Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01P—MEASURING LINEAR OR ANGULAR SPEED, ACCELERATION, DECELERATION, OR SHOCK; INDICATING PRESENCE, ABSENCE, OR DIRECTION, OF MOVEMENT
  • G01P15/00—Measuring acceleration; Measuring deceleration; Measuring shock, i.e. sudden change of acceleration
  • G01P15/02—Measuring acceleration; Measuring deceleration; Measuring shock, i.e. sudden change of acceleration by making use of inertia forces using solid seismic masses
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01P—MEASURING LINEAR OR ANGULAR SPEED, ACCELERATION, DECELERATION, OR SHOCK; INDICATING PRESENCE, ABSENCE, OR DIRECTION, OF MOVEMENT
  • G01P15/00—Measuring acceleration; Measuring deceleration; Measuring shock, i.e. sudden change of acceleration
  • G01P15/02—Measuring acceleration; Measuring deceleration; Measuring shock, i.e. sudden change of acceleration by making use of inertia forces using solid seismic masses
  • G01P15/08—Measuring acceleration; Measuring deceleration; Measuring shock, i.e. sudden change of acceleration by making use of inertia forces using solid seismic masses with conversion into electric or magnetic values
  • G01P15/0802—Details
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01P—MEASURING LINEAR OR ANGULAR SPEED, ACCELERATION, DECELERATION, OR SHOCK; INDICATING PRESENCE, ABSENCE, OR DIRECTION, OF MOVEMENT
  • G01P15/00—Measuring acceleration; Measuring deceleration; Measuring shock, i.e. sudden change of acceleration
  • G01P15/02—Measuring acceleration; Measuring deceleration; Measuring shock, i.e. sudden change of acceleration by making use of inertia forces using solid seismic masses
  • G01P15/08—Measuring acceleration; Measuring deceleration; Measuring shock, i.e. sudden change of acceleration by making use of inertia forces using solid seismic masses with conversion into electric or magnetic values
  • G01P2015/0805—Measuring acceleration; Measuring deceleration; Measuring shock, i.e. sudden change of acceleration by making use of inertia forces using solid seismic masses with conversion into electric or magnetic values being provided with a particular type of spring-mass-system for defining the displacement of a seismic mass due to an external acceleration
  • G01P2015/0822—Measuring acceleration; Measuring deceleration; Measuring shock, i.e. sudden change of acceleration by making use of inertia forces using solid seismic masses with conversion into electric or magnetic values being provided with a particular type of spring-mass-system for defining the displacement of a seismic mass due to an external acceleration for defining out-of-plane movement of the mass
  • G01P2015/0825—Measuring acceleration; Measuring deceleration; Measuring shock, i.e. sudden change of acceleration by making use of inertia forces using solid seismic masses with conversion into electric or magnetic values being provided with a particular type of spring-mass-system for defining the displacement of a seismic mass due to an external acceleration for defining out-of-plane movement of the mass for one single degree of freedom of movement of the mass
  • G01P2015/0828—Measuring acceleration; Measuring deceleration; Measuring shock, i.e. sudden change of acceleration by making use of inertia forces using solid seismic masses with conversion into electric or magnetic values being provided with a particular type of spring-mass-system for defining the displacement of a seismic mass due to an external acceleration for defining out-of-plane movement of the mass for one single degree of freedom of movement of the mass the mass being of the paddle type being suspended at one of its longitudinal ends

Definitions

• the present invention relates to an accelerometer of the type compensating for the effect of gravity. Compensating for the effect of gravity on the seismic mass of an accelerometer improves its sensitivity to a variation in acceleration.
• the invention is particularly applicable to devices of small dimensions. Its nature means that this accelerometer can be developed by technologies relating to mechanics, micromechanics or microelectronics (for example micromachining).
• the main field of application of an accelerometer according to the present invention is the study of the movement or of the behavior of environments subjected to gravity (seismology for example).
• the invention therefore makes it possible to design accelerometers of the monolithic type with compensation for the effect of gravity, whereas accelerometers of the monolithic type according to the known art do not allow this compensation.
• Such accelerometers according to the known art are for example described in the article "Development of micromachined silicon accelerometer" by M. Ueda, H. Inada, Y. Mine and K.
• An acceleration sensor of the most common type therefore consists of an inert mass, or seismic mass, generally supported by one or more springs. When this mass undergoes variations of acceleration, this one moves and the springs are deformed. The system returns to its initial position when the force due to acceleration is canceled.
• the current devices have a large mass M.
• this solution limits the miniaturization of the whole.
• the high-performance devices for example those capable of detecting a variation of a few nano-G under 1 G
• the smallest therefore weigh several kilograms and occupy a volume of a few tens of cm- ⁇ .
• the current vertical axis acceleration sensors are either not very sensitive, or heavy and bulky.
• Miniaturization of an efficient device requires reducing the seismic mass M and therefore increasing the quality factor.
• This can be obtained by making the entire sensor (the seismic mass and the spring in the case of gravity compensation by a spring), in a material having a high quality factor such as, for example, monocristralline silicon.
• the production of a compact device comprising a spring secured to the mass poses technological difficulties. Indeed, it is difficult to make small mechanical parts, such as the spring and the seismic mass, integral by mechanical means such as screws or by glue without causing areas where internal friction is high and cause damping phenomena detrimental to the quality factor. It is also necessary to preserve a great flexibility in the spring, this flexibility influencing the sensitivity of the sensor.
• the gravity force compensation technique proposed by the present invention is similar to the family of sensors whose seismic mass is supported by a spring. This solution has the advantage of reducing parasitic noise produced by the servo system necessary for other devices.
• the proposed compensation technique is based on the principle of a leaf-shaped spring produced by prestressing a surface of an element (a beam for example) supporting the seismic mass.
• the subject of the invention is therefore an accelerometer comprising a seismic mass capable of being subjected to a force induced by an acceleration to be measured, the seismic mass being connected to a support by means of mechanical connection capable of bending under the effect of said force, detection means being provided to enable the acceleration to be determined from the force induced in the seismic mass, compensation means being provided to compensate for the force exerted on the seismic mass by gravity, characterized in that the mechanical connection means comprise at least one part constituting said compensation means by inducing, in these mechanical connection means, a prestress opposing the force exerted on the seismic mass by gravity.
• the means of compensating for the force exerted on the seismic mass by gravity can be constituted by a surface layer deposited on the mechanical connection means, this surface layer being produced on one face of the mechanical connection means so as to oppose the force exerted by gravity on the seismic mass and being made of a material having the properties required to exert stress on said mechanical connection means.
• said material having a stress can be chosen from the group of materials constituted by chromium, molybdenum, tungsten, one of their alloys or a ceramic of the PZT type.
• the means for compensating for the force exerted on the seismic mass by gravity can be constituted by two surface deposits which have the mechanical connection means, these surface deposits being produced on two opposite faces of the mechanical connection means, one of the two deposits being made of a material inducing a tensile stress and the other deposit inducing a compressive stress, the combination of these two deposits inducing a stress gradient in the thickness of the mechanical connection means opposing the force exerted on the seismic mass by gravity.
• the two deposits can consist of thin layers of molybdenum deposited by different techniques so as to give them constraints of opposite signs.
• the means for compensating for the force exerted on the seismic mass by gravity can be constituted by a surface modification of the mechanical connection means, this surface modification inducing said prestress in the mechanical connection means.
• This surface modification can advantageously consist of surface doping of the material constituting the mechanical connection means.
• the mechanical connection means being made of monocrystalline silicon
• one of the faces of the mechanical connection means can be doped with a dopant chosen from the group consisting of phosphorus, boron, xenon, titanium, arsenic and argon.
• the surface doping can be carried out on two opposite faces of the mechanical connection means, each of these two faces being doped by different dopants, one of the dopants inducing a stress in extension and the other inducing a stress in compression with for result to induce a stress gradient in the thickness of the mechanical connection means s Opposing the force exerted on the seismic mass by gravity.
• one of the faces of the mechanical connection means can be doped with boron and the other face can be doped with argon.
• the mechanical connection means can consist of one or more beams.
• the invention therefore also relates to a method of manufacturing an accelerometer comprising a seismic mass connected to a support by mechanical connecting means capable of bending under the effect of a force induced in the seismic mass by an acceleration to be measured, compensation means being provided to compensate for the force exerted on the seismic mass by gravity, characterized in that it comprises the following steps: - masking of one of the main faces of a substrate, or first face, to delimit the seismic mass and the support there,
• FIG. 1 is a side view of an accelerometer compensating for the effect of gravity according to a first variant of the invention
• FIG. 2 is a partial perspective view of an accelerometer compensating for the effect of gravity according to a second variant of the invention
• FIG. 3 is a partial perspective view of an accelerometer compensating for the effect of gravity according to a third variant of
• FIG. 1 invention - Figure 4 is a top view of a support beam of a seismic mass coated with a constrained deposit and etched for an accelerometer according to the invention
• FIG. 5 is a perspective view of an accelerometer according to the invention in progress.
• accelerometers comprising a seismic mass distinct from its mechanical connection element to a support, this connection element possibly consisting of one or more beams. This is not limitative of the invention which also applies to the case where the seismic mass is not distinguished from the beam or is confused with the beam.
• Figure 1 illustrates quite schematically the principle of one invention for an accelerometer with vertical sensitive axis. There is shown a seismic mass 1 of mass M, connected to a support
• the seismic mass 1 is therefore cantilevered relative to the support 2.
• the center of gravity of the seismic mass 1 is sensitive to the force F at which is subject to the seismic mass following an acceleration g-
• the upper surface part 4 of the beam 3 is treated to induce a prestress which results in exerting on the mass seismic a force compensating for that induced by gravity in this seismic mass.
• This surface stress of the beam can be obtained in different ways. It can be obtained by depositing a thin layer on the surface of the beam or by modifying the surface of this beam large enough to produce a stress effect. Thus treated, the beam tends to bend by exerting a force opposite to that of gravity. The stressing conditions must be such that the end of the beam exerts a compensating force directed upwards as shown by the curved arrow in FIG. 1. The effort then allows, if the intensity of the stress is sufficient , to balance the force due to gravity.
• the usable deposition material can be chosen from metals or their alloys or even from materials known to present stresses such as certain piezoelectric materials, for example the ceramic of formulation Pb (Zr x Tij_ x ) ⁇ 3 also called PZT.
• the ceramic of formulation Pb (Zr x Tij_ x ) ⁇ 3 also called PZT.
• chromium, molybdenum, tungsten or one of their alloys is used.
• modification of the beam means the modification of the material constituting the beam itself, in the surface area, by an appropriate means.
• This means can be doping the surface of the beam over a small thickness.
• the beam is made of monocrystalline silicon, it is possible to use a dopant chosen from the following elements: phosphorus, boron, xenon, titanium, arsenic, argon.
• the solution proposed by the invention therefore makes it possible to design a spring linked to a seismic mass, forming an assembly which can be of small dimensions, without having recourse to a mechanical connection made of separate elements.
• Figure 1 illustrated an accelerometer in its simplest version, that is to say comprising a single beam.
• the invention applies to accelerometers with several beams.
• the principle of the invention can be applied to accelerometers comprising two, four or eight beams to limit the degrees of freedom (in translation and in rotation around the center of gravity) of the seismic mass in vertical displacement. These beams are then arranged in opposition two to two or four to four as appropriate.
• FIG 2 shows such an accelerometer.
• the seismic mass 10 is connected to the support 11, which has been leveled off at the level of the seismic mass for the sake of simplification, by four beams 12 arranged in opposition two by two.
• FIG. 3 represents, in the same mode of illustration as FIG. 2, an accelerometer with eight beams.
• This accelerometer can be obtained by sealing two structures 21 and 22 each comprising four beams (that is to say each being of the type shown in FIG. 2).
• This sealing can be carried out by known bonding or sealing methods, for example by the method described in the article "Application of oxygen plasma processing to silicon direct bonding "by O. Zucker, W. Langheinrich, M. Kulozik and H. Goebel, published in the journal Sensors and Actuators A. 36, 1993, pages 227-231.
• Superficial deposits must act to compensate for the force of gravity, these deposits 23, 24 are made on the faces of the beams to be directed upwards once the accelerometer is finished.
• a first solution consists in carrying out a discontinuous deposition on the beam as shown in FIG. 4 which is a top and partial view of an accelerometer according to the invention.
• FIG. 4 is a top and partial view of an accelerometer according to the invention.
• a second solution consists in exploiting the intrinsic anisotropy of the constraints of certain thin metallic layers to orient the maximum force in the most favorable direction, that is to say between the seismic mass and the support.
• a third solution consists in making one or more longitudinal slots in the beam, these slots having a direction going from the seismic mass to the support.
• the means for compensating for the gravitational force consist of a surface layer deposited on the beam, this layer would be split, preferably over its entire thickness, according to a pattern resembling that of FIG. 4.
• the surface stress can be exerted by a deposit or by a treatment which makes it possible to create a stress gradient in the thickness of the beam.
• the use of thin films offers the possibility of designing a two-layer system, ie, with reference to FIG. 1, a thin layer 4 and a thin layer 5 placed opposite each other on the beam.
• the upper thin layer must act in tension and the lower thin layer in compression.
• Certain materials produced in a thin layer (molybdenum for example) have constraints of opposite signs according to their production conditions.
• an accelerometer with four beams of the type represented in FIG. 2.
• This sensor is manufactured in silicon with orientation ⁇ 100>.
• the silicon is covered with a mask which can be a layer of silicon nitride Si3N4- Using the conventional photolithography methods, an opening in a zone delimiting the seismic mass is produced in the mask.
• Anisotropic etching of the silicon is then carried out, for example in a potassium hydroxide KOH bath (cf. for example the article "Develoment of micromachined silicon accelerometer” cited above). The etching time is expected to be long enough to obtain a thin silicon membrane around the seismic mass.
• FIG. 5 represents the result obtained at the end of this step of the method.
• the starting substrate 40 is shown in section.
• the section passes through the seismic mass 41 and gives an idea of the thickness of the membrane 42 remaining around the seismic mass 41.
• the face 43 of the substrate 40 located on the side of the membrane is then covered with a layer of silicon oxide Si ⁇ 2 • As previously, this coating is open so as to delimit the sides of the beams and the periphery of the seismic mass. Then, a physical etching technique (plasma etching) in a gaseous mixture of boron trichloride BCI3 and chlorine CI2 makes it possible to remove the silicon from the membrane which is not masked. After this operation, the seismic mass is released from the structure and is no longer supported except by the beams. The remaining silicon oxide layer is removed by plasma etching in a gaseous mixture of trifluoromethane CHF3 and oxygen O2 •
• plasma etching in a gaseous mixture of trifluoromethane CHF3 and oxygen O2 •
• the thin stress layer is then deposited on the blade by magnetron sputtering.
• the parameters for producing the metal film are adjusted by so as to produce tension stresses in the material.
• this type of structure can be made of silicon or quartz thanks to micro-machining techniques used in micro-electronic technologies.
• This manufacturing method allows the realization of a monolithic silicon assembly (that is to say that all the parts of the sensor are machined in a solid substrate) whose quality factor is high. It is then possible to design a lower mass and therefore a more compact structure.
• the force intended to balance the force of gravity is strongly affected by the shape of the beam.
• the inventors of the present invention have found that a rectangular shape of the bimetallic strip (when the beam is seen in the direction of the force of gravity) is less satisfactory for compensating for the force of gravity than a triangular shape whose base of the triangle is embedded in the support and the top of which is integral with the seismic mass.
• the accelerometer Given the simplicity of making the accelerometer according to the present invention, it is possible to envisage collective manufacture of the sensors and therefore to reduce their cost price.
• the invention can in particular be implemented to produce seismometers of small dimensions so that they can be introduced into a wellbore.

Landscapes

• Physics & Mathematics (AREA)
• General Physics & Mathematics (AREA)
• Measurement Of Mechanical Vibrations Or Ultrasonic Waves (AREA)
• Geophysics And Detection Of Objects (AREA)
• Pressure Sensors (AREA)

Priority Applications (4)

Applications Claiming Priority (2)

Publications (1)

Family

ID=9479980

Family Applications (1)

Country Status (6)

Cited By (2)

Families Citing this family (13)

Citations (6)

Family Cites Families (3)

• 1995
  • 1995-06-14 FR FR9507077A patent/FR2735580B1/fr not_active Expired - Fee Related
• 1996
  • 1996-06-13 DE DE69617890T patent/DE69617890T2/de not_active Expired - Lifetime
  • 1996-06-13 US US08/776,845 patent/US5922955A/en not_active Expired - Fee Related
  • 1996-06-13 WO PCT/FR1996/000904 patent/WO1997000451A1/fr not_active Ceased
  • 1996-06-13 EP EP96922919A patent/EP0776476B1/fr not_active Expired - Lifetime
  • 1996-06-13 JP JP50271897A patent/JP4139436B2/ja not_active Expired - Fee Related

Patent Citations (6)

Non-Patent Citations (1)

Also Published As

Similar Documents

Legal Events