Synchrotron-based X-ray Lithography Research Articles

Operational analysis of synchrotron-based X-ray lithography: comparison of 200 mm and 300 mm wafer flows

A companion paper describes a discrete-event simulation model that captures the processing of wafers through a next-generation X-ray lithography area employing multiple synchrotrons as the source of exposure radiation. The model incorporates the best current information on unit-cell design and processing times; implements the spectrum of events that interrupt the flow of wafers processing on the cell; and provides estimates of weekly throughput for the cell and the frequency of SEMI E-10 equipment states for the corresponding exposure tool. In this paper we apply the model to compare the performance of cells fabricating 200 mm wafers with that of cells fabricating 300 mm wafers. Results illustrate the dependence of average wafer throughput on lot size, exposure times, and assumptions regarding the number of die per wafer. For fixed assumptions regarding differential lot sizes for different wafer diameters, maximum throughput of wafers is achieved for 200 mm wafers with 25/spl times/25 mm field size. Ignoring wafer-sort losses, however, a maximum throughput of chips is realized for 300 mm wafers with 22/spl times/22 mm fields with 11/spl times/22 devices. Remarkably, the distribution of equipment states remains relatively unchanged across simulation experiments. These results suggest a useful analytical approximation for cell performance. When calibrated against the simulation results, this static model reinforces the conclusion that, all else being equal, acceptable throughput can be achieved for 300 mm wafers using the smaller wafer lots required to maintain acceptable cycle times.

Operational analysis of synchrotron-based x-ray lithography: Simulation model of wafer flows

Operational analysis of synchrotron-based x-ray lithography: Simulation model of wafer flows

Operational analysis of synchrotron-based X-ray lithography: simulation model of wafer flows

A discrete-event simulation is developed that captures the processing of wafers through an advanced X-ray lithography area employing multiple synchrotrons as the source of exposure radiation. The model incorporates the best current information on unit cell design and processing. Detailed model logic implements a range of unscheduled events that represent interruption of the flow of wafer processing on the cells, as well as recovery from these downtime events. Performance measures estimated from the simulation include the weekly wafer throughput for each unit cell and the frequency of equipment states for the corresponding exposure tool. Equipment states defined in the model are based on an expansion of the SEMI E-10 guidelines to account for downtimes associated with beam charge and preventive maintenance and for uptimes associated with processing send-ahead wafers. In this paper we discuss the rationale for the simulation and consider in detail the operating assumptions embedded in the simulation model. In a companion paper we describe a simulation study in which the model is applied to compare the performance of the X-ray lithography running 200 mm and 300 mm wafers.

Operational analysis of synchrotron-based X-ray lithography: comparison of 200 mm and 300 mm wafer flows

A companion paper describes a discrete-event simulation model that captures the processing of wafers through a next-generation X-ray lithography area employing multiple synchrotrons as the source of exposure radiation. The model incorporates the best current information on unit-cell design and processing times; implements the spectrum of events that interrupt the flow of wafers processing on the cell; and provides estimates of weekly throughput for the cell and the frequency of SEMI E-10 equipment states for the corresponding exposure tool. In this paper we apply the model to compare the performance of cells fabricating 200 mm wafers with that of cells fabricating 300 mm wafers. Results illustrate the dependence of average wafer throughput on lot size, exposure times, and assumptions regarding the number of die per wafer. For fixed assumptions regarding differential lot sizes for different wafer diameters, maximum throughput of wafers is achieved for 200 mm wafers with 25/spl times/25 mm field size. Ignoring wafer-sort losses, however, a maximum throughput of chips is realized for 300 mm wafers with 22/spl times/22 mm fields with 11/spl times/22 devices. Remarkably, the distribution of equipment states remains relatively unchanged across simulation experiments. These results suggest a useful analytical approximation for cell performance. When calibrated against the simulation results, this static model reinforces the conclusion that, all else being equal, acceptable throughput can be achieved for 300 mm wafers using the smaller wafer lots required to maintain acceptable cycle times.

Status and future of X-ray lithography

X-ray lithography is relatively mature, and may be used to manufacture complex integrated circuits at 130 nm ground rules and smaller. Overlay and critical dimension control for 180 nm ground rules have been demonstrated with synchrotron-based x-ray lithography. Significant engineering challenges remain in making x-ray masks for 180 nm and smaller ground rules. Extendibility of x-ray lithography to 100 nm ground rules and below will require ≤20 μm gaps and careful mask bias optimization.

Masked ion beam lithography for proximity printing

Optical and X-Ray proximity printing systems are resolution limited by diffraction and beam dispersion. Parallel dispersion free ion beam systems are therefore ideal to transfer stencil mask patterns onto all sorts of nonideal substrates. A feasibility study was performed with the existing Alpha ion projector of the Society for the Advancements of Microelectronics in Austria operated in the MIBL (Masked Ion Beam Lithography) mode with ≈ 10×10 mm2 exposure field. Structures as small as 0.2 μm in diameter could be transfered even with a gap of 1 mm between stencil mask and substrate. The widening of resist lines with 10% increase in dose was evaluated to be 14 nm for 2800 μm gap and 4 nm for 300 μm gap. This excellent exposure latitude favourably compares with synchrotron based X-ray lithography, where a widening of 20 nm with 10% overexposure has been reported for a 40 μm gap, and 10 nm for 10 μm gap. Promising applications of the MIBL technique include the fabrication of flat panel displays based on vacuum electronics (field emitter displays), surface acoustic wave and microoptic devices and - in combination with reactive ion etching - the fabrication of micro electro mechanical systems (MEMS). Prospects for MIBL steppers of printing fields > 100×100 mm2 are discussed.

Simultaneous optimization of spectrum, spatial coherence, gap, feature bias, and absorber thickness in synchrotron-based x-ray lithography

Of the many factors affecting the x-ray intensity distribution, the variables that can be controlled are the source spectrum, the proximity gap, the source spatial coherence, the mask linewidth bias, and the absorber thickness. To obtain the highest quality aerial image, all of these parameters must be optimized simultaneously. An optimization of the spectrum of the synchrotron Helios, located at IBM’s Advanced Lithography Facility is described. The optimum parameter space for proximity x-ray lithography at 0.1 μm minimum linewidth is then determined using the optimized spectrum by adjusting the free parameters. For maximum accuracy, a rigorous electromagnetic model that accounts for the dielectric properties of the absorber, the source partial coherence, and diffraction in the proximity gap is used to calculate the x-ray aerial image at the wafer. Descriptive figures-of-merit (FOMs) of the aerial image are the image contrast [(Imax−Imin)/(Imax+Imin)] and the exposure latitude. These two FOMs are maximized with respect to source spectrum, gap, source spatial coherence, feature size and bias, and mask absorber thickness. The global maximum of these FOMs is coarsely located in parameter space by determining the dependence of the FOMs on two variables at a time. The feature bias is then determined so that all feature types (gratings, lines, spaces) can be printed at the same dose with maximum average contrast and exposure latitude.

Deep X-ray lithography for micromechanics via synchrotron radiation

Micromechanics is an emerging field with many applications in microsensors and microactuators. The processing tool which is needed for these devices must produce three-dimensional structures from many materials with submicron tolerances. Synchrotron based X-ray lithography can be used to fabricate photoresist molds with structural heights to 500 μm and nearly perfect edge acuity. The plastic molds can be converted to metal molds via electroplating. Fabricated metal parts can be assembled to form complex structures. An example of this technique is a fully functional planar, magnetic micromotor which has been fabricated and is in the testing phase.

Optimal design of an x-ray lithography mask

The 0.25 μm 1:1, masks used in x-ray lithography have a very tight overlay requirement: it is not just sufficient to produce a perfect mask, it is also necessary not to degrade its characteristics by incorrect mounting. This is particularly true for synchrotron-based x-ray lithography where the masks are patterned and used in different configurations. The computational tools necessary to model mask distortions in the nanometer range with high accuracy and reliability are developed here and used to predict an optimal mask design. The results show that the overall mask distortion resulting from gravitational loading can be accommodated within the allocated error budget.

X-ray lithography two-state alignment system

We present a novel alignment system for x-ray lithography. A linear Fresnel zone plate, defined on the mask, is used to focus a laser beam on a wafer mark (a series of dots). A fast electro-optic modulator is used to make the laser beam hop between two positions at the mark edges. The light diffracted by the periodic mark is measured at each position and a misalignment signal is originated when the mark is not illuminated equally in the two states. A signal that is proportional to the misalignment is generated when the error is within the alignment range (±0.2 μm); outside that range a capture signal is present as long as the beam is on the mark, which would be typically 2–4 μm wide. The method is highly sensitive with resolution in excess of 0.003 μm, has a wide capture range, no moving parts, and is suitable for real-time alignment during exposure. Sensitivity to gap changes is highly reduced, making the system ideal for synchrotron-based x-ray lithography aligners.

State of the art of pattern placement accuracy of silicon X-ray master masks

A further increase in the integration density of SRAM and DRAM memories will be limited by resolution as well as overlay accuracy. In the case of synchrotron-based X-ray lithography, the main contribution to the total overlay budget is given by the achievable accuracy of the X-ray masks. In this paper, the pattern placement accuracy of silicon X-ray master masks will be shown, which are fabricated for real device application. The measurement results are obtained with an optimized and improved e-beam measurement system and discussed in terms of linear and nonlinear pattern placement errors. Considering a number of X-ray master masks with different absorber coverage within the step and repeat field (3 × 3 cm 2) and different levels of complexity, the average pattern placement error including both statistical and linear contributions can be reduced reproducibly down to about 70 nm, including the principal accuracy of the e-beam pattern generator and measurement system.

Application of X-ray steppers using optical alignment for synchrotron-based X-ray lithography

The results obtained with a first generation of synchrotron-compatible x-ray steppers are reported. Detection, alignment and overlay accuracy data are given. A number of devices have been manufactured with at least the critical mask levels made with x-ray lithography.

Experimental results with a scanning stepper for synchrotron-based x-ray lithography

A second generation of synchrotron-compatible x-ray steppers is described which uses the scanning exposure method to minimize overhead time between alignment and exposure. Available x-ray intensity is increased, since a thin exposure window, only a few mm high, can be used to separate the beam line vacuum from the exposure tool which operates in ambient air. First results indicate sufficient mechanical stability to allow the mask/wafer unit to be scanned in an aligned position.

Synchrotron-based X-ray lithography at Stanford University

In this paper we describe present and planned synchrotron-based X-ray lithography facilities at Stanford. We also present standard procedures used at the Stanford Synchrotron Radiation Laboratory (SSRL) for conducting experiments in lithography mask damage, device damage, and X-ray resist characterization.