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DEEP WET AND DRY ETCHING OF PYREX GLASS

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DEEP WET AND DRY ETCHING OF PYREX GLASS: A REVIEW CIPRIAN ILIESCU1, KWONG LUCK TAN1, FRANCIS E.H. TAY1,2, and JIANMIN MIAO 3 1Institute of Bioengineering and Nanotechnology, Singapore 2Department of Mechanical Engineering, National University of Singapore, Singapore 3 Micromachines Center, School of MPE, Nanyang Technological University, Singapore E-mail: [email protected] ABSTRACT This paper is a review of wet and dry etching of one of the most common types of glass: Pyrex. The paper analyzes the methods for increasing the glass etch rate in HF solutions, namely, annealing, concentration, ultrasonic agitation and temperature. The limitations of the wet etching of glass are also presented. Mashing layers commonly used for deep wet etching of glass are analyzed, in terms of the process time required until the defects are generated in the masking layer. The improvement of the surface roughness for deep wet etching of Pyrex glass is another subject that will be explored. The highest etch depth – 500 μm – of annealed Pyrex glass is achieved by wet etching in highly concentrated HF solution, using Cr/Au with the assistance of photoresist as a masking layer. The paper also reports a new technique of dry etching of Pyrex glass in C4F8 using bulk silicon mask. Keywords: glass etching, masking layer, 1. INTRODUCTION Glass is a widely used material in MEMS and biochip device fabrication. The classical piezoresistive pressure sensor chip is bonded on a glass substrate. A microgyroscope that is vacuum-sealed between two structured glasses at the wafer level is presented in [1]. A micro-XY-stage actuator made with glass has been reported in [2]. Most biochips are fabricated on a glass substrate due to its optical transparency and biocompatibility. A microflow of cells for the single molecule handling of DNA is presented in [3]. A microPCR for DNA amplification is shown in [4], while dielectrophoresis is represented in [5]. Three major groups of techniques are used in glass etching: mechanical, dry and wet. Mechanical methods include traditional drilling, ultrasonic drilling, electrochemical discharge or powder blasting. However, smooth surfaces cannot be generated using such methods. The dry etching technique of glass has been reported in [6] using SF6. However, the etching rate is relatively low. Wet etching is the most common method. The type of masking layer that should be used depends on the application and "thermal budget" of the fabrication process of the device. Photoresist is very often used as the mask layer [7-9], but its area of application is limited. A commonly used mask is Cr/Au [10, 11], where the Cr layer is used to improve the adhesion of gold to glass. Bu et al [11] reported the etching of a 500µm-thick glass wafer using multilayers of metal, Cr/Au/Cr/Au, in combination with a thick SPR220-7 photoresist, by etching from both sides of the wafer. Another commonly used mask material for glass etching is silicon – PECVD (amorphous silicon) [9,12], LPCVD (polysilicon) [10,13] or even bulk silicon [14] – all of which are deposited using different methods. The maximum reported depth was 320 µm by Bien et al [9], using a mask of polished polysilicon and SU8 as the mask. 2. EXPERIMENTS AND MATERIALS We focused our experiments on one of the most popular types of glass, Corning 7740. Two of its main properties make it suitable for microfabrication. Firstly, Corning 7740 is bondable to silicon and its thermal coefficient of expansion is similar to silicon. This can result in low residual stress being induced in the MEMS structure. Secondly, it contains a low concentration of oxides that gives insoluble products in HF solution. As presented in [15], these insoluble products can act as a micromasking layer and generate rough surfaces, or can drastically reduce the etch rate. The technique used for the stress characterization of the layers deposited on glass is described in [12]. This technique consists of the deposition of a thin reflective layer (50 nm Cr) on both sides of the wafer. The stress induced by the deposition on one side of the wafer is compensated by the stress introduced by the same Cr layer on the opposite surface. The experiment was performed on 500-µm thick, unannealed and annealed 4” glass wafers. The annealing was performed at 560OC in a N2 environment for 6 hours. The wafers were first cleaned in piranha solution (H2SO4/H2O2 2/1), then rinsed in DI water and spun-dried. The wet etching of the glass wafers was conducted in a sealed Teflon container with slow magnetic stirring. A silicon wafer was bonded to the glass wafer using wax to protect the side of the glass wafer that is uncovered by the masking layer. Dry etching was performed on an ICP system (Adixen) using SF6, CHF3, CH4 gases.3. WET ETCHING OF PYREX GLASS 3.1 Etch rate An important factor in the deep wet etching of glass is the etch rate. In some cases of wet process, the selectivity of the etching is the preferred parameter of the process. In the wet etching of glass, some materials used as masking layers (mainly silicon and gold) are inert in the HF-based etchant. The etching process is limited by the defects of the masking layer and the penetration of the etchant through these defects. For this reason, a fast etch rate of glass will lead to a deeper etching, while the defect generation will be maintained at the same rate each time. The main solution used for glass etching is based on HF. The etch rate is characteristic for each type of glass, especially due to the different oxides and compositions used during fabrication. The etch rate is determined by the concentration of HF enchants. To achieve a high etch rate, a standard (maxim) concentration of 49% should be used. Figure 1 presents the influence of HF concentration on the etch rate for Corning 7740 Pyrex glass. It should be noted that by increasing the HF concentration from 40% to 49%, a rapid increase of 50-60% of the etch rate can be achieved (4.4 µm/min to 7.6 µm/min for non-annealed glass). The annealing process has a strong influence on the etch rate of glass. Each type of glass has its optimum annealing point. The annealing influence is also presented in Figure 1. A similar variation was noted, but the increase in the etch rate was from 9.1 µm/min to 14.3 µm/min when the HF concentration was increased from 40% to 49%. We can conclude that annealing is an important parameter process not only for the


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