Method for forming thin silicon membrane or beam

Abstract: A method is disclosed for forming thin, suspended membranes of epitaxial silicon material. Silicon oxide strips (12) having a predetermined thickness are first formed on a silicon substrate (10). A gap (14) provided between adjacent pairs of strips (12) is preferably less than or equal to about 1.4 times the thickness of the silicon oxide strips (12). The underlying silicon substrate (10) is exposed within these gaps (14) in the silicon oxide layer, whereby the gaps (14) provide seed holes for subsequent epitaxial growth from the silicon substrate (10). Epitaxial silicon is grown through the gaps (14) and then allowed to grow laterally over the silicon oxide strips (12) to form a continuous layer (20) of epitaxial silicon over the silicon oxide strips (12). The backside of the silicon substrate (10), i.e., the surface opposite the surface having the silicon oxide strips (12), is then masked to delineate the desired diaphragm and microbridge pattern. The silicon is etched conventionally from the backside of the silicon substrate (10). Etching is terminated substantially automatically by the presence of the silicon oxide strips (12). The thin, single crystal silicon membranes thus formed are suitable for use as diaphragms and microbridges in a microaccelerometers or a pressure sensor.

Description: This invention generally relates to silicon microsensor fabrication techniques. More particularly, this invention relates to the formation of a thin silicon membrane or beam suitable for use in a microaccelerometer or pressure sensor using selective epitaxial growth, epitaxial lateral overgrowth and micro-machining techniques as specified in the preamble of claim 1, for example as disclosed in US-A-4,670,969. An accelerometer is one of the major sensors used in navigational systems, particularly inertial navigational systems, and on-board automotive safety control systems. Automotive examples of accelerometer use include various anti-lock braking systems, active suspension systems, and seat belt lock-up systems. Generally, an accelerometer is a device which measures acceleration and, in particular, an accelerometer measures the force that is exerted when a moving body changes velocity. The moving body possesses inertia, which causes the body to resist the change in velocity. It is this resistance to a sudden change in velocity that is the origin of the force which is exerted by the moving body when it is accelerated. This force is proportional to the acceleration component in the direction of the movement, and therefore may be detected by an accelerometer. In a typical accelerometer, a mass is suspended by two springs attached to opposite sides of the mass. The mass is maintained in a neutral position so long as the system is at rest or is in motion at a constant velocity. When the system undergoes a change in velocity in the direction of the axis of the springs or perpendicular to the axis of the springs, and therefore is accelerated in a particular direction, the spring-mounted mass will at first resist the movement along that axis because of its inertia. This resistance to the movement, or delay in the movement, will force the springs to be temporarily either stretched or compressed. The tensile or compressive force acting on each spring is related to the product of the weight of the mass and the acceleration of the mass. The acceleration is then correspondingly determined by the change in velocity experienced by the mass. Integrated circuit microaccelerometers having a proof mass suspended by pairs of microbridges are also known. An illustrative example of this type of accelerometer is disclosed in U.S. Patent No.4,901,570. In a microaccelerometer of this type, a proof mass is suspended by at least two pairs of microbridges. Each pair of microbridges is attached to opposite ends of the proof mass along a common axis. The acceleration of the mass is determined by the change in force acting upon each microbridge. This type of resonant microaccelerometer is attractive for precision measurements, because the frequency of a micromechanical resonant structure can be made highly sensitive to physical or chemical signals. A difficulty exists with regard to the manufacturing of these and other types of microaccelerometers. The microbridges are typically formed from extremely thin layers of material, generally silicon, which are suspended over the silicon substrate. These thin layers, or beams, are difficult to manufacture. Similarly, an alternative type of sensor, a pressure sensor, is characterized by a solid thin diaphragm, wherein the same difficulties are experienced in trying to obtain the thin diaphragm. Many techniques have been used to micromachine these components, however there are shortcomings associated with these previous methods. One common method has been to use an etchant, whereby the rate of etching is dependent upon the doping concentration of the silicon, to etch the silicon substrate and form the thin layers required for the membrane or microbridges. With this method a P+ buried layer which is disposed under an N-type epitaxial layer acts as an etch stop when the silicon wafer is etched from the backside. However, this method is problematic since a relatively thick layer of the N-type silicon epitaxy is required over the P+ buried layer so as to ensure a sufficient amount of the device quality epitaxy remains after etching. This relatively thick layer of epitaxy directly affects the thickness and thus the sensitivity of the silicon microstructure. It would be desirable to provide a method for micromachining the silicon which would eliminate the P+ etch stop, thereby eliminating the corresponding limit on the thickness of the silicon microstructure imposed by the quality of the overlaying epitaxial layer. Another method used to micromachine silicon microaccelerometers is to use a biased electrochemical etch wherein a biased junction acts as the etch stop. A biased junction is formed at the interface between a P-type silicon substrate and an overlaying N-type epitaxial layer. The biased junction serves as an etch stop when the silicon substrate is etched from the backside since the N-type epitaxial layer is passivated when exposed to the silicon etch. However, this method requires that electrical contact be made to each silicon wafer during etching of the silicon. Typically, there are many of these silicon microsensors formed on the silicon substrate, requiring constant electrical contact to each of the individual microsensors. Needless to say, maintaining these individual contacts is difficult in the harsh silicon etch environment. Therefore, it would be desirable to provide a method for micromachining the silicon wafer which does not require electrical contacts to be made. It would also be desirable to provide a method for machining the silicon which provides more uniform dimensional control when forming the thin silicon layers, rather than silicon-etch techniques which are relatively difficult to control. This is particularly desirable for automotive applications, where extremely large numbers of the sensors would be produced with the requirement that they all be dimensionally consistent. A method in accordance with the pre-amble of claim 1 is disclosed in US-A-4670969. Therefore, what is needed is a method for forming thin silicon membranes and beams which are suitable for use in a microaccelerometer or a pressure sensor which alleviates the above-mentioned detriments. In particular a method is needed which utilizes an etch stop technique which does not unduly affect the dimensional characteristics of the microaccelerometer or pressure sensor. A method for forming a thin silicon membrane according to the present invention is characterised over US-A-4670969 by the features specified in the characterising portion of claim 1. It is an object of the present invention to provide a method for forming thin silicon membranes and beams. It is a further object of this invention that such a method use epitaxial growth and micromachining techniques which utilize anisotropy and a silicon oxide etch stop. It is still a further object of the present invention that such a method for forming thin silicon membranes and beams should be useful for forming a microaccelerometer which has a proof mass suspended by thin silicon microbridges or for forming a pressure sensor which has a large, thin diaphragm. In accordance with a preferred embodiment of this invention, these and other objects and advantages are accomplished as follows. A method is disclosed for forming thin, suspended membranes or layers of single crystal silicon suitable for use as microbridges and diaphragms in microaccelerometers and pressure sensors. A layer of silicon oxide is first formed on a silicon substrate and then patterned. In the regions where the diaphragms and microbridges are desired, the silicon oxide is patterned so as to form flat, narrow strips of silicon oxide. The beams have a predetermined thickness, or height. The gap, or spacing, between adjacent silicon oxide strips is preferably less than or equal to about 1.4 the thickness of the silicon oxide strip, however this is not essential. The underlying silicon substrate is exposed within these gaps in the silicon oxide layer, thereby the gaps provide a seed hole for subsequent epitaxial growth from the silicon substrate. Epitaxial silicon is grown through the seed holes and then allowed to grow laterally over the silicon oxide strips to form a continuous layer of epitaxial silicon over the silicon oxide strips. The backside of the silicon substrate, i.e., the surface opposite the surface having the silicon oxide strips, is then masked to delineate the desired diaphragm and microbridge pattern. The silicon is then etched conventionally from the backside. An inventive feature of this method is that the silicon oxide strips act as etch stops to prevent further etching of the epitaxial silicon, thereby protecting the continuous layer of epitaxial silicon overlying the silicon oxide strips. In addition, anisotropic etching of the silicon within the gap regions terminates automatically within the gap at or below the level of the top surface of the silicon oxide when the width of the gap is less than or equal to about 1.4 times the thickness of the silicon oxide strips. The silicon is removed by the anisotropic etch, terminating on intersecting crystallographic planes within the gap. When the width of the gap is greater than about 1.4 times the thickness of the silicon oxide strips, the etching process does not terminate automatically at the level of the top surface of the silicon oxide and requires close control so as not to harm the thin, overlaying, continuous layer of epitaxial silicon. With this method, suspended regions of single crystal silicon are formed. The suspended regions are characterized by a continuous layer of epitaxial silicon overlaying the silicon oxide strips. The suspended regions are suitable for use as microbridges or diaphragms within a microaccelerometer or pressure sensor. The invention and how it may be performed are hereinafter particularly described with reference to the accompanying drawings, in which: Figures 1 to 7 are cross-sectional views depicting the various steps involved in a preferred method of the invention for forming suspended regions of silicon. Figures 8 to 10 are cross-sectional views depicting an alternative method of the invention for forming suspended regions of silicon. Figures 11 and 12 are cross-sectional and plane views respectively of a microaccelerometer having suspended regions of silicon formed in accordance with the preferred method of the present invention. Thin, suspended membranes of epitaxial silicon are formed using the methods illustrated in the drawings. These thin, suspended layers are suitable for use as a microbridge or diaphragm in a microaccelerometer or pressure sensor. As shown in Figure 1, a layer of silicon oxide is first formed on a silicon substrate 10 and then patterned so as to form flat, narrow strips 12 of the silicon oxide in those regions where a diaphragm or microbridge are desired. The silicon oxide strips 12 have a predetermined thickness, or height. The gap 14, or spacing, between adjacent strips 12 is preferably less than or equal to about 1.4 times the thickness of the silicon oxide strip 12, however this is not essential. The underlying silicon substrate 10 is exposed within these gaps 14 in the silicon oxide layer 12, and each of the gaps 14 provides a seed hole for subsequent epitaxial growth from the silicon substrate 10. As shown in Figure 2, epitaxial silicon 16 is grown from the underlying substrate 10 through the seed hole at each gap 14 between the silicon oxide strips 12. It is preferred that the underlying silicon substrate 10 be single-crystal-oriented along the 〈100〉 crystallographic plane for optimal subsequent etching and processing results, however suitable results are also obtained when the underlying silicon substrate 10 is oriented along the 〈110〉 crystallographic plane. As shown in Figure 3, the epitaxial silicon 16 within the gap 14 is continuously grown laterally over the silicon oxide strips 12 to form a layer 18 of epitaxial silicon over the silicon oxide strips 12. As shown in Figure 4, a continuous layer 20 of the epitaxial silicon is formed by allowing the lateral growth of the epitaxial silicon to continue until merged. In order to form the continuous layer 20 of epitaxial silicon, the single crystal epitaxial silicon is grown from the underlying substrate 10 through the seed hole at each gap 14 and then laterally over the silicon oxide strips 12. The preferred epitaxial growth conditions are as follows: an appropriate mixture of dichlorosilane and hydrogen chloride gases in a hydrogen carrier gas at a temperature varying between about 900°C to about 950°C, and a low pressure, i.e., less than approximately 19,998 Pa (150 Torr), in either a barrel or pancake-type epitaxy reactor. The preferred gas mixture for forming N-type epitaxial silicon is characterized by a mass flow ratio of the dichlorosilane to hydrogen chloride to flowing hydrogen of about one to three to 100; a specific example being a dichlorosilane mass flow of about 0.22 standard litres per minute (slm), a hydrogen chloride mass flow of about 0.59 slm and hydrogen flowing at about 60 slm at a temperature of about 920°C to about 950°C. These exemplary processing parameters result in the N-type epitaxial silicon being grown at about a rate of 0.1 micrometres per minute. The N-type epitaxial silicon is preferred because it results in the most controlled dimensions during subsequent processing and etching of the silicon, however other dopant species may be introduced into the epitaxial silicon with satisfactory results, if desired. A preferred thickness for the selective epitaxial growth silicon is about 1 micrometre. Therefore about 10 minutes at these processing conditions is required to produce the epitaxial silicon to this preferred thickness within each gap region 14, however further exposure to these processing conditions is required to form the laterally grown, continuous layer 20 of epitaxial silicon overlaying the silicon oxide strips 12. About 5 micrometre to 10 micrometre thickness is preferred since a membrane of this thickness will be highly sensitive and responsive to movement in a microaccelerometer or pressure sensor, yet relatively durable also. Theoretically, almost any membrane thickness may be formed with this method. For continuous epitaxial silicon films 20 laterally grown in this manner, an aspect ratio of about 1 to 1 has been obtained. Therefore, it is preferred that the silicon oxide strips 12 be not wider than twice the desired aspect ratio so that the laterally grown epitaxial silicon 18 may form a continuous layer 20 over the silicon oxide strips 12. This assumes that seed holes in the gaps 14 to the underlying substrate 10 are provided on each side of the silicon oxide strip 12 in those regions where a continuous layer 20 of epitaxial silicon is desired. As shown in Figure 5, the backside of the silicon substrate 10, i.e., the surface opposite the surface having the silicon oxide strips 12 and overlaying epitaxial film 20, has a layer of silicon oxide 22 grown on it which is then conventionally masked and patterned to delineate those regions where the subsequently formed suspended layers of epitaxial silicon are desired. As shown in Figure 6, the silicon substrate 10 is etched conventionally from the backside to remove the substrate 10 material under the desired suspended regions. It is preferred that a wet etch, such as potassuim hydroxide (KOH) or ethylenediamine pyrocatechol (EDP), be used initially to etch the silicon substrate 10 until the silicon oxide strips 12 are exposed, as shown in Figure 6 and enlarged in Figure 7. As shown in these Figures, the width of the gap 14 between silicon oxide strips 12 is less than about 1.4 times the thickness of the silicon oxide strips 12, whereby the etching process within the gap region 14 will terminate automatically. A second etching step using either plasma or reactive ion-etching techniques could be used to etch the beams from the front side since these techniques result in a more controlled etching process than the wet electrochemical etch, however these methods are also much slower and more time-consuming than the wet-etching technique and therefore are not desirable for the first etching step for removal of the bulk of the silicon substrate 10. An inventive feature of this invention is that the silicon oxide strips 12 act as etch stops to prevent further etching of the epitaxial silicon 18. In addition, as stated above, etching of the epitaxial silicon 18 within the gaps 14 between silicon oxide strips 12 terminates automatically within the gap 14 when the width of the gap 14 is less than 1.4 times the thickness of the silicon oxide strip 12, as shown in Figures 6 and 7. This is because the epitaxial silicon is etched anisotropically; terminating along the 〈111〉 crystallographic plane, or more simplistically, the silicon is removed by the anisotropic etch along crystallographic planes 24, as shown in Figure 6 and enlarged in Figure 7. Within the gap 14, these crystallographic planes 24 intersect, and thereby prevent subsequent removal of epitaxial silicon. Suspended regions 30 of the continuous layer 20 of epitaxial single crystal silicon are formed with this method, as shown in Figure 6. The suspended regions 30 are useful as thin membranes or diaphragms for use in microaccelerometers or pressure sensors. As shown in Figure 8, if desired for a microaccelerometer, a gap 114 between adjacent silicon oxide strips 112 may be greater than twice the thickness of the silicon oxide strips 112. If so, then a second layer of silicon oxide 126 may be deposited over epitaxially-grown, continuous silicon layer 120 prior to etching from the backside of silicon substrate 110 through silicon oxide masking layer 122 as shown in the cross-sectional view in Figure 8. This second overlaying oxide layer 126 acts as the etch stop for removal of epitaxial silicon 120 within the gap region 114, as shown in Figure 9. With this alternative method, the overlaying layer of silicon oxide 126 may then subsequently be removed as shown in Figure 10 or may be left as it is depending on the desired final characteristics of the suspended membrane. It should be noted that, in Figure 10, suspended regions 128 are attached to the substrate 110 at regions not shown in the particular cross-section depicted in the illustration. With this general method, suspended regions of epitaxial single crystal silicon are formed. The suspended regions are characterized by a continuous layer of epitaxial silicon overlaying the silicon oxide strips. The suspended regions are suitable for use as diaphragms or microbridges within a microaccelerometer or a pressure sensor. As an illustrative example of how the suspended regions of epitaxial silicon may be used in a microsensor, a microaccelerometer having thin regions of suspended silicon, or microbridges, is discussed. This invention may also be use to form a thin single crystal silicon diaphragm in a pressure sensor. An illustrative example of a microaccelerometer 200 having a suspended proof mass in the form of membrane 230 attached to a substrate 210 by two pairs of microbridges 220 and 222, is shown in Figure 11 and cross-sectionally in Figure 12. The microbridges 220 and 222 may be formed with this method. The microaccelerometer 200 measures the components of acceleration in the plane of the silicon substrate 210. Preferably the silicon microaccelerometer 200 comprises two pairs of microbridges 220 and 222 which are orthogonally attached to the silicon proof mass membrane 230. In order to cancel temperature and material effects to first order response of the microaccelerometer 200, the microbridge elements 220 and 222 which are attached to opposite sides of the proof mass membrane 230 should be matched, so that they experience differential axial loads during acceleration. The inertial force on the proof mass membrane 230, due to acceleration in the plane of the substrate 210, generates the differential axial loads on the opposing microbridges in each pair 220 and 222 thereby causing a corresponding change in the piezoresistors formed in the beams. The method for forming thin single crystal silicon membranes disclosed herein provides several advantages. The thickness and width of a membrane may be controlled to very small dimensions (such as even less than one micrometre), and therefore the sensitivity of any resulting device is increased. In addition, the thickness of the membrane is uniform and predictable since the thickness is determined by the crystal growth rate, i.e., about 0.1 micrometres per minute, thereby enhancing the reproducibility and reliability of any resulting device. Lastly, with this method there is great flexibility in the design parameters of an accelerometer or pressure sensor or other device having a thin silicon region, since the dimensions of the suspended membrane or microbridges may be increased or reduced rather easily. Although this invention discloses a preferred method for forming suspended layers of epitaxial silicon, which are suitable for use in a microaccelerometer or a pressure sensor, it is to be understood that various modifications and changes may be made in the processing parameters, use and construction without departing from the concept of the invention, such as by varying the conductivity type of epitaxial silicon grown and accordingly the gas composition within the epitaxial reactor or by modifying the processing parameters and materials within the scope of the art. For this reason, these variations are properly considered within the scope of this invention and accordingly the scope of the present invention is to be limited only by the following claims.

Claims: A method for forming a thin silicon membrane comprising the steps of providing a silicon substrate (10;110) which is oriented essentially along a single crystallographic plane having front and back surfaces which are substantially parallel to each other; forming a blanket layer of silicon oxide over said front surface thereof; patterning said blanket layer of silicon oxide so as to form a plurality of apertures (14;114) in the blanket layer of silicon oxide connecting with said silicon substrate (10;110) depositing a crystalline layer (20;120) of silicon on said blanket layer of silicon oxide; masking said back surface of said silicon substrate (10;110) so as to expose a desired pattern of said back surface of said silicon substrate (10;10); removing exposed silicon from said back surface of said silicon substrate (10;110) in said desired pattern; and terminating said removing step for the exposed silicon at said silicon oxide layer, characterised in that the patterning step forms a plurality of silicon oxide strips (12;112) in spaced relationship with each other, said silicon oxide strips (12;112) being spaced from one another so that the underlying silicon substrate (10;110) is exposed at gaps (14;114) disposed between adjacent pairs of said strips (12;112); monocrystalline silicon is selectively epitaxially deposited onto said silicon substrate (10;110) exposed at said gaps (14;114) and laterally from said gaps (14;114) over said silicon oxide strips (12;112) so as to form said crystalline layer (20;120) of silicon as a substantially continuous layer (20;120) of epitaxial silicon over said silicon oxide strips (12;112); and the terminating step forms suspended regions (30;128) containing said epitaxial silicon overlaying said silicon oxide strips (12;112). A method for forming a thin silicon membrane according to claim 1, in which each of said silicon oxide strips (12) is characterized essentially by said predetermined thickness of the blanket layer of silicon oxide; said silicon oxide strips are spaced from one another so that the width of each gap (14) is less than or equal to about 1.4 times said predetermined thickness of said silicon oxide strips (12); single crystal epitaxial silicon is selectively grown on said silicon substrate (10) up through said gaps (14) by vertical epitaxial deposition and then laterally over said silicon oxide strips (12) so as to form said substantially continuous layer of epitaxial silicon (20); and said removing step substantially terminates automatically at said silicon oxide strips (12) and within said gaps (14) so as to form said suspended regions (30) containing said epitaxial silicon overlaying said silicon oxide strips (12). A method for forming a thin silicon membrane according to claim 1 or 2, in which said silicon substrate (10) is oriented along the 〈100〉 crystallographic plane. A method for forming a thin silicon membrane according to claim 3, in which said epitaxial silicon is characterized by N-type conductivity. A method for forming a thin silicon membrane according to claim 1, in which said silicon oxide strips (112) are spaced from one another so that the width of each gap (114) is more than about 1.4 times said predetermined thickness of said silicon oxide strips (112); single crystal epitaxial silicon is selectively grown on said silicon substrate (110) up through said gaps (114) by vertical epitaxial deposition and then laterally over said silicon oxide strips (112) so as to form said substantially continuous layer (120) of epitaxial silicon over said silicon oxide strips (112); and the method includes forming a second blanket layer (126) of silicon oxide over said substantially continuous layer (120) of epitaxial silicon; whereby said removing step is substantially terminated at said second blanket layer (126) of silicon oxide so as to form said suspended regions (128) containing said epitaxial silicon overlaying said silicon oxide strips (112). A method for forming a thin silicon membrane according to claim 5, in which said silicon substrate (10;110) is substantially a single crystal oriented along the 〈100〉 crystallographic plane.

Inventors: DE ROO DAVID WILLIAM, LOGSDON JAMES HOUSTON, NEUDECK GEROLD WALTER, STALLER STEVEN EDWARD, DE ROO, DAVID WILLIAM, LOGSDON, JAMES HOUSTON, NEUDECK, GEROLD WALTER, STALLER, STEVEN EDWARD
Applicants: DELCO ELECTRONICS CORP, PURDUE RESEARCH FOUNDATION, DELCO ELECTRONICS CORPORATION
Application year: 1991
Main classification: G01L9
IPC codes: G01P15, H01L29, G01L9, H01L21