The optical performance of Markle-Dyson projection optics is now well established. Here we describe options for 1X reflective optical masks that might achieve the desired linewidth control. One option is the use of aluminum as the reflecting material. A film less than 50 nm thick has nearly twice the reflectivity of the silicon used until now, and so it should be possible to develop an etching process (for such a thin film) that is adequately precise. Moreover options exist for repairing both opaque and clear defects. An interesting alternative configuration, that eliminates the need to etch the aluminum, is to use a patterned absorber on the substrate and to deposit the aluminum over the patterned absorber.
The concept of Cpk has been a long accepted method for indicating manufacturing process capability and making critical judgments about expected performance. Within this paper Cpk's are calculated for a set of test masks generated by divergent photomask lithography tools. The MSTM (Manufacturing Simulation Test Mask) uses different address sizes (1.0, 0.5, 0.25, and 0.1 micrometers ) allowing all address structures to be tested on the same mask. A Leica LMS- 2000 was used for the registration measurements. Through critical observation of the data sets' mean and range variance, an insight about contributing error sources is gained and a common ground for unbiased comparison is found.
The actual photomask surface formation during all relevant stages of manufacturing and application generally is determined by substrate flatness, stress introduced by the coating layers and, in particular, by gravity bending determined by substrate dimensions and actual clamping configuration. Dependant on the source of surface deformation, the actual pattern positions may vary in different ways compared to the design and lead to a registration error of up to 0.1 micrometers . Approaching these problems from the mask metrology point of view this paper presents a study based on modelling and experimental verification utilizing the Z- mapping feature of Nikon XY 3I mask metrology tool. The two error types, cosine error and pattern shift by lateral surface compression or stretching, will be calculated for two basic clamping configurations. Derived from the results and under consideration of current flexure compensation schemes the impact on the printed wafer image will be estimated leading to a discussion of possible correction schemes.
This paper describes how the new Hoya Micro Mask APTCON 4045 optical mask processor was used in conjunction with the ETEC CORE 2564 reticle writer in the optimization of a 64 Mb DRAM process. Exposure conditions, develop/etch parameters, and processor variables were individually optimized; results from the ensuing process are analyzed. The major components of the Convac-APT APTCON 4045 processor are described. The system description, physical layout, automation, chemistry handling, and adjustment versatility are all covered. Cassette-to-cassette full automation is used, in support of an extremely tight control over process variability. Fully automatic chemistry handling is implemented, with supply auto- switch, in an isolated safety enclosure. The on-line chemistry and D.I. water are kept under accurately controlled conditions. Great adjustment versatility in the chemistry dispense is provided through unusually maneuverable devices. In addition, other process variables (such as the air velocity and spin speed) can be controlled with unique fineness and range.
Chrome masks have traditionally been wet etched in an acidic solution of cerric ammonium nitrate. The etchant is commonly sprayed on the mask while the mask is slowly rotated, using an APT-914 or equivalent processor. While this process is well-understood, relatively trouble- free and inexpensive, the isotropic nature of wet etching results in an undercut of the chrome relative to the resist etch mask of approximately equals 150 nm per edge. Compensation for the undercut, in order to maintain control of the mean critical dimension (CD), is done by adjusting the printed feature size such that the undercut grows the printed feature to the desired final size. This sizing can be performed by manipulating the computer aided design database, which can be expensive and time consuming. In this paper, we present a comparison of wet and dry chrome etch processes using plates printed with the CORE-2564 in OCG-895 i resist. The differences in CD performance and resolution are illustrated.
Poly(trimethylsilylmethyl methacrylate-co-chloromethylstyrene) P(SI-CMS) has been designed to function as a negative-acting electron beam sensitive resist. This material is shown to be applicable to processes in which the chrome layer of 1X, 5X, and phase shifting masks is patterned via plasma etching. P(SI-CMS) containing 90 mole % trimethylsilylmethyl methacrylate and 10 mole % chloromethylstyrene exhibits thermal properties (Tg > 80 degree(s)C; Td > 250 degree(s)C) which provide stability in reactive ion etch environments. In a Cl2 - O2 plasma P(SI.90-CMS.10) resist etches a factor of 4.25 slower than chromium, and 16 times slower than novolac based resists.
Automation is widely discussed in IC- and mask-manufacturing and partially realized everywhere. The idea for the automation goes back to 1978, when it turned out that the operators for the then newly installed PBS-process-line (the first in Europe) should be trained to behave like robots for particle reduction gaining lower defect densities on the masks. More than this goal has been achieved. It turned out recently, that the automation with its dedicated work routes and detailed documentation of every lot (individual mask or reticle) made it easy to obtain the CEEC certificate which includes ISO 9001.
This paper discusses work done at Intel to meet CD specifications for a 0.50 micron generation photomask process using 6' 250 mil photoblanks coated with PBS resist. Initial CD uniformity results indicated that the develop tool was the main source of non-uniformity. To reduce the contribution of the develop tool, two strategies were pursued simultaneously to ensure that the within-plate linewidth uniformity (3 sigma) of the etched CD's would be 50 nm. The first strategy was to improve the capability of the current developer and to determine if any write parameters could be modified to allow for more process latitude. The second strategy was to evaluate new developers and identify a possible replacement. Both of these strategies will be presented with accompanying results for CD uniformity of the developed and etched CD structures.
This document develops a 1D self-calibration method which calibrates a measuring tool with an uncalibrated artifact. The artifact is measured in more than one position, achieving a calibration that is approximately equal to the reproducibility of the measuring tool. The self- calibration method is targeted for the micro-lithography industry.
This paper addresses the effect of blank material topography on positioning accuracy due to design variance of the mask holders of any two pieces of equipment, and it highlights positioning accuracy issues seen on wafers resulting from mask holders and alignment methodology differences. If manufacturers primarily concern themselves with equipment-error budgeting, it is unlikely position accuracy goals can be attained. The mask and blank manufacturers must study different surface topography effects on all related tools. The writing tool and the stepper manufacturers must include the difference effects in their performance study. The metrology tool manufacturer must study not only the P/T (precision/tolerance) ratio to minimize the effects of the uncertainty of the measurement but also study the difference between the writing tool and stepper tool metrology. They must then define the best method and work with all the suppliers to carry it out.
This paper introduces the theory of operation for `through-quartz' metrology as it applies to endpoint detection in an automated chemical processor, the Siscan IMPACT 7000. Further, a statistical analysis of actual manufacturing results will be presented with a focus on improvements in manufacturing process capability over standard manufacturing techniques as well as improved reticle performance in the wafer lithographers' environment through tighter image fidelity to design rules.
Critical dimension metrology of a phase-shifting mask must include depth as well as width measurements because both the phase and the lateral position of the transmitted photons must be controlled. Scanning probe microscopes are well suited to perform these measurements because they achieve high resolution simultaneously in all three dimensions. As with other microscopes, the probe-sample interaction strongly affects critical dimension measurement. The shape of the probe mixes with the measured object in an intrinsically nonlinear manner. We present measurements of phase-shifting masks performed with a scanning force microscope, and we discuss how they illustrate the capabilities and limitations of the technique.
Lasertec has developed a novel phase-shift measurement system 1PM11, which uses a differential heterodyne interferometer. In 1PM11, phase information is converted into a low- frequency heterodyne beat that is easily measurable by an electric current. Two-frequency laser beams which are oscillated from a He-Ne laser (632.8 nm) tube are used. When a 40X objective lens being used, the beam is 2.3 micrometers in diameter and the distance of the two beams is adjustable from 3.0 to 6.0 micrometers . The performance of 1PM11 for three types of phase-shift masks: (1) the shifter of etched quartz, (2) spin-on-glass shifter on etch- stop/quartz, and (3) the attenuating shifter, is reported.
Application of an Aerial Image Measurement System (AIMSTM) to binary and phase- shift mask fabrication and evaluation is described. The AIMS tool, an optical system which measures the aerial image directly from a mask, provides rapid feedback on lithographic performance for a variety of stepper configurations through modifications of the wavelength, numerical aperture, and illuminator design. The AIMS tool has been applied during the implementation of an alternating phase-shift mask (PSM) fabrication process in order to understand the impact of the etched-quartz sidewall on lithographic performance. AIMS measurements were used to extract the effective phase and transmission as a function of phase- etch depth as well as post-etch treatment condition. A set of basic test structures are proposed which can be used in conjunction with the AIMS tool to automate the extraction of transmission, phase, and second-level overlay for phase-shifting processes such as alternating and attenuating PSM.
Previous studies described the Depth Resonance Function (DRF) capabilities to understand and control defocus effects of a CD metrology system. This paper discusses the important properties of the DRF and how these properties affect the linewidth metrology process for photomasks built for submicron lithography. The DRF is used to investigate how various metrology process parameters change the response of the CD metrology system. The response of the CD metrology system is then presented as `Focus-Illumination' curves. Such CD results are compared to typical Bossung F-E curves to determine to what extent the metrology Focus- Illumination curves can simulate mask aerial image degradation that occurs during normal stepper exposure process.
Recently Leica introduced a new X/Y Metrology system called the LMS 2020. Performance results indicate an approximate 30% improvement over the existing model--the LMS 2000. Through-put, design grid accuracy, and precision have all been incrementally improved to varying degrees. Information presented includes: brief background description of the tool, a description of the measurement algorithms used to obtain the data, and the specific performance results obtained through measurements on both photomasks and wafers.
The aerial image measurement system is an optical system for measurements on phase shift masks under chosen stepper characteristics of NA, sigma, wavelength and depth of focus. The present tool operates at I-line or DUV (248 nm) and commonly 5 or 6 inch reticles can be handled. The image obtained is optically equivalent to that incident on resist, but is highly magnified so that it can be recorded using an UV CCD camera. Typically, features of interest are recorded as a through focus series; image intensity is digitized and may be analyzed in a variety of ways so as to produce intensity contours or profiles. Combined with simple models for predicting resist behavior a great deal of information may be obtained on the expected printing performance of a given reticle as a function of intensity and depth of focus prior to actual resist tests.
In order to assure that only defect free masks will reach Hoya Micro Mask's customers we need clean water, an excellent cleaning system and, of course, the means to inspect the mask for lingering particles and to certify that the product is defect free within the customer's specifications. We have designed our own original method for checking the QC Optics API- 3000/5, for accuracy, precision and sensitivity by depositing a group of metallic points using our Seiko FIB and evenly spreading polystyrene spheres of various sizes over our test mask. The depositions provided a vehicle for determining the precision of the API-3000/5 and establishing defect placement. The polystyrene spheres determine the inspection system's accuracy and limits of detection. We have added to our cleaning capability by working together with Convac-APT in the design of a new state-of-the-art standalone cleaning system and have solved many of our water problems by introducing a custom designed point-of-use water treatment system.
The design concept for 64 M is 0.35 micrometers and finer than wavelength of i-line (365 nm), and i-line is not applicable to 64 M production. Various methods have been investigated to achieve 0.35 micrometers resolution: (1) To improve the resolution by using the phase shift, or special illumination technique, etc. (2) To improve the resolution by using short wavelength light of excimer laser or deep UV of UV-II. Mitsui began the development of the pellicles for KrF-excimer laser (248 nm) and deep UV of UV-II (245 - 252 nm) in the fall of 1989. This report describes the deep UV pellicles, which Mitsui has developed for excimer laser and deep UV focusing on light resistance properties.
The evaluation of three different phase-shifting inspection test masks will be described. Each reticle was used to evaluate the defect detection capabilities on an automated inspection station for a different phase shifting technique. Two test reticles (alternate aperture and sized rim shifter) were fabricated using standard chromium on quartz blanks and one attenuated test reticle was made using thin chromium on a quartz blank provided by Hoya. A comprehensive study was conducted to determine the defect detection rates of phase defects achievable on these reticles using optimized settings for a KLA219HRL-PS automated inspection station. Results show that considerable gains in sensitivity and false detection reduction could be achieved using optimized settings, but phase defect detection and below 0.5 micrometers will require moving to the next generation tool.
The relation between the accuracy of the database representation and the inspection quality is discussed. In order to visualize the problems, a simplified model of an inspection machine is described. Using this model various aspects of database accuracy are presented. It is shown that some of the conventional methods for digitization of trapezoids, e.g. Brezenham method, may cause pixel-positioning errors and dropouts between figures. A better approach that is based on high precision subpixel addressing is proposed and its implication on reducing the database inaccuracy is proved.
Transmission defects are localized variations in the nominal transmission value of a photomask or reticle. Experience has shown that random transmission defects on reticles and photomasks have caused both device failures and reliability problems. In order to characterize the nature of transmission defects and the ability to detect them, the fabrication and evaluation of a programmed transmission defect test mask was undertaken. Using the programmed transmission test mask, quantitative data was obtained regarding the detectability of transmission errors on contact geometry.
Phase shifting masks (PSMs) have proven to increase resolution in optical lithography. However, the production of defect free PSMs still remains a challenge. The increase in resolution not only decreases the maximum allowed chromium defect size, but also introduces phase defects which print at even smaller sizes than conventional defects. This paper will describe typical defects on quartz etched Rim shifting and Attenuated PSMs as well as the minimum requirements for repairing these defects during the process development phase. Finally, possible PSM repair methods using conventional mask repair techniques such as focused ion beam sputtering and laser ablation will be discussed.
Results of a study to develop methods, based on UV laser ablation and chemical etching, for repairing defects in phase shifting masks will be discussed. The application of these techniques to a variety of candidate phase shifting mask types including attenuating Cr, embedded shifter Cr and SOG PSM will be discussed. The repair processes will be characterized in terms of process laser wavelength and energy flux, precursor gas type and material removal rate, transmission in the repaired area, and the phase shift, if any, introduced at the repair site. The effectiveness of various optical and gas delivery techniques will be compared with the types of defects likely to be encountered in the candidate masks. The results presented will include repair rate, etching uniformity and transmission data along with SEM and optical micrographs before and after repair events.
The introduction of phase shift masks presents a great challenge in the area of defect repair. Previously the number of variables was small. Defects consisted of extra chrome or missing chrome. These were either sputtered away or an opaque deposition was created to take the place of the missing chrome. These repairs really only had to be accurate in two dimensions-- X and Y. The Z dimension was somewhat important--it is necessary to remove all the chrome and not create significant glass damage--but the requirements were fairly loose. This has all changed with phase shift masks. The number of materials has doubled. We now must be concerned with missing and extra phase defects, and the Z dimension is immensely important and must be controlled to a few hundred Angstroms.
A perturbational model which describes the printability of defects in phase-shift masks (PSM) as a function of focus position is presented and used along with the SPLAT to generate design data on tolerable defect size as a function of defect phase. Defects in a clear field mask with phases other than 180 degrees have the greatest tendency to print when out of focus. From a reformulation of the perturbational model, the tendency to print when out of focus can be explained by the combination of the phase contributed by defocus in the defect image with the phase of the light passing through the defect. To assess the printability of defects, the impact of defects near features in thin chrome attenuating PSM and embedded attenuating PSM were evaluated by calculating the image linewidth variation.
This paper will discuss a new universal format for representing the pattern data and other information which define integrated circuits and the process of making them. The format, and the applications to be built around it, are designed to greatly save in cost, reduce production time, eliminate errors, and simplify procedures. It is called a `universal' format because the pattern data for a mask or a wafer can be encoded independently of the method of manufacturing it, and all tools can access the same data file without data conversion. The format encompasses all aspects of production, including mask layout, critical dimensions, e- beam and optical stepper proximity corrections, phase shift masks, and overlap removal. This paper discusses technology for connecting the universal format data in real time to high-speed tools for lithography (both raster and shaped beam technologies) and inspection.
This paper describes the calibration of an automatic inspection system to size 5X reticle defects down to half-micron resolution, the defect printability limit of the stepper lens with which the 5X reticles are to be used. An enhanced technique using image analysis for defect sizing is also described. Whilst enabling more accurate defect sizing, this method is resolution limited in automatic operation.
Applicability of conventional Focused Ion Beam photomask repair techniques is discussed in relation to chrome-level repair of phase-shift masks. Attention is paid to the underlying glass damage caused during chrome removal as well as the residual gallium staining. A technique used to control these issues is presented that does not require depth measurements prior to phase etch of the glass substrate. The applicability of this technique is discussed as it relates to full thickness chrome used in rim and alternating PSMs as well as the thin `leaky' chrome used for attenuated masks.
IBM Burlington has made the transition from a purely IBM MVS Host based architecture for writing and inspecting photomasks to an innovative hybrid host and local area network (LAN) solution. This involved going from a pure 9 track host generated tape environment to a IBM RISC/6000 LAN solution. An 80% tape reduction has been realized to date. Other IBM sites utilizing the same architecture include IBM Essonnes, France; IBM San Jose, CA and IBM Rochester, MN. The transition of the IBM Burlington photomask operational environment will be detailed to show the migration from tape to LAN for current and new equipment. Decision points and rationale that were used in the process will be given.
This paper describes the history and experiences of Du Pont Photomasks in their efforts to integrate the continuous improvement philosophy and practices embodied in the Malcolm Baldrige National Quality Award criteria into their way of doing business. A case study of key learnings in this almost four year long process is presented. Specific topics discussed include the process applied to achieve ISO 9000 certification, the quality systems deployed in this effort, and the use of a balanced set of business and quality metrics to assess and improve upon performance.
In 1989, Intel's internal mask operation limited itself to research and development activities and re-inspection and pellicle application of externally manufactured masks. Recognizing the rising capital cost of mask manufacturing at the leading edge, Intel's Mask Operation management decided to offset some of these costs by manufacturing more masks internally. This was the beginning of the challenge they set to manufacture at least 50% of Intel's mask volume internally, at world class performance levels. The first step in responding to this challenge was the completion of a comprehensive operation capability analysis. A series of bottleneck improvements by focus teams resulted in an average cycle time improvement to less than five days on all product and less than two days on critical products.
We present a general analysis of cost of ownership for an integrated circuit production lithography system. We illustrate the method with examples from i-line and deep ultraviolet lithography, as well as soft x-ray projection lithography. Tool utilization is emphasized as well as system throughput. Our analysis suggests that with 20 wafer per hour throughput, which may be attainable with soft x-ray projection lithography, lithography costs will rise to four times today's i-line costs, or higher. In addition to throughput, reticles and photoresist will be cost drivers for this technology.
Product quality and timely delivery are two of the most important parameters, determining the success of a mask manufacturing facility. Because of the sensitivity of this data, however, very little is known about industry performance in these areas. Using Arthur Andersen & Co. to protect contributor identity, the authors have conducted a blind quality survey of mask shops which represents over 75% of the total merchant and captive mask volume in the US. Quantities such as return rate, plate survival yield, performance to schedule and reason for return were requested from 1988 through Q2 1993. Data is analyzed and conclusions are presented.
The choice of a fabrication technique for phase-shifting masks is described by comparing approaches in which the shifter patterns are defined by etching the substrate (subtractive approach) or by adding a layer to the substrate (additive approach). By reviewing the various fabrication alternatives, a subtractive methodology that allows the fabrication of any type of phase-shift pattern (alternating aperture, assist-slot, rime, chromeless) on the same mask with only three lithographic steps was adopted. This process was extended to the fabrication of attenuated phase-shifting masks. The current processing capabilities are reviewed.
Even though rim PSM, unlike Levenson Phase Shift Mask, has no design restriction that neighboring patterns should be altered, its benefit is modest in both resolution and DOF enhancement, which masks it difficult to be applied to high resolution patterns. In order to improve the performance of rim PSM, we propose a new simple method called rim+ATOM, which combines rim PSM with ATOM (Advanced Tilted illumination On Mask). By combining rim PSM with ATOM, we could obtain resolution improvement by 20% compared to that of rim PSM with i-line stepper (5X, NA equals 0.45). And rim+ATOM showed less pattern degradation than ATOM for 45 degree patterns.
In this paper we report a new type of phase shifting mask (PSM), namely, halftone-rim, which is a combination of rim and halftone. Based on our simulation study using DEPICT-2 simulation tool, the aerial image of halftone-rim PSM in i-line for 0.35 micrometers contact hole pattern has better contrast and larger total depth of focus (DOF) than other mask techniques, such as conventional, subresolution, rim, and attenuated (halftone). Except attenuated mask, halftone-rim also has higher aerial image intensity among these masks. The preliminary contact hole pattern transfer studies using this new type PSM indicate a resolution down to 0.31 micrometers and a total DOF of 0.9 micrometers for contact holes by 5X i-line stepper (NA: 0.5, coherence: 0.6). Further experimental works on optimization of lithographic processes, especially in reactive ion etching of shifter layer and wet etching of halftone chrome, are needed to improve both resolution and total DOF.
An unexpected resist pattern due to the phase transition at the shifter edge is one of the problems for applying phase shifting masks. We have developed a novel subtractive process with a wet-etching technique. By using a controlled conventional resist process with some modifications, a gradually sloped shifter edge over 2 micrometers wide was produced when the quartz substrate was etched to 380 nm for a 180 degree(s) phase shift. By this etching process, the quartz covered with the chrome becomes only a 380 nm undercut and this chrome edge was durable during the physical cleaning process. This fabricated mask could easily provide 0.3 micrometers L/S patterns on the wafer with 160 mJ/cm2 on i-line stepper and the unexpected resist patterns totally disappeared at 110 mJ/cm2 and higher doses.
The use of electron-beam lithography for x-ray mask making requires an advanced system optimized for geometries of 0.25 micrometers and below. IBM's advanced mask facility in Essex Junction, Vermont, uses a variable-shaped-spot electron-beam lithography system known as EL-3+ to fabricate 1X x-ray membrane masks. The specifications of this system include 0.35-micrometers minimum features and 100-nm image placement (3(sigma) to absolute grid). Image-placement distortion and image-resolution errors on the E-beam system have been analyzed and minimized using unique methods, extending the system's capability into the 0.25-micrometers -feature-size range and improving placement to under 70 nm. This paper gives a brief overview of the x-ray mask-making process and methods used in the E-beam lithography area to correct for known errors; some results obtained using these methods are also presented.
LithoGraphTM, a PC-based lithography simulation software package developed by Etec Systems, is examined for its ability to be a useful tool in lithography process development. We evaluated conventional binary masks, attenuated phase shift masks (PSMs), alternate PSMs, and quadrupole illumination techniques at varying stepper parameters. As boundary conditions for the optimization, we considered image contrast, exposure-defocus latitude, exposure dose level, and optical proximity corrections. To quantify the extent of optical proximity effects, we calculated the overall exposure latitude by overlapping exposure-defocus diagrams generated at various pattern pitches.