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Method for manufacturing a silicon sensor and a silicon sensorRelated Patent Categories: Semiconductor Device Manufacturing: Process, Manufacture Of Electrical Device Controlled PrintheadMethod for manufacturing a silicon sensor and a silicon sensor description/claimsThe Patent Description & Claims data below is from USPTO Patent Application 20060046329, Method for manufacturing a silicon sensor and a silicon sensor. Brief Patent Description - Full Patent Description - Patent Application Claims
CROSS-REFERENCE TO A RELATED APPLICATION [0001] This application is a Divisional application of application Ser. No. 10/472,465, filed on Nov. 24, 2003, which is the national phase under 35 U.S.C. .sctn.371 of PCT International Application No. PCT/F102/00241 which has an International filing date of Mar. 21, 2001, which designated the United States of America. BACKGROUND OF THE INVENTION [0002] 1. Field of the Invention [0003] The invention is particularly intended for use in the manufacture of silicon-based acceleration and angular velocity sensors. [0004] 2. Description of Background Art [0005] As to its basic principles, silicon micromechanics utilizes so-called planar technology, that is, a kind of thin-film technique and the manufacturing methods developed thereon. Micromechanical fabrication techniques are tailored by combining basic processes in different fashions, adjusting their parameters to suite the intended application and designing desired structures that are implemented by transferring the layout patterns onto substrates, generally in a plurality of manufacturing steps. Among others, the basic processes include the deposition of different kinds of thin films, etching the same and modifying their properties by way of, e.g., controlled heat-treatments. [0006] The basic processes of thin-film technology include patterning of thin films and, more generally, transfer of patterns into structures using techniques in which patterning and etch processes are integrated with each other. The most commonly employed patterning technique is based on the use of a photosensitive resist applied on the surface of a wafer or other substrate by spraying on or rotation (spinning) of the wafer. After the steps of baking the resist, exposing the patterns by means of ultra-violet light from a mask onto the substrate and, in the case of a positive resist, removal of the exposed areas of the resist by rinsing with a developer, the patterns thus formed on the deposited mask layer can be further transferred into the underlying layers of the wafer. The latter operation is performed using a sacrificial process known in the art as etching that, according to the nature of its working mechanism, is called either a wet etch process or a dry etch process. [0007] Wet etch takes place in an etch solution, whereby the sacrificial material dissolves into the liquid etch solution, typically forming a plurality of different intermediate compositions. Most ones of the etch processes are isotropic, which means that their etch rate is the same in different orientations. However, monocrystalline silicon may also be etched using an anisotropic etch solution, whereby the etch rates in the different crystal orientation of the substrate material can be widely different from each other. This feature is utilized in conventional silicon micromechanics for forming precision structures aligned according to the crystal orientations. Anisotropic etching of silicon is also characterized by a very uniform depth of the etched volume and a smooth surface structure of the etched surface. Typically, the surface roughness of the etched volume is in the order of a tenth of one per cent. [0008] In a dry etch process, the reactive components of the gas-phase atmosphere react on the wafer surface so as to form volatile compounds, whereby the solid material is converted into a gaseous form. Typically, the reactor chamber is operated under a partial vacuum, whereby the gaseous reaction products are removed by pumping therefrom. [0009] In wet etch techniques, pattern transfer is implemented using inorganic thin-film hard masks in addition to conventional resist masks. This procedure is typical of, e.g., anisotropic etching of silicon that is carried out in a concentrated alkaline solution. Since a photoresist cannot stay intact in such circumstances, conventionally oxide or nitride hard masks are used as the actual etch masks. Hereby, these masks of an inorganic material are first patterned using a photoresist in combination with dry or wet etch processes. [0010] Three-dimensional micromechanical structures can be fabricated either by utilizing the entire thickness of the substrate, whereby the technique is known as bulk micromechanics, or, alternatively, by forming the structures to be released by etching so that they are located substantially in the surface layers of the substrate or a thin-film layer deposited onto the surface of the substrate. The latter technique is called surface micromechanics. Bulk micromechanical structures are fabricated by using a thin-film patterning process to fabricate the etching masks onto the wafer surfaces and then transforming the patterns by means of different etch processes into the substrate itself. [0011] Dry etching is a term commonly used in conjunction with gas-phase etch processes in order to make a distinction from wet etch processes carried out in a liquid environment. The normal dry etch method is to use a plasma discharge, typically a corona discharge in a gas atmosphere under a partial vacuum, whereby the discharge is excited by an electric AC field or, less frequently, a DC field. The basic techniques of controlling an etch process are to adjust the composition of the gas atmosphere, its pressure, the excitation power of the plasma discharge and the geometry of the plasma etch chamber. Etching is terminated either at the lapse of a predetermined etch time, by measuring the impedance of the plasma discharge or using an optical etch end point system based on monitoring the emission of the plasma discharge. Also the optical detection of structures becoming visible from the monitored specimen can be used for determining the end point of sufficient etching. [0012] As process gases are used such that can react with the material to be etched so as to form gaseous reaction products. These include, e.g., sulfur hexafluoride (SF.sub.6), tetrafluoromethane (CF.sub.4) and chlorine (Cl.sub.2). In the plasma, the gas dissociates partially, whereby the reactive fluorine, chlorine or other radicals can react with the substrate being etched. The plasma process may be modified by introducing into the reaction chamber along with the reactive gas some inert gas or gases such as argon (Ar) or helium (He). These gases serve to stabilize the etch reaction or improve the thermal conductivity of the process atmosphere. The properties of the plasma discharge may further be controlled by adding certain gases such as oxygen (O.sub.2) that are capable of altering the reaction equilibrium. Thereby it is possible for instance to elevate the level of free fluorine in certain reactions thus increasing the etching rate of the material being removed or, alternatively, to control the sidewall contour in the layer being etched when a photoresist mask is used. A third group of agents affecting the plasma process are passivating or polymerizing gases such as formaldehyde (CHF.sub.3) or octafluorocyclobutane (C.sub.4F.sub.8). [0013] To etch deep trenches in silicon, a particular dry-etch method is used based on pulsed or alternating etch cycles. Herein, an almost isotropic etch is applied in alternating step for rapidly machining into silicon patterns as deep as 1 .mu.m. Subsequently, the substrate is subjected to a passivation step, whereby all the surfaces of the substrate including those just etched are covered by a polymer layer deposited from plasma gas. The next step of a rapid isotropic etch phase punches the polymer layer at the bottom of the pattern trenches thus deepening the patterns by one increment. Simultaneously, the polymer layer deposited on the sidewalls of the pattern protect them from further etching. The etch gas is typically sulfur hexafluoride, while the passivating gas is octafluorocyclobutane. This succession of alternating etch/passivation steps is continued until the desired depth of etched pattern is attained. By means of this technique it is possible to etch narrow slots through the entire thickness of a silicon wafer so that the aspect ratio, that is, the depth-to-width ratio of the etched trench can attain values from 10 to 40. Such a dry etch process is very anisotropic particularly in regard to the surface being etched. As to the behavior of etch masks, the method gives in a routine fashion excellent selectivity defined as the ratio of the etch rate of the substrate to the etch rate of the mask material. Using an oxide mask, etch selectivity ratios as high as 200 to 300 can be attained, while even a resist mask may reach etch selectivity ratios in the range of 50-100. Typically, a conventional unpulsed plasma etch process can provide an etch selectivity ratio which is only one-tenth of these values. The etch surface roughness values herein are generally in the order of a few per cent. [0014] Etch processes that are anisotropic in regard to the surface orientation of the substrate being etched, including such processes as anisotropic dry etch processes (wherein the etch rate in an orientation perpendicular to the substrate surface is much faster than in the an orientation parallel to the substrate surface), facilitate the fabrication of openings extending in almost undefined shapes as deep as even through the entire silicon wafer with sidewalls that are substantially vertical. However, the instantaneous etch rate due to the inherent nature (ARDE, Aspect Ratio Dependent Etching) of the method is dependent on the geometry of the opening being made. Resultingly, it becomes imprecise or even impossible to fabricate narrow-aspect slots of a desired depth or, e.g., thin spring elements in the central plane of a wafer. [0015] Spring elements located in the central plane of a wafer can be fabricated by using anisotropic etch processes in combination with isotropic etch processes (the latter having equal etch rate in all orientations). However, a good isotropic etch process has not yet been disclosed for silicon. Only the use of XeF.sub.2 as isotropic etch gas has been reported in the art [Esashi et al.] [0016] Single-crystal silicon wafers having their surfaces substantially aligned parallel to the {100} planes of the crystal structure can be used for fabricating spring elements located in the central plane of the wafer by using a wet etch process (typically based on a potassium hydroxide solution) that etches anisotropically along the different crystal orientations of silicon. This method gives a good surface quality of the spring element and a uniform etch rate. [0017] One of the shortcomings of the prior art has been that dry etching as a single process has not been suitable for fabricating spring elements of high dimensional precision in the central plane of a silicon wafer. On the other hand, wet etching requires a large wafer area in the fabrication of deep structures by etching. The use of conventional wet etching techniques in the manufacture of elongated spring elements in particular makes the spring elements to be surrounded by large-area holes in which the etched surfaces are formed by {111} crystal planes. By the same token, it is extremely difficult to control the dimensional precision of the spring elements particularly as to their width that in conventional manufacturing decreases at a rapid rate when the end value of the spring element thickness is approached during the etch process. Even the smallest variations in the manufacturing process or in the initial thickness of the wafer can cause a large deviation in the end width of the spring element. Hence, it may become impossible to attain desired spring element qualities using this kind of geometry in the cross section of the spring element. [0018] Acceleration sensors may also be manufactured based on multilayer structures of the SOI type comprising a set of superposed layers: thick silicon substrate--dielectric layer--thin silicon layer--dielectric layer--thick silicon layer. A drawback of this technique is that it involves a complicated and expensive manufacturing process. SUMMARY AND OBJECTS OF THE INVENTION [0019] It is an object of the present invention to overcome the problems of prior-art techniques and to provide an entirely novel manufacturing method and a silicon sensor based thereon. [0020] The goal of the invention is achieved by way of using a dry etch process for making the through-the-wafer etchings and then finalizing the high-precision structures located in the central plane of the wafer by means of a wet etch process. Advantageously, the straight sidewalls of spring elements in a sensor structure, as well as the straight portions of angled spring legs or tangents of the curved portions thereof are askew away from the <110> orientation of the silicon substrate, advantageously by at least 15.degree., preferably by 45.degree. which is equivalent to the <100> orientation, or, if the shape of the spring element is different from that of a rectangular parallelepiped, having the sidewalls or tangential planes of the spring elements askew approximately by 45.degree. from the <110> orientation. In the case silicon wafers are used having a differently cut orientation, the general idea is to minimize the areas of inclined surfaces oriented in the {111} crystal planes within the volume occupied by the spring elements. [0021] It is a feature of the invention to combine the advantages of two different kinds of etch processes. Continue reading about Method for manufacturing a silicon sensor and a silicon sensor... Full patent description for Method for manufacturing a silicon sensor and a silicon sensor Brief Patent Description - Full Patent Description - Patent Application Claims Click on the above for other options relating to this Method for manufacturing a silicon sensor and a silicon sensor patent application. ###  How KEYWORD MONITOR works... a FREE service from FreshPatents1. Sign up (takes 30 seconds). 2. Fill in the keywords to be monitored. 3. Each week you receive an email with patent applications related to your keywords. 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