In world-leading semiconductor manufacturing, the device feature size keeps on reducing and with it processes become more challenging in the next technology node. The On Product Overlay (OPO) budget is therefore required to reduce further. Alignment is one of the key factors in reducing overlay wafer to wafer (W2W) variations. To save product area and reduce scribe line width, small alignment mark is evaluated to achieve the similar results as reference mark and to optimize the OPO performance. In this work, we will show the experimental results of small alignment mark and investigate the on product overlay performance by simulation.
In the MEMS world, increasing attention is being given to 3D devices requiring dual-sided processing. This requires
lithography tools that are able to align a wafer to both its back side as front side. Overlay describes how well front and back side layers are positioned with respect to each other. Currently there is no simple and fast method to qualify the overlay. This paper covers a method of measuring the overlay between front- and back side patterns using a glass substrate. We describe the methods used, special process requirements and measurement data. The main advantages of the presented method are the simplicity of the concept and the need for only basic fab processing equipment. The substrate employed is re-usable and low cost. The results are as follows: 1. Glass wafers can be used to measure front to back side overlay. The accuracy of the proposed method is better than 100 nm (3σ) on ASML PAS 5000/5200 machines. On ASML PAS 5500 steppers, the expected accuracy is better
than 80 nm (3σ). 2. The proposed method of measuring the absolute glass shift, from a glass-on-silicon stack, yields unreliable
information. This is due to deformation of the glass. An alternative method is described which builds on result 1 (above). 3. Processing of glass wafers has been established, and a glass overlay measurement wafer has been defined. 4. The benefit of Anti Reflective (AR) coatings is suspected, but not yet proven. Minimizing bi-refringency does not play a role in the measurement accuracy of glass wafers for overlay measurements.
To validate the Front- To Backwafer Alignment (FTBA) calibration and to investigate process related overlay errors, electrical overlay test structures are used that requires FTBA [1]. Anisotropic KOH etch through the wafer is applied to transfer the backwafer pattern to the frontwafer. Consequently, the crystal orientation introduces an overlay shift. A double exposure method is presented to separate the process-induced shift from the FTBA shift. The process induced overlay shift can run up to 3 μm, large compared to the expected FTBA error (around 0.1 μm). The measured overlay distribution is 0.45 μm (3σ), this includes both waferstepper and process related overlay errors. The overlay distribution, corrected for waferstepper related overlay errors, like lens distortion, resembles the overlay distribution of the bulk micromachining (BMM) process; 0.26 μm (3σ). The procedures described in this work provide a quantitative method of describing the waferstepper and process related front to backwafer overlay errors.
The increased demand for high throughput lithography with Front To Back-side Alignment (FTBA) capability has led ASML to develop the FTBA functionality within its current platform. This option is named 3DAlign and aims at an FTBA overlay of 500 nm. To relate a position on the back side of a substrate to its front side, two back-side marks are projected from the back to the front side by two optical branches. In this way, the images of the back-side marks have the proper orientation and magnification to be aligned by the standard ASML alignment hardware. The metrology challenge is to calculate the back-side mark positions in front-side (exposure) coordinates and to compensate for the systematic errors introduced by the optical branches. An absolute relation between the front- and back-side positions on the substrate is obtained by calibrating the system with a special 3DAlign calibration wafer. This is a wafer that has marks on both sides with a known offset. In this paper, the basic ideas and algorithms for the FTBA metrology are discussed.
A novel front-to-back alignment method, which does not require additional alignment sensors, is being developed for optical projection lithography tools. The system is designed such that it can be easily retrofitted to existing systems. By embedding a pair of tiny optics into the wafer stage, the existing Through-the-Lens (TTL) and/or Off-Axis (OA) alignment systems can be utilized, thereby avoiding the added complexity of supplementary alignment electronics and hardware. The front-to-back alignment is accomplished by imaging the alignment marks on the wafer back surface to the front and then using the standard front-side alignment system. To calibrate the front-to-back alignment module and to minimize the alignment errors, new metrology software is introduced. The front-to-back alignment accuracy is specified to be <EQ 500 nm, which provides typically twice the improvement compared to current state-of-the-art implementations. The front-to-back alignment capability is being developed in response to the requirements of the MEMS/MOEMS market.
From several single arthropods methane and water vapor release is measured with a time resolution of 15 seconds by a liquid nitrogen cooled CO-laser intracavity photoacoustic arrangement. Atmospheric methane is an important component in the process of global warming and ozone depletion. Four taxa of arthropods are thought to be responsible for up to 25% of the total mondial CH4 release to the atmosphere. The measurement of water vapor release together with CO2 emission is of importance for understanding the complex breathing pattern of arthropods, in view of evolutional adaptations these animals have undergone to survive terrestrial conditions.
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