Digital Scanner (DS) is an optical maskless exposure tool with a SLM (Spatial Light Modulator) and a DUV solid-state laser with wavelength of 193 or 248 nm. There are two configurations of SLM: a tilt SLM, in which each micro-mirror pixel tilts to change the amplitude of reflected light; and a piston SLM in which each micro-mirror pixel moves along optical path to change the phase of reflected light. Both types are applicable for DS, but piston SLM has a better image contrast due to strong phase shift effect. A DS proof-of-concept tool (DS-POC) with piston SLM and exposure wavelength of 193 nm was developed, which has a similar imaging resolution with the DS248, a tool planned as the first DS product for lithography of 180 nm node or below. Exposure results of 180 nm node logic patterns by DS-POC are presented. Process window analysis of the logic patterns by simulation shows better performance of piston SLM than tilt SLM on exposure latitude. CD accuracy of less than 10% was experimentally demonstrated for resolution chart of L/S with CD from 150 nm through 300 nm using piston SLM at DS-POC.
Nikon has been developing the Digital Scanner, an optical maskless exposure tool with a DUV light source. The Digital Scanner uses a spatial light modulator and rasterized pattern data, instead of glass photomasks, to project an optical image. The modulator is a micromirror array and each micromirror takes one of two possible states, so the pattern data are essentially equivalent to a one bit per pixel bitmap image. In spite of the one-bit depth input similar to a black-and-white bitmap, the Digital Scanner can control projected patterns in subpixel resolution because the pixel size is chosen to be smaller than the resolution of the projection optics. Besides the projection hardware, we have also developed special pattern data preparation system for the Digital Scanner in order to realize the subpixel controllability. Polygons from GDSII or OASIS files are rasterized by dedicated pixel-based algorithms so that the optical image of the resulting pixel data becomes equivalent to that of input polygons. Another pattern data converter with optical proximity correction (OPC) capability is also being developed and available for large area conversion. We explain the exposure system of the Digital Scanner and report the progress of the pixel-based data preparation system including recent demonstration printing results of exposure data generated by the new converter that has OPC capability.
Digital scanner (DS), a deep ultraviolet optical maskless exposure tool is being developed. DS uses a micromirror-type spatial light modulator to create the “mask” pattern combined with a solid-state laser with a wavelength of 193 or 248 nm. The exposure concept of DS and advantage of solid-state laser as an exposure light source is described. DS proof-of-concept tool with resolution of half-pitch 80 nm L/S was developed. The exposure results of maskless unique application, such as large area printing and chip ID printing for security purposes, are shown.
The first planned Digital Scanner product, DS248, will have the optical resolution of 110 nm and overlay accuracy of less than 10 nm, the same level as a KrF mask scanner. In addition, DS248 has more application areas, such as individual chip customization and large-area printing up to wafer size, with KrF resolution, which are not possible with the current mask scanner but will be beneficial for performance enhancement of semiconductor devices in future. The latest exposure results of DS-POC, which has the similar imaging performance with DS248, are introduced including chip ID exposure on entire 200 mm wafer and exposure of wafer scale integration substrate. Simulation data of high aspect ratio patterning with high resolution by means of integration of multiple heads of solid-state laser is described. Development progress of DS’s pixel mask conversion software that directly generates pixel mask from target pattern with OPC is reported.
Nikon has been developing the Digital Scanner (DS), an optical maskless exposure tool with a DUV light source and a micromirror-type spatial light modulator (SLM). Rasterized digital data, essentially huge bitmap files, are used to drive the SLM. The DS enables new applications such as large area printing and chip customization because its digital pattern data are easily modified. Flexible and fast data preparation software was developed for the new applications. As a standard operation of DS data preparation software, a CAD file (GDS or OASIS) is converted into bitmap files. In addition, bitmap file generation by a scripting language is available without a CAD file. This is useful when the CAD file includes a lot of polygons in which each polygon is similar but not identical, resulting in a huge file. As an example of application, a metasurface consists of sub-wavelength periodic patterns with various shapes, which are arranged to achieve the desired optical effect. The shape of each pattern at a grid point can be determined by a computer program, i.e., a pattern generator script. On the other hand, data preparation time can be shortened for periodic pattern which is often seen in semiconductor circuits. We report those data preparation methods for the DS, which have been used for our recent exposure experiments.
Maskless exposure makes possible of individual chip design customization and large area chip fabrication that are impossible with mask exposure.
We are developing DUV optical maskless exposure tool named as Digital Scanner (DS) that uses a spatial light modulator as a pattern generator and a DUV solid-state laser as a light source (193 or 248 nm).
We will report technology development progress of DS including the latest experimental data. Sub-pixel patterning capability by DS will be presented. Finally, we will discuss on the DS production tool with 248 nm exposure wavelength that are being prepared to release in mid-2020s.
The Digital Scanner (DS) being developed by Nikon is an optical maskless exposure tool with DUV light source and a micromirror-type spatial light modulator (SLM). The SLM forms a pixelated image; although each micromirror operates in a binary mode, the DS is capable to control pattern edges with subpixel resolution. This is because the pixel size on the wafer plane is smaller than the optical resolution, and therefore multiple pixels can contribute to each point in a projected image. We report simulation results of subpixel edge placement controllability of the DS. Actual exposure results on our experimental tool are also presented.
Digital Scanner (DS), a DUV optical maskless exposure tool is being developed. It uses a micromirror-type spatial light modulator (SLM) to create the “mask” pattern combined with a solid state laser with wavelength of 193 or 248 nm. The exposure concept of DS and advantage of solid state laser as an exposure light source is described. DS proof-of-concept tool with resolution of half-pitch 80 nm L/S was developed. The exposure results of maskless unique application such as large area printing and chip ID printing for security purposes are shown.
Double patterning requires extremely high accuracy in overlay and high uniformity in CD control. For the 32 nm half
pitch, the CDU budget requires less than 2 nm overlay and less than 2 nm CD uniformity for the exposure tool. To meet
these requirements, Nikon has developed the NSR-S620D. It includes a new encoder metrology system for precise stage
position measurement. The encoder system provides better repeatability by using a short range optical path. For CD
uniformity control, various factors such as focus control, stage control, and dose control affect the results. Focus
uniformity is evaluated using the phase shift focus monitoring method. The function of "CDU Master" provides dose and
focus correction across the exposure slit, along the scan direction, and across the wafer. Stage synchronization variability
will also influence CD control. In this paper, we will show the actual results and analysis of the overall performance of
S620D, including the exposed result of pitch splitting double patterning. S620D has sufficient performance for the 32 nm
half pitch double patterning generation and shows potential for double patterning at the 22 nm half pitch node.
Advanced scanners need an extremely high accuracy wafer alignment system, and nowadays it is also necessary that the alignment marks occupy a smaller area in order to expand the available area for IC patterns. Therefore, narrower lines with a smaller pitch must form the alignment marks. In this paper, a higher Numerical Aperture (NA) and lower aberration alignment optical system are studied for these requirements. At first the small alignment marks are shown, and suitable NA in the optical system is then discussed. As a result, the necessity for higher NA is shown. As for low aberration, the necessary specification of wavefront aberration is discussed. Assuming it is possible to suitably select the NA and the illumination NA in the optical system, the results of simulation -- that simulate image signals and perform image processing -- are reported. These results show the optical system that has aberration causes position shift, so that the specification of wavefront aberration is estimated in order that the position shifts may be sufficiently small. To make sure that with such a strict specification the system will be possible, a trial optical system has been made. Finally the techniques of manufacturing and the results of evaluation are reported.
Wafer alignment plays a significant role in the advancement of microlithography and has been constantly improved to meet various situations. As a result, its configuration is very dynamic and it sometimes requires considerable cost for process optimization.
Software has been developed which evaluates the alignment performance in a variety of conditions from the minimal data set. It allows the user to perform off-line optimization, essentially reducing the amount of interruption toward production. This article illustrates the simulation method implemented in the software, OverLay EValuation program (OLEV).
Outliers in measurement often interfere with alignment. They are caused by sudden damages in the alignment mark, and existence of particles, resist damages and so on. In a conventional way to identify outliers, the observations that have larger residual than previously determined threshold are identified as outlier. It works well only with the operator’s labor of adjusting the threshold according to the deviation of ordinaries (non-outliers). However, labor is a problem especially in Small-Quantity Large-Variation fabrication such as for ASIC, System-LSI and so on. A novel method for elimination of the labor has been developed. It utilizes normal mixture models whose number of components is determined based on the Maximum Penalized Likelihood (MPL) method. It can be regarded as an identification method that determines threshold adaptively using ordinaries’ deviation. Simulation results show that the penalty coefficient, the only parameter of the method, can be a constant in the variation of ordinarie's deviation. It also shows that in the absence of outliers, the accuracy of the method is comparable with the maximum likelihood estimation that is commonly considered to be the best method when the observations follow the normal distribution. The method performs better than conventional ones when there are a sufficient number of observations (no less than ten) in the standard Enhanced Global Alignment (EGA). Superiority of the adaptive method is dependent upon the probability of outlier occurrence, variation of the situation, the number of observations and the complexity of the model fitted to the observations.
Advanced stepper or scanner needs extremely high accuracy alignment system. This alignment accuracy is mainly affected by the errors caused by mark deformations and by optical system. To improve the alignment accuracy of our wafer alignment system called 'FIA' we have developed a method called the 'FFO'. Our studies have already shown that FFO has the effect of reducing the errors caused by mark deformations. To examine the errors caused by optical system, new approaches are adopted. In the new approaches a simulation method and a suitable experimental are used. The simulation results by the new method, Spatial Frequency Analysis of Image, show the relation between defocus and the errors caused by optical system and the superiority of FFO. Suitable experimental system brings us the same results as the simulation method. As a result, FFO also has a positive effect on the errors caused by optical system. FIA with FFO is much more accurate alignment sensor for ULSI production.
Detecting position of the wafers such as after CMP process is critical theme of current and forthcoming IC manufacturing. The alignment system must be with high accuracy for any process. To satisfy such requirements, we have studied and analyzed factors that have made alignment difficult. From the result of the studies, we have developed new optical alignment techniques which improve the accuracy of FIA (alignment sensor of Nikon's NSR series) and examined them. The approaches are optimizing the focus position, developing an advanced algorithm for position detection, and selecting a suitable mark design. For experiment, we have developed the special wafers that make it possible to evaluate the influence of CMP processes. The experimental results show that the overlay errors decrease dramatically with the new alignment techniques. FIA with these new techniques will be much accurate and suitable alignment sensor for CMP and other processes of future generation ULSI production.
As semiconductor design rules decrease, tighter tolerances are required for alignment. Improvement of the measurement algorithm can make a considerable contribution to reduction of the overlay error. An algorithm makes the alignment accuracy greatly improved that utilizes wavelet transform and uses information about image asymmetry. Experimental result using the Alignment Data Logging System shows that there is a process that the algorithm reduces the overlay error from over 100nm (3(sigma) ) to under 50nm. Two other algorithms are also introduced that are an interpolation method that reduces error from image sampling and a mark recognition method that reduces measurement failures focusing on some kinds of symmetry of the alignment mark.
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