WO2003014667A1 - Method and device for trimming sensors with oscillating structures - Google Patents

Abstract

The invention relates to a method for trimming sensors with oscillating structures according to which a sensor element (14) that has an oscillatory structure for detecting a measured quantity is machined using a laser beam (17a) in order to remove material in a specific manner, whereby at least one resonance frequency of the sensor element (14) and/or an imbalance of the sensor element (14) are/is adjusted. During removal of the material, the sensor element oscillates and a state change is instantaneously measured. A femtosecond laser (17) is used for generating the laser beam (17a). A device (10) for trimming sensors has a measuring device (13) for determining a resonance frequency and/or an imbalance of the sensor element (14). Said device also has a laser (17) provided with a control device (18) for the specific removal of material from the sensor element, and has a comparison device in order to compare a measured value, which represents the current resonance frequency and/or the current imbalance, with a given value.

Description

 Method and device for trimming sensors with vibrating structures

11. The present invention relates to a method and a device for trimming sensors with vibrating structures according to the preamble of claims 1 and 11, respectively.

Sensors with structures capable of oscillation are suitable for measuring various physical quantities such as rotation rates or accelerations. In order to obtain the highest possible measurement accuracy, the sensors must be manufactured very precisely.

Such sensors, however, often cannot show the ideally expected properties due to manufacturing tolerances. For this reason, attempts are made to improve the sensor properties by making slight corrections to the geometry or by removing mass or by changing the mechanical tension of thin layers.

Known methods for this are, for example, mechanical trimming by milling or grinding. However, these methods have the disadvantage of high tool consumption. They are also very time consuming.

JP 09287956 A therefore suggests trimming an oscillating structure of a sensor by means of a laser in order to set a resonance frequency. This is intended to improve the measuring sensitivity of the sensor and to make the sensor compact and light.

However, even with this known method there is the general problem that a high expenditure of time is required in order to detect the individual sensors. to be trimmed precisely and precisely adjusted in their sensor properties. Another problem is that when the frequency is set using a laser trimmer, an imbalance of the sensor element can occur or an existing unbalance is often changed in an uncontrolled manner.

The article "MICROMACHINING OF SEMICONDUCTORS WITH FEMTOSECOND LASERS" by H. K. Tönshoff et.al, Published on Proceedings of ICALEO 2000, Dearborn (USA) describes the ablation process when using femtosecond lasers. Silicon structures are processed with a femtosecond laser.

The object of the invention is to enable a contactless and wear-free adjustment of mechanical properties of sensors, with which a high throughput and a high degree of automation can be achieved and the time expenditure and the associated cost expenditure can be reduced.

This object is achieved by the method for trimming sensors according to patent claim 1 and by the device for trimming sensors according to patent claim 11.

Further advantageous features and details of the invention result from the dependent claims, the description and the drawings.

In the method according to the invention for trimming sensors with oscillating structures, a sensor element which has an oscillatable structure for recording a measured variable is processed with a laser beam in order to remove mass in a targeted manner, at least one resonance frequency of the sensor element and / or imbalance of the sensor element being compared , where- After the mass has been removed, the sensor element is measured with regard to the resonance frequency and / or the unbalance.

The method according to the invention allows mechanical properties of sensors to be adjusted in a contactless and wear-free manner, a high throughput and a high degree of automation being made possible at the same time.

The method according to the invention considerably reduces the time required for the exact setting of the sensor properties, thereby saving costs. Natural frequencies and imbalances of the sensors or sensor elements can be set specifically and independently of one another.

A femtosecond laser, which generates laser pulses in the femtosecond range, is advantageously used to generate the laser beam. These measures in particular avoid thermal coupling. The material does not melt during processing. There is no thermal coupling, since the exposure time is extremely short. H. no change in the material properties is caused. In addition, there is no mechanical stress caused by the removal process. In addition, there is little or no material deposition in the vicinity of the site of removal. Furthermore, the beam path does not necessarily have to be in a vacuum.

The adjustment is preferably carried out at the wafer level. This results in even more effective time and cost savings. This means that online measurement and trimming at the wafer level of sensors with vibrating structures is possible, which results in lower processing costs, higher throughput and thus lower costs. Furthermore, a preselection and a first function test of the sensor elements on the wafer level is possible using a test method that only uses standard equipment, such as needle probes, lasers, etc Furthermore, the sensors can be pre-classified with regard to selected sensor properties. In the case of rotation rate sensors, this is done, for example, with regard to the sensitivity by setting the frequency difference between an excitation mode of an oscillation and the readout mode, and with regard to the zero point stability, which is dependent on the balancing of the unbalance.

The sensor is preferably made of silicon. He can e.g. B. a micromechanical rotation rate sensor, the sensor element being, for example, a tuning fork. With a tuning fork-shaped sensor element, e.g. Resonance frequencies of the prongs of the tuning fork structure can be set using laser ablation. The process according to the invention is preferably carried out under vacuum conditions.

The method is preferably carried out in such a way that at least one resonance frequency of the sensor element is determined and this at least one resonance frequency is changed by laser ablation or laser ablation until a predetermined value for the at least one resonance frequency or a predetermined difference between two resonance frequencies is reached.

It is also possible to carry out the method in such a way that the tines of a tuning fork are excited to vibrate at a first resonance frequency fz and at a rate of rotation a torsional vibration is caused due to the Coriolis force, which has a second resonance frequency ft, with the removal of at least one tine first resonance frequency fz is increased in order to set a predetermined difference value between the two resonance frequencies. In the method, an unbalance of the sensor element is advantageously determined and the unbalance is compared by laser ablation until a sensor signal caused by the unbalance becomes minimal.

According to another aspect of the invention, a device for trimming sensors with vibrating structures is created, which comprises a measuring device for determining a resonance frequency and / or an unbalance of the sensor element, and a laser with a control device for the targeted removal of mass of the sensor element, and a Comparison device to compare a measured value, which represents the current resonance frequency and / or the current unbalance, with a predetermined value. With this device, the method according to the invention can be carried out quickly and effectively in a cost-effective manner. The particular advantages that result from this have already been mentioned above in connection with the method according to the invention.

The laser is advantageously a femtosecond laser. The device according to the invention preferably comprises a vacuum chamber for receiving a sensor element to be trimmed or a wafer with a plurality of sensor elements. Furthermore, the device according to the invention advantageously comprises a device for carrying and / or holding a wafer, which comprises one or more of the sensor elements.

The carrying or holding device is preferably arranged in the vacuum chamber. It is, for example, a known x-y-z-θ table for holding wafers or semiconductor elements in the form of disks or chips.

In summary, the invention enables, in particular, a fully automated method for setting sensor properties of sensors with vibrating structures, since in particular an input characterization with design trimming, subsequent trimming and initial characterization in situ with the same system.

This means that it is no longer necessary to remove the sensor or to provide the sensor for trimming and measuring in different devices or positions.

The trimming process and / or the characterization or the measurement is preferably controlled by suitable software.

The cycle of the method can, if it should be necessary, run through several times and in particular be carried out at the wafer level.

The decisive advantages are effective handling, high throughput, low costs, a possible pre-selection and a possible pre-classification.

Another decisive advantage is that the trimming of frequency and imbalance can be geometrically separated very well, which is particularly relevant when trimming tuning fork rotation rate sensors.

The fully automated in-situ trimming process can be performed online, i.e. during the measurement, both for the unbalance and for trimming the frequency at the wafer level.

In order to set the properties of a vibrating sensor element with the method according to the invention, it is also possible, in addition to the mass loss on the vibrating mass elements or elements themselves, to change the spring constant and thus the natural frequency by changing the cross section of a suspension of vibrating elements. Preferred exemplary embodiments of the invention are described below by way of example with reference to the figures in which

1 schematically shows a device for trimming sensors according to a particularly preferred embodiment, and

2 shows a tuning fork rotation rate sensor which is subjected to a trimming method according to the invention as a preferred example.

1 shows, as a preferred embodiment of the invention, a device 10 for trimming sensors with vibrating structures in a schematic representation. The device 10 comprises a holder 11 in the form of an x, y, z, Θ table for carrying or holding a wafer 12, which is made of silicon, for example. The wafer 12, which is in the form of a silicon wafer, is in electrical contact with a measuring device 13 during the measuring operation, which is used for mechanical and electrical characterization, in particular for determining a resonance frequency and / or an unbalance of a sensor element 14. For this purpose, manipulators 15a, 15b are connected to the measuring device 13, the needles 16a, 16b of which contact one or more sensor elements 14. In other words, the measuring device comprises test cards for testing chips.

The device 10 further comprises a laser 17 in the form of a femtosecond laser. In operation of the device 10, i.e. H. When performing the trimming process, the laser 17 is directed at the sensor element 14 in order to remove mass from there in a targeted manner by means of a laser beam 17a. A control device 18 is coupled to the laser 17 and to the measuring device 13 in order to control the laser 17 as a function of the measured values for the resonance frequency and / or the unbalance of the sensor element 14.

The xyz-θ-measuring table, which forms the holder 11, is together with the manipulators 15a, 15b and the needles 16a, 16b in the interior of a vacuum chamber 19 arranged. There is electrical contact with test probes, manipulators or test cards.

When performing the trimming method, the sensor elements 14 are trimmed at the wafer level. In other words, a silicon wafer is provided for processing, which can contain a multiplicity of sensors or sensor elements 14. The sensor or sensors 14 are excited to vibrate during the measurement, the resonance frequency and / or the unbalance being measured. In order to excite the vibrations of the sensors or sensor elements, they are provided with an actuator system, which is not shown in the figure.

The sensors or sensor elements can thus be brought into their excitation state or in their shrinkage mode during the removal process, in order to immediately measure the change in their characteristic properties.

During the measurement of the sensor elements 14, these are processed with the laser beam of the laser 17 in such a way that mass of structures that can vibrate is removed. During the removal of the mass, the sensor properties, i.e. H. Resonance frequency and / or imbalance measured so that the result of the laser ablation is immediately recognizable and is used by a suitable control within the control device 18 for determining the further ablation.

The control can take place, for example, in such a way that laser ablation is carried out by illumination with the laser beam 17a until a predetermined resonance frequency is reached.

The frequency adjustment and the unbalance adjustment are explained in more detail below using the example of a tuning fork rotation rate sensor. FIG. 2 shows a schematic view of a tuning fork rotation rate sensor 30. The sensor 30 comprises two opposing tines 31a, 31b in the form of plates which are aligned parallel to one another. Actuators 32a, 32b, for example in the form of piezoelectric elements, are located on the tines 31a, 31b in order to excite the tines 31a, 31b to vibrate in the z direction. The tines 31a, 31b are fastened to a torsion bar 33 which comprises a measuring element 34 for measuring a torsional movement. The torsional movement is generated when the tines 31a, 31b vibrate in the z direction and when the system is rotated about the axis of rotation A at the same time due to the Coriolis force fc acting in this state. Such a sensor is described in detail in DE 195 289 61 C2.

If there is an unbalance, which can be present, for example, due to an asymmetry of the tines 31a, 31b, a torsional movement is generated within the torsion bar 33 even if there is no rotational movement of the system.

In the present case, the unbalance is influenced, reduced or eliminated by laser ablation on the prong 31a in the removal areas 35. The natural frequency of the rotation rate sensor 30 shown here is trimmed or set in the further removal region 36 by laser ablation or mass removal. The ablation area 36 for frequency adjustment is located in the area of the free end of the tine 31a or 31b.

During the frequency adjustment, the prongs 31a 31b are excited to oscillate in their resonance frequency fz by the actuators 32a, 32b. Then, at a rate of rotation of the sensor 30, a torsional vibration about the axis of rotation A is excited. This torsional vibration has the resonance frequency ft. B. by rotating the bracket 11 (see Fig. 1). Now the resonance frequency fz is increased by abrasion on the tines 31 a, 31 b. Here, for example, by increasing the resonance frequency, a predetermined difference frequency Δf = fz-ft is set. The sensitivity of sensor 30, which is set in this way, depends on this difference frequency Δf.

An unbalance present on the sensor 30 is compensated for by an asymmetrical mass removal with respect to the axis of rotation A in the removal areas 35. The imbalance arises, for example, due to manufacturing tolerances that lead to asymmetries, which causes a swing about the axis of rotation A.

An exemplary sequence of the method for sensor adjustment is described below. B. on the rotation rate sensor 30 can be carried out by means of the device shown in Fig. 1.

In order to carry out the method for sensor adjustment, a wafer or a silicon wafer is first installed in the vacuum chamber 19. Then the wafer 12 or the wafer is aligned and the vacuum chamber 19 is evacuated. Now a sensor or sensor element 14 as part of the wafer 12 is approached with an xyz thetatic and contacted with the needles 16a, 16b. In this case, the xyz-thetic forms the holder 11. The resonance frequencies fz and ft and the unbalance are determined via an electronic unit and the measuring device 13. However, some of these characteristics can also be determined or a mechanical electrical plausibility check can be carried out, and the resonance frequency fz is changed by laser ablation until a desired Δf as the difference between the two resonance frequencies fz and ft is reached. Here, fz is adjusted step by step, ie fz is measured and then material is removed using laser 17. These steps are repeated until Δf is reached. Now the unbalance is adjusted by laser ablation until the effect of the unbalance on the sensor signal is minimal. The next sensor 14 of the wafer 12 is then approached and contacted with the needles 16a, 16b in order to perform the trimming process. The process of mass removal can take place seamlessly on the vibrating sensor structure for measurement.

Claims

claims 1. A method for trimming sensors with vibrating structures, in which a sensor element (14, 30) which has a vibratable structure (31a, 31b) Detection of a measured variable, processed with a laser beam (17a) in order to remove mass in a targeted manner, at least one resonance frequency of the sensor element (14, 30) and / or an imbalance of the sensor element (14, 30) being compared, characterized in that the Sensor element (14, 30) vibrates during the removal of the mass or is in the operating state and a change in state due to the mass removal is measured instantaneously. 2. The method according to claim 1, characterized in that a femtosecond laser (17) is used to generate the laser beam (17b) Laser pulses generated in the femtosecond range. 3. The method according to claim 1 or 2, characterized in that the adjustment is carried out at the wafer level, and that during the removal of the mass, a measurement of the sensor element (14, 30) is carried out with regard to the resonance frequency and / or the unbalance and the change in state to eat. 4. The method according to any one of the preceding claims, characterized in that the sensor (14, 30) is made of silicon. 5. The method according to any one of the preceding claims, characterized in that the sensor (14, 30) is a micromechanical rotation rate sensor, the sensor element (14, 30) being in the form of a tuning fork. 6. The method according to claim 5, characterized in that the tuning fork-shaped sensor element (14, 30) comprises prongs (31a, 31b), the resonance frequency of which is set by means of laser ablation. 7. The method according to any one of the preceding claims, characterized in that it is carried out under vacuum conditions. 8. The method according to any one of the preceding claims, characterized by the further steps: - determining at least one resonance frequency of the sensor element (14, 30); and Changing the at least one resonance frequency by laser ablation until a predetermined value for the at least one resonance frequency or a predetermined difference between two resonance frequencies is reached. 9. The method according to any one of the preceding claims, in which prongs (31a, 31b) of a tuning fork are excited to vibrate in a first resonance frequency fz and at a rate of rotation due to the Coriolis force a torsional vibration is caused which has a second resonance frequency ft, whereby by Removal of at least one prong (31a), the first resonance frequency fz is increased in order to set a predetermined difference value between the two resonance frequencies. 10. The method according to any one of the preceding claims, characterized by the further steps: Determining an imbalance of the sensor element (14, 30); and aligning the imbalance by laser ablation until a sensor signal caused by the imbalance becomes minimal. 11. Device for trimming sensors with vibrating structures, characterized by a measuring device (13) for determining a resonance frequency and / or an unbalance of the sensor element (14, 30); a laser (17) with a control device (18) for the targeted removal of the mass of the sensor element; and a comparison device in order to compare a measured value, which represents the current resonance frequency and / or the current unbalance, with a predetermined value. 12. The device according to claim 11, characterized in that the laser (17) is a femtosecond laser. 13. The apparatus of claim 11 or 12, characterized by a vacuum chamber (19) for receiving a sensor element to be trimmed (14); 14. The apparatus according to claim 13, characterized by a device (11) for carrying or holding a wafer (12), which comprises one or more of the sensor elements (14), in the vacuum chamber (19).