Mark Lawliss

Learning from native defects on EUV mask blanks

Defects in the EUV mask blank are one of the largest hurdles to achieving manufacturing readiness of EUV masks. For defect-free masks, the obvious approach is to order blanks that do not have defects or to shift the pattern so that remaining defects do not create a printed defect on wafer. The approach during development should be different. At this learning phase, it is wise to study the defects as they occur naturally on the EUV mask blank. This paper outlines a comprehensive approach to building a mask specifically to showcase the native defects so that they can be studied and repairs can be attempted. The method applied to mask build, defect inspection and characterization will be reviewed in detail. Printability of the mask defects of interest are characterized using both wafer printing and EUV microscope data. Repairs are attempted and characterized. In the end, the impact of native defects is discussed along with the viability of various repair methods.

Repairing native defects on EUV mask blanks

Mask defectivity is a serious problem for all lithographic masks, but especially for EUV masks. Defects in the EUV blank are particularly challenging because their elimination is beyond control of the mask fab. If defects have been identified on a mask blank, patterns can be shifted to place as many blank defects as possible in regions where printing impact will be eliminated or become unimportant. For those defects that cannot be mitigated through pattern shift, repair strategies must be developed. Repairing defects that occur naturally in the EUV blank is challenging because the printability of these defects varies widely. This paper describes some types of native defects commonly found and begins to outline a triage strategy for defects that are identified on the blank. Sample defects best suited to nanomachining repair are treated in detail: repairs are attempted, characterized using mask metrology and then tested for printability. Based on the initial results, the viability of repairing EUV blank native defects is discussed.

Through-focus EUV multilayer defect repair with nanomachining

Defects within the multilayer mirrors of EUV photomasks have been a leading challenge for EUV lithography for quite some time. By creating non-planar surfaces, they distort both the amplitude and phase of reflected light. Amplitude errors generally create a CD error on wafer, whereas phase errors tend to cause asymmetric printing through focus. Since defect-free mask blanks are not expected to be available for initial high volume EUV manufacturing, defect mitigation, compensation, and repair strategies are essential. This paper describes a technique to repair both the amplitude and phase effects of multilayer defects. For a bump defect, the phase effect (i.e. tilted Bossung curve behavior) is corrected by removing multilayer material in the vicinity of the defect. This creates a phase effect opposite to that of the defect and the two effects cancel. The amplitude error (i.e. CD error) caused by both the defect and by the phase repair is then corrected by modifying the surrounding absorber pattern. The repairs in this paper are performed by nanomachining with an AFM repair tool. The concept is validated by a combination of simulation and experimental studies with data from the Actinic Inspection Tool (AIT) at the Lawrence Berkeley National Laboratories, the EUV Alpha Demo Tool (ADT) in Albany, New York, and an AFM repair tool. The process for a complete multilayer repair is described using an example native defect repair. Encouraging results indicate that nanomachining is capable of creating the complex nano-scale three dimensional topographies required for the repair. Repair strategies for both bump and pit defects are addressed. Multiple simulation studies are used to understand the requirements for such a repair and what type of repairs may be possible.

Impact of the OMOG Substrate on 32 nm Mask OPC Inspectability, Defect Sensitivity and Mask Design Rule Restrictions

Aggressive optical proximity correction (OPC) has enabled the extension of advanced lithographic technologies to the 32nm node. The associated sub-resolution features, feature-feature spacings, and fragmented edges in the design data are difficult to reproduce on masks and even more difficult to inspect. The patterns themselves must be differentiated from defects for inspectability, while the ability to recognize small deviations must be maintained for sensitivity. This must be done without restricting necessary OPC design features. The semi-transparent nature of industry-standard 6% attenuated phase shift substrates introduces a host of problems relative to inspectable dimensions and subsequent defect sensitivities. The result is a reduction in inspectability, defect sensitivity and the inability to inspect smaller critical dimensions and OPCed features. The introduction of a binary-type attenuated phase shift film improves the ability to inspect smaller critical dimensions and smaller OPC features without loss of inspectability and sensitivity extending the capability of existing inspection hardware for 32nm ground rule masks. This paper introduces inspection characterization results for this new film, opaque MoSi on glass (referred to as OMOG in this paper) and draws a correlation between the films transmission qualities and inspectability of 32nm OPC features. The paper will further show a correlation between OPC feature size and defect sensitivity for 32nm ground rule designs. Aerial Image (AIMS) analysis will be used to identify areas where the enhanced inspection capability can be leveraged to avoid unnecessary restrictions on OPC.

Patterning-induced image placement distortions on electron beam projection lithography membrane masks

Membrane masks are needed for charged particle lithography and can include both stencil masks and masks with thin continuous membranes. Producing accurate image placement on membrane masks requires careful control of mask shape, pattern writing, and stress control of the mask materials. Pattern density and pattern density gradients also affect image placement (IP) control. This article discusses IP distortions on electron projection lithography masks caused by patterning the imaging layers with low and high density patterns and patterns with large gradients in the density. The process-induced distortion has been found to be largest with the largest vector distortion at the boundary when high pattern density gradients are present. The anisotropic stiffness of the unit cell also affects the process-induced distortion. Qualitatively, the results between continuous membrane and stencil masks show similar characters. The results provide distortion information that could be used to determine the maximum allowab...

Overlay enhancement with product‐specific emulation in electron‐beam lithography tools

Electron‐beam image–placement errors, commonly expressed as registration and overlay errors, are becoming increasingly critical as the device dimensions shrink into the subhalf‐micrometer range. Contributors to image‐placement errors include: (1) system limitations, i.e., noise and minimum exposure increment or least‐significant bit (LSB); (2) column and resist charging; (3) substrate and carrier clamping; (4) resist stress; (5) thermal effects; and (6) mask‐processing effects. This article attempts to quantify the magnitude of positional errors attributable to these effects using experimental data from x‐ray membrane exposures under various conditions; when possible, a comparison to theoretical data is made. Although the image–placement errors from many of the previously mentioned contributors are typically very repeatable, they are difficult to eliminate. Methods such as discharge layers, decreased exposure time, and alternate resist systems can minimize the effects but may cause other problems such as ...

Wafer Plane Inspection Evaluated for Photomask Production

Wafer Plane Inspection (WPI) is a novel approach to inspection, developed to enable high inspectability on fragmented mask features at the optimal defect sensitivity. It builds on well-established high resolution inspection capabilities to complement existing manufacturing methods. The production of defect-free photomasks is practical today only because of informed decisions on the impact of defects identified. The defect size, location and its measured printing impact can dictate that a mask is perfectly good for lithographic purposes. This inspection - verification - repair loop is timeconsuming and is predicated on the fact that detectable photomask defects do not always resolve or matter on wafer. This paper will introduce and evaluate an alternative approach that moves the mask inspection to the wafer plane. WPI uses a high NA inspection of the mask to construct a physical mask model. This mask model is used to create the mask image in the wafer plane. Finally, a threshold model is applied to enhance sensitivity to printing defects. WPI essentially eliminates the non-printing inspection stops and relaxes some of the pattern restrictions currently placed on incoming photomask designs. This paper outlines the WPI technology and explores its application to patterns and substrates representative of 32nm designs. The implications of deploying Wafer Plane Inspection will be discussed.

Next-generation lithography mask development at the NGL Mask Center of Competency

Mask fabrication is one of the difficult challenges with all Next Generation Lithography (NGL) technologies. X-ray, e-beam projection, and ion-beam projection lithography all use some form of membrane mask, and extreme ultraviolet (EUV) lithography uses a reflective mask. Despite some differences, the various mask technologies share some common features and present similar fabrication difficulties. Over the past several years, the IBM Advanced Mask Facility (AMF) has focused on the fabrication of x-ray masks. Several key accomplishments have been demonstrated including fabricating masks with critical dimensions (CD) as small as 75 nm, producing line monitor masks in a pilot line mode to evaluate mask yields, and fabricating masks to make working microprocessors with the gate level defined by x-ray lithography. The experience on fabricating 1X x-ray masks is now being applied to the other NGL mask technologies. Progress on membrane and absorber materials can be applied to all the technologies, and patterning with advanced e-beam writing with chemically amplified resists utilizes learning from writing and baking on x-ray membrane masks.

EUVL mask repair: expanding options with nanomachining

Mask defectivity is often cited as a barrier to EUVL manufacturing, falling just behind low source power. Mask defectivity is a combination of intrinsic blank defects, defects introduced during the mask fabrication and defects introduced during the use of the mask in the EUV exposure tool. This paper works towards minimizing the printing impact of blank defects so that the final EUVL mask can achieve a lower defectivity. Multilayer defects can be created by a step or scratch as shallow as 1nm in the substrate. These small defects create coherent disruptions in the multilayer that can generate significant variations in mask reflectivity and induce clearly-defined, printable defects. If the optical properties of the defect can be well understood, nanomachining repair processes can be deployed to fix these defects. The purpose of this work is to develop new nanomachining repair processes and approaches that can repair complex EUVL mask defects by targeted removal of the EUVL mask materials. The first phase of this work uses nanomachining to create artificial phase defects of different types and sizes for both printability evaluation and benchmarking with simulation. Experimental results validate the concept, showing a reasonable match between imaging with the LBNL Actinic Inspection Tool (AIT) and simulation of the mask topography measured by AFM. Once the printability of various nanomachined structures is understood, the second phase of the work aims to optimize the process to repair real EUVL mask defects with surrounding absorber patterns.

Impact of transmitted and reflected light inspection on mask inspectability, defect sensitivity, and mask design rule restrictions

The application of aggressive optical proximity correction (OPC) has permitted the extension of advanced lithographic technologies. OPC is also the source of challenges for the mask-maker. Sub-resolution features, small shapes between features and highly-fragmented edges in the design data are difficult to reproduce on masks and even more difficult to inspect. Since the inspection step examines every image on the mask, it is required to guarantee the total plate quality. The patterns themselves must be differentiated from defects, and the ability to recognize small deviations must be maintained. In other words, high inspectability at high defect sensitivities must be achieved simultaneously. This must be done without restricting necessary OPC designs features. Historically, transmitted light has been deployed for mask pattern inspection. Recently, the inspection challenge has been both enhanced and complicated by the introduction of reflected light pattern inspection. Reflected light reverses the image contrast of features, creating a new set of design limits. This paper introduces these new reflected inspection limits. Multiple platform capabilities will be incorporated into the study of reflected and transmitted inspection capability. The benefits and challenges of integrating a combination of transmitted and reflected light pattern inspection into manufacturing will be explored. Aerial Image Measurement System (AIMS) analysis will be used to help understand how to leverage the enhanced inspection capability while avoiding unnecessary restrictions on OPC.