MINIATURIZED PRESSURE SENSOR

Abstract: Miniaturized pressure sensor for physiological in situ measurements, and having a shell (5), and elastic diaphragm (2), an optical fibre (4), a silicon body (5) inside the shell, and a reflecting surface (6) on the silicon body. The body has a thin, cantilevering, short beam structure (10) arranged for actuation by the diaphragm (12) for moving the reflecting surface (6) in a direction across the end surface of the optical fibre (4). The thickness of the beam structure is in the order of magnitude 5-30 mu m.

Description: Miniaturized Pressure Sensor The present invention relates to a miniaturized pressure sensor with a shell of an eleastic material, an elastic diaphragm arranged over an opening situated in the surface of the shell, a light conductor arranged inside the shell, a silicon body arranged inside the shell and a reflecting surface arranged on the body such as to be opposite of the light conductor. A device of the kind mentioned in the introduction is known from our Swedish patent specification 86 02836-2. There is a need of still further miniaturzing this known pressure trans¬ ducer, but problems then occur which are directly related to the small dimensions. The known pressure transducer is a so-called relative pressure transducer, which means that it measures the pressure dif¬ ference between ambient pressure and a reference pressure. The reference pressure is in contact with the atmospheric pressure via an air duct inside the shell. This air duct has a constriction formed by a silicon body situated in the region of the diaphragm. The problem with the known pressure transducer is that the resistance of the air duct to volume changes caused by the diaphragm moving is not sufficiently low, and for rapid pressure changes this leads to the occurrence of a certain excess pressure in the distal air volume nearest the diaphragm, this air volume being limited on one side by the constriction. This excess pressure falsifies the measuring result. The present invention has the object of providing a minia¬ turized pressure sensor of the kind described in the introduc¬ tion, which allows maintenance of atmospheric pressure in the distal air volume nearest the diaphragm in spite of the diaphragm moving quickly. Another object of the invention is to achieve a pressure sensor of the kind described in the introduction, and which has reduced compliance. Compliance is herein defined as volume change per pressure change. In other words, a small deflection or movement of the diaphragm shall provide a small volume change. This can be accomplished by either reducing the deflection or the size of the diaphragm. In accordance with the invention,, the above-mentioned objects are achieved by the body comprising a thin, cantilevering, short beamlike structure of silicon arranged for actuation by the diaphragm, and by the reflecting surface being arranged at the free end of the cantilever part of the beam structure, and upstanding from it, such that the pressure changes acting on the diaphragm will move the beam structure and thereby the reflecting surface, which then will move in front of the end of the light conductor or optical fibre. By making the beam structure short, in accordance with the invention, and thereby decreasing the deflection of the diaphragm for an applied pressure, an acceptable signal-to- noice ratio is retained, while at the same time the pressure sensor has great sensitivity to rapid pressure changes. By reducing the opening over which the diaphragm is arranged, in accordance with the invention, and thereby the volume change per pressure change, there is obtained the advantage that the diaphragm will be stiffer and afford good protection for the sensor. However, there is an adhesion effect resulting in the tendency for the diaphragm to stick to the beam struc¬ ture when the structure moves. This gives rise to an undesired hysteresis in the pressure measuring result. In accordance with another embodiment of the invention, the adhesion effect associated with a reduced opening is avoided by the cantilevering beam structure having a hump on its back part situated under the diaphragm, this hump projecting into the opening in the shell surface of the sensor. Two embodiments of the invention will now be described in more detail and with reference to the accompanying drawings, where Figure 1 illustrates a miniaturized pressure sensor according to the state of the art as appears in the above- mentioned Swedish patent, Figure 2 is a cross-sectional view of a miniaturized pressure sensor in accordance with the present invention, Figure 3 is a perspective view of an assembly means for ensuring a free path for the volume of air deciding reference pressure, Figure 4 is a cross-sectional view of a miniaturized pressure sensor having a reduced area of an opening for a diaphragm, and Figure 5 is a cross-sectional view of a miniaturized pressure sensor in accordance with a second embodiment of the present invention. The known pressure sensor includes a shell 1 of elastic material, and an elastic diaphragm 2 arranged over an opening 3 situated in the shell surface. A light conducor 4 and a silicon body 5 are arranged inside the shell. A reflecting surface 6 is arranged opposite the end surface of the light conductor 4. The amount of reflected light in the light conduc¬ tor 4 is responsive to the amount with which the ambient pressure P deflects the diaphragm 2 and thereby the end surface of the light conductor or optical fibre 4 in relation to the reflecting surface 6. As will be seen from Figure 1, the silicon body 5 forms a constriction between the air volume 7 at the distal end of the diaphragm and the air duct 8 inside the proximal part of the shell. This constriction is a disadvantage in miniaturization. In addition, it is different from one sensor to the next, due to reasons concerned with manufacturing techniques. Figure 2 illustrates the pressure sensor in accordance with the present invention. The silicon body 5 is provided with a thin, cantilevering, short beam structure 10 arranged for actuation by the diaphragm 2. The structure is produced from single crystalline silicon by anisotropic etching of the 100- plane or the Ill-plane, which have a lower etching rate than surrounding crystal planes. A vertical mirror can be etched in this way at the end of the beam structure. By selective etching, the thickness of the beam can be defined within the area of magnitude from about 5 to about 30 μm. By using single crystalline silicon the beam structure is provided with extremely good bending properties, and since it can have the mentioned thickness its length can be made less than the length of the free end of the optical fibre in the embodiment according to Figure 1. The length of the complete sensor can thus be reduced in this way. The silicon body 5 is cemented to the light conductors or optical fibre 4, which is in turn cemented to the shell 1. Selective etching means that the etching rate is dependent on doping. The silicon can be p-doped or p-doped silicon can be n-doped. Different etching liquids can be used, inter alia postassium hydroxide. The silicon body illustrated in Figure 3 is used to provide a well-defined air duct and for facilitating assembly of the optical fibre in the sensor. The body 5 is substantially quadratic in cross section and is formed partly by the beam structure 10, which is provided at its end with the reflecting surface 6, and partly by an upwardly open elongate box with a bottom wall 11 and two side walls 12, 13. The two side walls are each provided with an upwardly open cutaway part, of which only one, 14, is shown. These parts 14 only extend a short distance along the length of the side walls, this distance corresponding to the length of the cantilevering part of the beam structure 10. The beam structure is joined to the box by an atomic bonding process (see Tenerz, Hϋk, Electronic Letters, May 22, 1986, Vol. 22, No. 11, pages 615-616) or anodic bonding by deposit¬ ing a glass layer in the joint. When the sensor is assembled, the box 10-13 is inserted in the shell 1 up to the position corresponding to the one in Figure 2, the beam structure then being fixed to the shell, e.g. by cementing, or in some other way. The light conductor or optical fibre is then inserted in the end opening of the box in the manner illustrated in Figure 3. The position of the fibre relative the reflecting surface is easy to determine with great precision since the bottom, side and roof walls of the box serve as guides. It will be understood that the air duct mentioned above is formed between the optical fibre and teh four walls 10-13. These walls are smooth and have well defined sizes, in comparision with the rather random constric¬ tions which can occur in the embodiment according to Figure 1 in connection with the body 5 being cemented in the shell. Although not described in more detail above, the side walls 12, 13 and bottom wall are either made from flat wafers which are joined to each other, or the walls 11, 12 and 13 are made integral using such a selective etching procedure of the kind mentioned above. Figure 4 illustrates a miniaturized pressure sensor similar to the one in Figure 2. In the embodiment according to Figure 4, the area of the opening 3 has been reduced in relation to the one in Figure 2, with the object of increasing the comp¬ liance of the sensor. This embodiment further affords the advantages that the diaphragm 2 will be stiffer compared with the one in Figure 2, and therefore provide good mechanical protection to the sensor. However, there is the disadvantage that an adhesion effect occurs, i.e. the diaphragm tends to stick to the beam structure 5 in the region denoted by the arrow 15. This falsifies the measuring result. Figure 5 illustrates a miniaturized pressure sensor in accor¬ dance with a second preferred embodiment of the invention. Here as well, the opening 3 for the diaphragm 2 has a reduced area compared with the embodiment in Figure 2. Here, the diaphragm is substantially flat and does not bulge down into the opening as in Figure 2. The beam structure 10 of the body 5 has a hump 16 on its back part in the region of the latter situated under the opening 3, this hump projecting into the opening 3 such as to be in contact with the diaphragm 2. This configuration results in that the adhesion effect no longer occurs or is at least considerably reduced. The upper surface of the hump is flat and its contour, seen from above in relation to Figure 5 is somewhat less than the contour of the opening so that the hump does not jam in the wall of the opening 3 when the beam structure moves. The hump 16 is preferably made integral with the body and is preferably produced simultaneously with the beam structure 10 by selective etching of the silicon as described above. This embodiment of the body 5 can also have the form of a box similar to the one with open end walls, as illustrated in Figure 3, the beam structure 10 being provided with the hump 16 on its back. It should be noted that in all the embodiments of the invention described above the beam structure 10 is only in contact with the diaphragm 2, and is not cemented to the diaphragm. The above described embodiments of the invention can be modified in many ways and varied within the scope of the inventive concept.

Claims: CLAIMS 1. Miniaturized pressure sensor for physiological in situ measurements including: a shell (1) of an elastic material, an elastic diaphragm (2) arranged over an opening (3) situated in the surface of the shell, a light conductor (4) arranged inside the shell, a body (5) of silicon or gallium arsenide arranged inside the shell, a reflecting surface (6) arranged on the body, such that the reflecting surface is arranged opposite the end surface of the light conductor, characterized in that the body has a thin, cantilevering, short beam structure (10) arranged for actuation by the diaphragm, in that the reflecting surface is arranged at the free end of the cantilevering beam structure, perpendicularly extending therefrom, such that pressure changes acting on the diaphragm will move the silicon or gallium arsenide beam structure and thereby the reflecting surface, which then will move in front of the end of the optical fibre. 2. Miniaturized pressure sensor as claimed in claim 1, characterized in that the beam structure (10) has a hump (16) on its back, thin hump projecting into the opening (3). 3. Miniaturized pressure sensor as claimed in claim 1 or 2, characterized in that the body (5) has a through opening intended to accommodate the light conductor (4). 4. Miniaturized pressure sensor as claimed in claim 3, characterized in that the through opening accommodates the light conductor with play such as to form an air duct between the air volume situated in the sensor region in front of the body (5) and the air volume which is situated inside the shell behind the body (5). 5. Miniaturized pressure sensor as claimed in claim 4, characterized in that the body (5) is cemented to the inside of the shell. 6. Miniaturized pressure sensor as claimed in claim 1, 2 or 3, with the body produced from silicon, characterized in that the cantilevering beam structure (10) is formed by selective etching of a single crystaline silicon in the 100- plane or the Ill-plane. 7. Miniaturized pressure sensor as claimed in claim 6, characterized in that the beam structure is from about 5 to about 30 μm thick and about 0.3-0-6 mm long. 8. Miniaturized pressure sensor as claimed in claim 5, characerized in that the exterior diameter of the shell in the distal part of the sensor is in the order of magnitude less than 0.5 mm. 9. Miniaturized pressure sensor as claimed in claim 1, characterized in that the body (5) is substantially quadratic in cross section. 10. Miniaturized pressure sensor as claimed in claim 9, characterized in that the body has the form of an elongate box, the top wall of which is formed by the cantilevering beam structure (10), and the side and bottom walls of which are formed by flat silicon wafers, the side walls (12, 13) having upwardly open cutaway parts (14), which extend a distance along the length of the " side walls, said distance corresponding to the free length of the cantilevering beam structure (10). 11. Miniaturized pressure sensor as claimed in claim 10, characterized in that the beam structure is joined to the box by atomic or anodic bonding.

Inventors: HOEK BERTIL, TENERZ LARS, HOEK, BERTIL, TENERZ, LARS
Applicants: RADI MEDICAL SYSTEMS
Application year: 1990
Main classification: A61B5
IPC codes: A61B5, G01L9