Design of Convex Corner Compensation Pattern in Manufacturing of Si Diaphragms

Convex corner cutting under the etch mask of convex corner structures anisotropically etched in (100) single-crystal silicon arises in the fabrication process of three-dimensional microstructures. This paper presents a novel design of complex corner compensation mask with small geometrical dimensions for microstructures with high packing density. These complex patterns possess defined convex corners and slits. During etching the undercut process begins at the convex corners of the etch mask and reaches after a defined time the mask openings in the compensating patterns forming new convex corners. In this way the etch front of the undercut process will be reflected for one or more times. Experimental results of an example for making V-groove-crossings show satisfactory corner compensation.

This paper investigates the anistropic etching characteristics and convex corner undercut mechanism of (110)‐oriented silicon in aqueous potassium hydroxide solutions. First, the crystal planes governing the etch front of the undercut are determined, and their etch rates are measured. Then, based on the measured data, several methods for convex corner compensation techniques are examined. Conventionally, 〈111〉 direction beams are used to compensate for the undercut on convex corners. This method is found to produce good results for acute convex corners, but results in the emergence of large residue structures with (111) crystal planes on the obtuse convex corners. To alleviate this problem, new corner compensation methods that use triangular and rhombic patterns are developed based on the measured etch front planes. The effectiveness of the proposed corner compensation patterns in reducing the undercut and residues is evaluated in detail. It is found that for obtuse convex corners the rhombic corner compensation provides the best result, and for acute convex corners both the rhombic and 〈111〉 beam patterns are effective.

In the present study, chemical dicing methods were developed as an alternative solution for thin silicon chips separation. Dicing streets were fabricated by wet and dry etching. The dry etching approach is reliable but also an expensive process. On the other hand, the wet etching approach offers a faster and low-cost process compared with the dry etching approach. However, serious undercutting usually occurs and damages the chip corners. During the course of the present study, a convex corner compensation scheme was implemented to resolve the undercutting problem. A parametric study was performed with various kinds of redundant [1 1 0]-oriented beam patterns at the corners of intersections. Experimental results showed that relatively sharp corners could be achieved with certain compensation patterns. A relation between the compensation pattern and the etched depth was established. The experimental result of the wet etching approach will be discussed in detail in this paper.

This paper presents etching of convex corners with sides along <n10> and <100> crystallographic directions in a 25 wt% tetramethylammonium hydroxide (TMAH) water solution at 80 °C. We analyzed parallelograms as the mask patterns for anisotropic wet etching of Si (100). The sides of the parallelograms were designed along <n10> and <100> crystallographic directions (1 < n < 8). The acute corners of islands in the masking layer formed by <n10> and <100> crystallographic directions were smaller than 45°. All the crystallographic planes that appeared during etching in the experiment were determined. We found that the obtained types of 3D silicon shape sustain when n > 2. The convex corners were not distorted during etching. Therefore, no convex corner compensation is necessary. We fabricated three matrices of parallelograms with sides along crystallographic directions <310> and <100> as examples for possible applications. Additionally, the etching of matrices was simulated by the level set method. We obtained a good agreement between experiments and simulations.

This paper reports corner compensation methods for fabricating the intact mesa structure in MEMS (Micro-Electro-Mechanical System). To investigate the undercutting problem in the mesa structure, over ten corner compensation patterns are designed by computing the relations among a series of parameters, e.g. etching rates in different crystal planes, etching depth, etching times, etc. The compensation patterns are then simulated by the simulation software Anisotropic Crystalline Etch Simulation (ACES) beta 2, the 3D etching simulations are gotten. Various new compensation structures preventing the undercutting of convex corners of (100) silicon in THAH solution are redesigned and optimized based on the simulation results, the fabrication are conducted to verify the feasibility of the corner compensation patterns.

In wet anisotropic etching based silicon bulk micromachining, undercutting, which has both advantage and disadvantage, takes place at the convex corners of microstructures. In order to retain the desired shape of fabricated structure, corner compensation method is most commonly used to protect the convex corners. The design and shape of the compensation geometry depend on the type of etchant. In this work, various types of corner compensating structures to protect convex corners on Si{110} and Si{110} are studied in potassium hydroxide (KOH) modified by adding NH <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> OH solution. Silicon etch rate in NH <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> OH-added KOH is 3-4 times more than that in pure KOH solution, which is very useful for industrial application to improve productivity. Mesa shape structures are fabricated using different shapes corner compensating geometry to optimize the design to obtain best shape convex corner. Triangular shape and beam shape geometries are found most appropriate structures to obtain well-shaped convex corner on Si{100} and Si{110}, respectively.

Experimental study and analysis of corner compensation structures for CMOS compatible bulk micromachining using 25 wt% TMAH

This paper reports a simple method of convex corner compensation for (1 0 0) silicon wafer etching in a KOH solution. A pattern-induced effect is introduced and utilized in the new compensation design. This compensation structure comprises three square patterns which are joined to the convex corner apex. The three squares surround the corner and prevent it from exposure to the etchant during KOH etching. The design is confirmed by comparing the graphic analysis of etched shapes with SEM photographs at different etching depths. Experimental results prove the high accuracy of this method. Using this new corner compensation design, excellent convex corners have been fabricated. Compared to conventional compensation structures, the new structure has the advantages of easy design and less requirement of the extra area.

In this paper a methodology for the three dimensional (3D) modeling and simulation of the profile evolution during anisotropic wet etching of silicon based on the level set method is presented. Etching rate anisotropy in silicon is modeled taking into account full silicon symmetry properties, by means of the interpolation technique using experimentally obtained values for the etching rates along thirteen principal and high index directions in KOH solutions. The resulting level set equations are solved using an open source implementation of the sparse field method (ITK library, developed in medical image processing community), extended for the case of non-convex Hamiltonians. Simulation results for some interesting initial 3D shapes, as well as some more practical examples illustrating anisotropic etching simulation in the presence of masks (simple square aperture mask, convex corner undercutting and convex corner compensation, formation of suspended structures) are shown also. The obtained results show that level set method can be used as an effective tool for wet etching process modeling, and that is a viable alternative to the Cellular Automata method which now prevails in the simulations of the wet etching process.

Maskless etching with convex corner compensation in the form of a 〈1 0 0〉 oriented beam is investigated using both experiments and simulations. The maskless convex corner compensation technique is defined as a combination of masked and maskless anisotropic etching of {1 0 0} silicon in 25 wt% TMAH water solution at a temperature of 80 °C. This technique enables fabrication of three-level micromachined silicon structures with compensated convex corners at the bottom of the etched structure. All crystallographic planes that appear during etching are determined and their etch rates are used to calculate the etch rate value in an arbitrary crystallographic direction necessary for simulation by an interpolation procedure. A 3D simulation of the profile evolution of the etched structure during masked and maskless etching of silicon based on the level set method is presented. All crystallographic planes of the etched silicon structures determined in the experiment are recognized in the corresponding simulated etching profiles obtained by the level set method.

A maskless convex corner compensation technique in a 25 wt% TMAH water solution at the temperature of 80 °C is presented and analyzed. The maskless convex corner compensation technique is defined as a combination of masked and maskless anisotropic etching with convex corner compensation in the form of a 〈1 0 0〉 oriented beam. This technique enables the fabrication of three-level micromachined silicon structures with compensated convex corner at the bottom of the etched structure. All the planes that appear during the etching of (1 0 0) silicon in the 25 wt% TMAH water solution at the temperature of 80 °C are determined. Analytical relations have been found to explain the etching of all exposed planes and to calculate their etch rates. Analytical relations are determined and empirically verified in order to obtain regular shapes of the three-level silicon mesa structures. A boss for a low-pressure piezoresistive sensor has been fabricated as an example of the maskless convex corner compensation technique.

Silicon bulk micromachining using the wet anisotropic etching process is widely employed for the development of commercial products such as an inkjet printer head, a pressure sensor, accelerometers, infrared sensors, etc using (1 0 0) silicon wafers. In wet anisotropic etching, the resultant shape and size of the microstructures are restricted by crystallographic properties of silicon. If structures such as seismic mass in an accelerometer are required to be created, convex corners will emerge in the etching process. Considerable deformation occurs at convex corners resulting in poor control on the shape and size of the microstructure. Various methods/techniques are developed to overcome the problem of undercutting at convex corners in a (1 0 0) silicon wafer. Here, we have reviewed the fabrication techniques for the realization of convex corners in silicon bulk micromachining technology. The review is restricted to the wet anisotropic etching process which is usually performed in potassium hydroxide solution, ethylenediamine pyrocatechol solution, tetramethylammonium hydroxide, etc. The corner compensation method is the most widely used technique for the fabrication of convex corners. Various types of corner compensating design have been proposed by different research groups. The corner compensation method gives nearly sharp corners. Recently developed techniques, which do not use any compensating design, give perfect convex corners. The limitations and advantages of all the techniques have been discussed.

Due to the corner rounding effect in litho process, it is hard to make the wafer image as sharp as the drawn layout near two-dimensional pattern in IC design 1, 2 . The inevitable gap between the design and the wafer image make the two-dimensional pattern correction complex and sensitive to the OPC correction recipe. However, there are lots of different two-dimensional patterns, for example, concave corner, convex corner, jog, line-end and space-end. Especially for Metal layer, there are lots of jogs are created by the rule-based OPC. So OPC recipe developers have to spend lots to efforts to handle different two-dimensional fragment with their own experience. In this paper, a general method is proposed to simplify the correction of two-dimensional structures. The design is firstly smoothed and then simulation sites are move from the drawn layer to this new layer. It means that the smoothed layer is used as OPC target instead of the drawn Manhattan pattern. Using this method, the OPC recipe tuning becomes easier. In addition, the convergence of two-dimensional pattern is also improved thus the runtime is reduced.

We combine experiment, theory and simulation to design and fabricate 3D structures with protected edges and corners on Si{1 1 0} using anisotropic wet chemical etching in 25 wt% tetramethylammonium hydroxide (TMAH) at 71 °C. In order to protect the convex corners formed by <1 1 2 > and <1 1 0 > directions, two methods are considered, namely, corner compensation and two-step etching. The mask design methodology for corner compensation is explained for various microstructures whose edges are aligned along different directions. The detailed geometry of each compensation pattern is shown to depend on the desired etch depth. The two-step wet etching process is explored in order to realize improved sharp convex corners. Using the same etchant concentration and temperature, the second etching is carried out after mask inversion from silicon nitride (Si3N4) to silicon dioxide (SiO2), obtained by local oxidation of silicon (LOCOS) followed by nitride etching. Based on the use of the continuous cellular automaton (CCA), the simulation results for both corner undercutting and two-step etching show that the CCA is suitable for the analysis and prediction of anisotropic etching on Si{1 1 0} wafers.

Undercutting is a common problem in wet anisotropic etching. This problem in turn, influences the performance and sensitivity of MEMS devices. This paper investigates the use of corner compensation to prevent convex corner undercutting in a MEMS piezoresistive accelerometer. The Intellisuite CAD simulation software was used for designing the mask with corner compensation and for analysing wet anisotropic etching profiles in potassium hydroxide (KOH) and tetra-methyl-ammonium-hydroxide (TMAH) solutions at different concentrations and temperatures. Perfect 90 degrees corners on the proof mass was successfully etched using a corner compensation design at etching temperature of 63 °C for KOH and 67.7 °C for TMAH with 25 wt% and 10.3 wt% concentration levels, respectively. Etching in TMAH required lower concentration level, thus making the etching process safer. However, TMAH required longer time to etch perfect convex corners compared to KOH. Nevertheless, both KOH and TMAH etchants have been successfully used to etch perfect convex corners by using the designed corner compensation mask.