For the successful reduction of chip production costs, the usage of more advanced designs with lower area consumption by manufacturing angled line structures is one possibility. The usage of conventional vector shaped electron beam writers does only allow writing Manhattan-like structures as well as 45 degree angled structures. There are several approximation possibilities for writing any angled lines, e.g. they could be approximated by writing only small rectangles or small rectangles in combination with small 45 degree triangles. This method introduces a very pronounced line edge roughness due to the written uneven edges. The critical dimension uniformity on the mask and the printing behavior are directly influenced by this synthesized line edge roughness. This paper addresses the investigation of critical dimension of the angled mask structures as well as the influence on the printing behavior. The different masks used in the experiment were patterned at the Advanced Mask Technology Center (AMTC). Measurements of pattern line widths were performed by using scanning electron microscopy techniques. The printing behavior of different structures was investigated by running AIMS measurements and performing exposure experiments. Comparing the mask structures and the final printed wafer structures, estimations on the transfer function of the synthesized line edge roughness could be performed.
Monitoring long-term performance of projection optics in lithographic exposure systems will become more and more important, especially for 193nm wavelength. Various effects influence the quality and long-term stability of a lens projection system. Using the well known and established blazed phasegrating method, it is possible to identify lens degradation before it becomes a significant detractor in a manufacturing process. A two beam interferometer formed by a blazed grating reticle is used to measure the aberration values. This works for all DUV tools, and therefore it allows a comparison of tools from different suppliers. The test can be run after regular preventive maintenance or as daily monitor checks, in order to evaluate lens aberration over time. By storing the results, it is easy to generate a tool individual database. With this paper, we will show aberration data over time and the possibility to increase tool performance and stability.
The concept of developing a Two Beam interferometer through the use of a Blazed Phase Grating reticle design was introduced several years ago. Although this technique showed great promise and capability in the lab environment, introduction of this product into the manufacturing processes has been slow. We will introduce some of the newest findings and techniques that have allowed us to bring this technique into use in the day-to-day manufacturing environment. We will discuss advancements made in the design of the hardware elements and inspection routines, new detections analysis routines and integration techniques that have allowed us to introduce the Blazed Phase Grating into our R2R focus control. The results of this technique will be presented and we will show the correlation to our existing focus monitoring techniques as well as the sensitivities to processing effects on the determined best Focus Values. We will attempt to show some of the imaging hardware effects on printed best focus, including the effects of scan direction as well as the introducing a monitoring technique used for determining wafer and e-chuck flatness.
Various methods of printing small contact holes are discussed. Although the resolution capability is one key object for printing small contacts, it does not always reflect the process window. This paper compares resolution as well as process windows for several contact printing techniques. It shows the huge benefit of ring-type contacts with respect to process window even when compared to Bessel like contacts.
Maximized use of exposure fields is essential for achieving high stepper throughput rates and high productivity in semiconductor manufacturing. For today’s low-k lithography, very often lens quality is limiting the imaging performance and can eat up overlay budget and allowable CD tolerance. As a result, decreasing yield at extreme slit positions, is a potential danger. Finding an optimum product field size, considering stepper productivity and product yield, is difficult and often based on non-measurable engineering experience. This paper investigates the effect of lens aberrations on misplacement and CD deviations of two critical patterns in a DRAM cell. It can be shown that, depending on the exposure tool, the biggest error can even occur close to the middle of the exposure slit. Also, model calculations based on PMI numbers underestimate the actual overlay degradation. Therefore, smaller exposure fields do not necessarily avoid pitfalls caused by high lens aberrations or other effects resulting in a reduced overlay budget.
A method is described to assess the influence of lens aberrations on the image by analyzing the interaction of specific aberrations and diffraction patterns resulting from corresponding mask structures. In order to establish a correlation between the amplitude of individual diffraction orders and specific aberrations, the sensitivity of each diffraction order is investigated separately. The resulting information is used in order to find means to reduce the influence of the aberrations on the diffraction pattern. Several possibilities such as various mask biases, serifs and non-printable assist features can either enhance or decrease the sensitivity to specific aberrations. This method of diffraction order sensitivity study is described and experimentally tested.
A reticle with phase-only blazed gratings of varying azimuthal orientations diffracts light into only two orders, 0 & +1, discretely illuminating a lens pupil. The image of each grating is a sinusoidal interference pattern and is recorded as a surface relief in a highly absorbing photoresist. The maximum image contrast occurs when focus is set such that the RMS wavefront error over the two beams is minimized. This maximum contrast vs focus is recorded by a CCD array mounted on a dark-field optical microscope and the aberrations are obtained from an analysis of this record. Repeatability of equivalent primary aberrations of less than 0.001(lambda) RMS are achieved and used to monitor lens stability.
A double exposure technique, so called nano-stepping, was investigated to evaluate its benefit for very dense features to reduce line shortening, improve pattern fidelity and resolution capability. The technique involves relaxing the pitch of dense patterns in one dimension and filling in the missing patterns by exposing the same reticle again, offset by an appropriate amount. This method suffers only small throughput loss compared to conventional dual reticle exposure techniques. For 1D patterns, 100 nm lines and spaces can be printed with a 248 nm exposure tool and a half tone mask. Dense 2D contacts with various length to width ratios can be achieved with minimum distance to adjacent neighbors.
Multiple contact hole resist samples from a variety of DUV resist suppliers, including both acetal and ESCAP chemistries are evaluated on an organic anti-reflective under layer (ARC) using an attenuated phase shift mask (APSM). One sample exhibited excellent surface inhibition and superior lithographic performance for patterning contact holes of 0.2 micrometers imaging size. For most of resists, the process windows are limited by unwanted sidelobe printing through focus. The sensitivity of sidelobe printing to focus can be attributed to lens aberrations. For the first time, we prose to use Depth-of-focus (DOF) loss PWLdof and Exposure latitude (EL) loss PWLel to characterize resists surface inhibition, as well discovered that DOF loss is a sensitive measure of surface inhibition. Similar lithographic performance is obtained from acetal and ESCAP based materials. The two ESCAP resists EB3 and EA2 have better oxide etch resistance than the acetal resist AC1. The top surface reticulation is observed on ESCAP resist EB3 and EA2 during the oxide etch, but not on the acetal resist AC1. 110 nm underexposed resolutions achieved with the resist EA4 at a mask size of 250 nm. Faster resists generally exhibit better resolution but have smaller process windows when side lobe printing is included as a criterion. Selection of a resist formulation for attenuated phase shift applications has to face a compromise between resolution, photospeed, process window and surface inhibition. Finally, ARC operational modes and optical properties had little effect on sidelobe printing, and optimization of PEB temperature is important in suppressing sidelobe printing.
We report on a systematic study of the transport characteristics of YBa2Cu3O7 (YBCO) step-edge Josephson junctions as a function of the step angle. The microstructure of a YBCO film depends very critically on the step angle (alpha) in a SrTiO3 or LaAlO3 substrate. Briefly, on shallow steps (0 < (alpha) < 44 degree(s)) the film grows epitaxially across the step. On steep steps (46 degree(s) < (alpha) < 85 degree(s)) two grain boundaries occur on the step. In this paper it is shown that the I-V curves of the step-edge junction reflect the microstructure of the YBCO film on the step. The I-V curves on shallow steps are of flux-flow type. On 45 degree(s) steps there are several Josephson junctions with different critical currents, while on steep steps two critical currents are found in the I-V curve. Summarizing all data, we conclude that the grain boundaries formed on steep steps are responsible for the Josephson behaviour of the junctions.
We have investigated the Josephson radiation from different types of YBa2Cu3O7 (YBCO) thin film junctions: step-edge (SEJ), biepitaxial, and superconductor-normal conductor- superconductor (SNS) with N=Au and PrBa2Cu3O7. The radiation was detected using a nonresonant radiometer system with a receiving frequency of 11-12 GHz. The current-voltage characteristics were measured simultaneously with the radiation spectra in the temperature range from 4.2 to 90 K. All junctions exhibited a large emission peak at a voltage which was related to the frequency through the second Josephson relation. Typically, for high temperatures, and, therefore, small critical currents, the experimental data of the radiation linewidth agreed well with the theoretical predictions of the RSJ model. At lower temperatures the experimental linewidths deviated from the theoretical values due to additional noise sources in the junctions. Some of the SEJs showed a nonmonotonic dependence of the linewidth on temperature. Such SEJ data will be discussed in terms of a model which treats the SEJ as an interferometer consisting of a parallel array of Josephson junctions.