization in the first layer is depicted. In the sacrificial twin mask40, a first plurality of sacrificial patterns and a second plurality of sacrificial patterns are present in the first layer62. The first plurality of sacrificial patterns are dummy flat patterns84. The second plurality of sacrificial patterns are dummy grating patterns86.

The dummy flat patterns84provide flat areas to clear out the irregular patterns82in the first layer62exactly below the second layer repeating-pattern grating, such that the second layer repeating-pattern grating may be measured and the second layer intra-field CD uniformity metrology requirement may be satisfied. While the dummy flat patterns84are preferably implemented in the first layer62, the dummy flat patterns84may be implemented at any other layer, including the second layer, or other layers, to provide flat areas for irregular patterns, without departing the spirit and scope of the present disclosure. Dummy grating patterns86, on the other hand, are regular or repeating patterns, which enable the first layer resist grating CD data to be measured and the first layer intra-field CDU to be optimized.

Referring toFIG. 12, a diagram illustrating another embodiment of the sacrificial twin mask for CDU optimization in the second layer64is depicted. In the sacrificial twin mask40, a plurality of sacrificial patterns are present in the second layer64. The plurality of sacrificial patterns are referred to as a dummy grating88. The dummy grating88provides grating-like or repeating patterns. The dummy grating88may be used to measure second layer resist grating and optimize second layer intra-field CDU.

Referring toFIG. 13, a diagram illustrating a combination of the sacrificial twin masks in the first layer and the second layer is depicted. In the first layer62, the dummy flat pattern84provides flat areas to clear out the irregular patterns82in the first layer62exactly below the dummy grating88of the second layer64. In this example, the dummy flat pattern81in the first layer62provides a flat area to clear out the irregular pattern82in the first layer62exactly below the dummy grating83in the second layer64. Similarly, dummy flat pattern85in the first layer62provides a flat area to clear out the irregular pattern82in the first layer62exactly below the dummy grating87in the second layer64.

Referring toFIG. 14, a graph90of inter and intra CD uniformity optimization for the second layer using sacrificial twin mask is depicted. The Y-axis of graph90for the bars indicates CD uniformity (3× sigma) of the second layer in nm. The X-axis of graph90indicates the slot number of the wafers. In this example, prior to CDU optimization using the sacrificial twin mask, the CDU is 4.22 nm, as evidenced in the CDU of slot16. With the advantages of sacrificial twin mask of the present disclosure, CDU is optimized to 1.94 nm as evidenced by the CDU of slot24. Thus, both the inter and intra-field CDU is optimized for the second layer with the use of sacrificial twin mask.

Referring toFIG. 15, a graph92of intra CD uniformity optimization for the second layer using sacrificial twin mask is depicted. The Y-axis of graph92indicates the intra-field CDU of the second layer in nm. The X-axis indicates the die numbers within a wafer. In this example, prior to CDU optimization using sacrificial twin mask, the intra-field CDU of die38is 1.84 nm, as evidenced by the CDU of slot16. With the advantages of sacrificial twin mask of the present disclosure, the CDU of die38is optimized to 0.79 nm as evidenced by the CDU of slot24. Thus, the intra-field CDU is optimized for the second layer with the use of sacrificial twin mask.

In summary, aspects of the present disclosure provide a sacrificial twin mask for optimizing intra-field CD uniformity. The sacrificial twin mask comprises a plurality of sacrificial patterns. The plurality of sacrificial patterns may include grating like patterns or dummy patterns. Aspects of the present disclosure also allow selection of local patterns for focused CDU optimization. In addition, the plurality of sacrificial patterns provides systematic CD measurements of OCD and average line width measurements (ALW) by CD SEM. Furthermore, the CDU of the second layer, may be optimized with dummy patterns that provide flat areas for irregular patterns in the first layer and the CDU of the dummy grating patterns in the second layer may be optimized with the dummy grating patterns. The CDU of a specific layer, such as the second layer, may also be optimized with dummy grating patterns. For average CD measurement by CD SEM, the plurality of sacrificial patterns enable small dummy gratings to be used. With the sacrificial twin mask, intra-field CD uniformity may be optimized without the limitation of sampling size and the location of OCD patterns.