Design of a multi-physics coupling MEMS pressure sensor

The pressure measurement of explosion shock wave puts forward high requirements on the temperature resistance and response speed of the sensor. In this paper, a multi-physics coupling pressure sensor is designed to meet requirements. The sensor mainly consists of a Fabry-Perot cavity optical fiber sensing unit, a piezoelectric pressure sensing unit and a thermometric resistance temperature measurement module. Firstly, the structure of the sensor is designed. The top and bottom surfaces of the Fabry-Perot cavity are composed of a silicon diaphragm and a quartz glass. The piezoelectric pressure sensing unit is an AlN film including its electrodes. Pt thermometric resistance is developed to realize real-time temperature monitoring. Secondly, the fabrication process of the sensor is discussed, especially three key technologies. An Al film is sputtered to increase the reflectivity of the quartz glass. The SU-8 photoresist is applied to accurately control the thickness of Fabry-Perot cavity. Etching is adopted to work out the graphics of the AlN piezoelectric film. At last, a complete fabrication process of the sensor is described. The fabrication of multilayer films begins with a double-sided polishing silicon wafer, while Fabry-Perot cavity was from a double-sided polishing quartz glass. Then two parts of the sensor are combined to ensure the multi-physics coupling sensor to achieve complete function.


Introduction
The research object in the field of explosion shock wave is high-frequency dynamic information.When the pressure parameters are obtained during testing, it is often accompanied by the interference of strong electromagnetic field, strong light and high temperature.The experimental test is a transient signal with extremely fast change, and the fundamental wave, first harmonic wave and high harmonic wave should be considered.Therefore, sensors should have a wide response frequency band, high natural frequency and response speed.There is an urgent need to develop a miniature high temperature pressure sensor to meet the requirements.
According to different structure and principle, existing high temperature pressure sensors are mainly divided into SOI sensors [1,2], SOS sensors [3], SiC sensors [4,5] and fiber optic sensors [6,7].SOI sensors are difficult to work for a long time in an environment above 500 ℃, due to factors such as high-temperature creep and high-temperature leakage current increase of silicon.As for SOS sensor, there is a large lattice mismatch between the epitaxial monocrystalline silicon film with sapphire.It leads to a large mismatch stress, which limits the use temperature of this type of sensor.Although SiC sensors can work in ultra-high temperature (above 800°C) environments, the fabrication of SiC is extremely hard to work out.
In order to further break through the performance of high temperature pressure sensor, MEMS pressure sensor with multi-physics coupling and multi-parameter composite will become the future development trend.For example, there is a potential to combine a fiber optic sensor, a piezoelectric sensor and a thermometric resistance into an integrated sensor.There would be characters of high temperature resistance and anti-electromagnetic interference from fiber optic part, character of fast response from piezoelectric materials and character of in-situ temperature measurement.Based on the multi-physics coupling sensor, the ultrafast accurate measurement of pressure of explosion shock wave can be realized.the sensor connected with a fiber.In the Fabry-Perot cavity sensing unit, pressure sensing is realized by converting the decreasing of the cavity length into the change of the optical signal.The optical signal is emitted and collected by a single-mode fiber, which is showed in Figure 1(b).A glass sleeve is used to fix the fiber to the sensor.Multilayer films consist of a piezoelectric pressure sensing unit and a thermometric resistance temperature measurement module.

Design of the Fabry-Perot cavity sensing unit
Figure 2(a) presents the details of the Fabry-Perot cavity sensing unit.The Fabry-Perot cavity is designed as a cylindrical air cavity, whose top and bottom surfaces are respectively the bottom surface of a silicon diaphragm and the top surface of a quartz glass.The end face of the fiber coincides with the bottom of the quartz glass, which ensures the light path not changing here.Through the quartz glass, the incident light is reflected and refracted on the two surfaces of the Fabry-Perot cavity, which is showed in Figure 2

Design of the multilayer films
Figure 3 presents the details of multilayer films.The piezoelectric pressure sensing unit is realized by an AlN film including its top and bottom electrodes.They grow onto the top of the diaphragm.The top electrode is designed as a circular ring.Its location coincides with the edge of the Fabry-Perot cavity, where maximum pressure is loaded so that most remarkable piezoelectric signal can be detected.As for the thermometric resistance, Pt is adopted for its stable linear change of resistance along with temperature.A complex fractal pattern is designed to increase the resistance.
Figure 3: The composition of multilayer films.

Fabrication of the sensor
The fabrication steps of the sensor are mainly based on the MEMS technology, several key technologies are essential and worth analyzing.

Analysis of key technologies
During the fabrication of the Fabry-Perot cavity sensing unit, sufficient interference light intensity and accurate Fabry-Perot cavity thickness need to be guaranteed.In addition, when multilayer films are fabricated, the graphics of AlN film is considered.4 presents several results of reflectivity of different situations according to the ultraviolet-visible-infrared spectrophotometer.It's apparent that during all wavelength, quartz glass has a lower reflectivity than silicon diaphragm.Moreover, we can read that in the visible and near-infrared bands, the reflectivity of quartz glass is only between 5 and 10 percent.It will result in the weakness of received optical signal.
In view of this, magnetron sputtering is adopted to plate Al film (thickness of 20nm) on the top surface of the quartz glass.The reflectivity of quartz glass is increased obviously because of the existence of the Al film.In the near-infrared bands, the reflectivity of quartz glass with Al film is close to or even exceeds the silicon diaphragm's reflectivity.Besides, it is kept in a reasonable range so that the transmitted light intensity is not too low.Figure 5 shows the condition of quartz glass with Al film.

Accurate Fabry-Perot cavity thickness: SU-8 photoresist. A conventional method to fabricate
Fabry-Perot cavity is the anisotropic etching of glass.Based on this method, the Fabry-Perot cavity is usually generated a round platform rather than a cylinder.Moreover, the instability of the etching rate lead to the difficulty of thickness control.In addition, the surface roughness of the etched glass is too large, which affects the performance of the reflection increasing film.
To overcome shortcomings of the conventional method, SU-8 photoresist is spun on the top surface of the quartz glass to realize accurately control the thickness of Fabry-Perot cavity.The one-to-one correspondence between spin coating parameters and photoresist's thickness ensures the stability of thickness.In the meantime, the high aspect ratio and high strength of SU-8 photoresist ensure that the cavity is cylindrical and will not deform.Figure 6 shows the condition of quartz glass with graphical SU-8 photoresist.3.1.3.Graphics of the AlN piezoelectric film: etching.Two methods are usually applied in the graphics of films: lift-off and etching.In this condition, reactive magnetron sputtering is adopted to grow the AlN film.The temperature would be up to 400 ℃ during this technology.There is no type of photoresist could resisit it without deterioration.Therefore, etching is applied after the deposition of AlN film and the spin coating, lithography, development of photoresist of masking layer.Figure 7 shows the consequence of the multilayer films after etching of AlN film.

Conclusions
A multi-physics coupling pressure sensor for explosion shock wave measurement is designed.In the phase of structure design, the optical surfaces of Fabry-Perot cavity are composed of a silicon diaphragm and a quartz glass.As for the multilayer films, the piezoelectric pressure sensing unit is achieved by an AlN film.Pt is used for material of the thermometric resistance.The fabrication of the sensor is also analysed.The reflectivity of the quartz glass is increased by deposition of an Al film.The SU-8 photoresist is applied in order to accurately control the thickness of Fabry-Perot cavity.
Besides, the graphics of the AlN piezoelectric film is finished by etching.In the end, a complete fabrication process of the sensor is designed.

Figure 1 (
a) presents the structure of a multi-physics coupling sensor.The sensor mainly consists of a Fabry-Perot cavity sensing unit and multilayer films.

Figure 1 :
Figure 1: (a) The diagram of the sensor and (b) the sensor connected with a fiber.In the Fabry-Perot cavity sensing unit, pressure sensing is realized by converting the decreasing of the cavity length into the change of the optical signal.The optical signal is emitted and collected by a single-mode fiber, which is showed in Figure1(b).A glass sleeve is used to fix the fiber to the sensor.Multilayer films consist of a piezoelectric pressure sensing unit and a thermometric resistance temperature measurement module.
Figure 2: (a) The Fabry-Perot cavity sensing unit and (b) the cross-sectional view with the optical path.

Figure 5 :
Figure 5: The microscopic image of the reflection increasing film.

Figure 6 :
Figure 6: The microscopic image of the Fabry-Perot cavity made by SU-8 photoresist.

Figure 7 :
Figure 7: The microscopic image of the multilayer films after etching of AlN film

Figure 8 :
Figure 8: Fabrication processes of the sensor.