RU215318U1 - Thermal gas flow sensor of calorimetric type - Google Patents

Abstract

The utility model relates to measuring technology - the field of microelectronic and microelectromechanical (MEMS) devices, namely to thermal gas flow sensors. The thermal gas flow sensor of the calorimetric type contains a sensitive element, which is a thin dielectric membrane over a hole in a silicon crystal with temperature-sensitive elements on the membrane. The essence of the utility model is to increase the sensitivity and expand the range of measured flow rates of a thermal gas flow sensor by placing four temperature-sensitive elements on the sensor membrane, which simultaneously perform the functions of heaters and temperature sensors. The temperature-sensitive elements are placed two in two groups, and in each group the temperature-sensitive elements are placed in close proximity to each other to provide better thermal contact, and the groups of temperature-sensitive elements are spaced apart to minimize heat transfer through the membrane and the predominance of heat transfer through the gas. The utility model was created as part of the implementation of the LIC "Trusted Sensory Systems" program and can be used to overcome technological barriers in accordance with the roadmap of end-to-end digital technology "robotics and sensors components". 2 ill.

Description

The utility model relates to measuring technology - the field of microelectronic and microelectromechanical (MEMS) devices, namely to thermal gas flow sensors.

Known implementation of the gas flow sensor with a silicon substrate and a recess in it /1/. A silicon membrane hangs over the recess, on which a thin-film heater and thermistors are located. The silicon membrane is perforated to reduce its thermal conductivity to minimize heat dissipation from the heater and temperature sensing elements to increase sensor sensitivity. A temperature-sensitive temperature correction resistor is located on the sensor substrate outside the membrane. The disadvantage of such a sensor is the use of a heater, which increases the energy consumption of the sensor, as well as the use of silicon as a membrane material, which does not allow the membrane thermal conductivity to be reduced as much as possible (150 W/m⋅K versus 1.4 W/m⋅K for silicon oxide).

Known implementation of the gas flow sensor, which minimizes the turbulence of the surrounding gas flow based on silicon /2/. The sensor is made on a semiconductor substrate and includes a dielectric membrane, a thin-film heater, and temperature-sensitive elements. The temperature sensitive elements are formed in pairs and are located before and after the gas flow. The heater and at least one sensitive element are located on a flat dielectric diaphragm. It is formed from three alternating layers of silicon oxide and silicon nitride in a multilayer structure and provides thermal insulation for the sensing and heating elements of the sensor. In addition, one thermistor is located on the semiconductor package and is able to measure its temperature. Sensitive thermistors are covered with top layers of silicon oxide and nitride, which protect the sensitive thermistors from environmental influences, and also compensate for the mechanical stresses of the sensor's dielectric membrane after the manufacturing process of the entire structure. The disadvantage of the sensor is the need to use a separate heater element, which reduces the energy efficiency of the sensor, as well as limiting the current supplied to sensitive thermistors due to self-heating of these elements. This requires large gains of the elements in the signal processing circuit of the sensor and leads to additional noise and interference.

The closest technical solution to the proposed one is a gas flow sensor on a membrane in a silicon crystal, which uses two groups of temperature-sensitive elements without the use of a separate heater element /3/. The temperature sensitive elements in the gas flow sensor function as both heaters and flow sensors and are mounted on a dielectric membrane in a silicon crystal. The temperature-sensitive temperature correction element is placed on the silicon crystal of the sensor itself. The disadvantage of this implementation of the gas flow sensor is the placement of temperature-sensitive elements at a great distance from each other. This feature can adversely affect the provision of thermal contact of the elements, which will reduce the accuracy of the sensor measurements.

The objective of this utility model is to improve the accuracy and expand the range of measurement of the gas flow rate.

This is achieved due to the fact that in the proposed sensor a temperature-sensitive element of temperature correction is placed on a crystal, and four temperature-sensitive elements are placed on the sensor membrane, which simultaneously serve as heaters and temperature sensors and are placed two in two groups along the flow of the measured gas, and in each group there are temperature-sensitive the elements are placed closely to each other to ensure good thermal contact, and the groups are spaced apart to minimize heat transfer through the membrane and the predominance of heat transfer through the gas, and the temperature-sensitive elements form a Wheatstone measuring bridge, in the arms of which temperature-sensitive elements from different groups are placed.

In FIG. 1 shows an image of a thermal gas flow sensor of the calorimetric type;

in fig. 2 shows an image of the sensor membrane with groups of temperature-sensitive elements placed on it.

The numbers in the figures indicate

1 - gas flow sensor crystal;

2 - multilayer dielectric membrane of the gas flow sensor;

3 - temperature-sensitive elements of the group input along the flow;

4 - temperature-sensitive elements of the output group;

5 - temperature-sensitive element of temperature correction.

The gas flow sensor crystal 1 is made of silicon. On the sensor crystal there is a thin dielectric membrane 2 with minimal thermal conductivity, which can consist of several alternating layers of oxide and silicon nitride. The number of layers should not be very large in order to avoid strong sagging under its own weight, but at the same time ensure sufficient membrane strength. Temperature-sensitive "input" and "output" resistors 3 and 4 combined into a thermal bridge are formed on the dielectric membrane of the sensor and can be made of materials such as platinum, permalloy, etc. The temperature correction thermistor 5 is placed on the silicon chip of the sensor.

Calorimetric type gas flow sensor operates as follows.

When a gas flow flows through the temperature-sensitive elements 3 and 4 located on the membrane 2, the thermal cloud created by the temperature-sensitive elements is displaced relative to the center of the membrane, which causes a change in their temperature, and, consequently, the resistance. An unbalance signal occurs in the thermal bridge of the flow sensor, which then must be transmitted to the signal processing circuit. The temperature-sensitive resistor 5 on the sensor chip is introduced to correct the temperature error of the calorimetric method for measuring the signal received from the temperature-sensitive elements in the signal processing circuit. The use of four thermistors makes it possible to reduce the requirements for amplifying the temperature difference signal when using the calorimetric method for measuring the gas mass flow rate. Since the current in the thermistors of the heaters is quite high and amounts to a few milliamps, the signal about the temperature of the thermistor is large enough and does not require significant amplification to obtain a normalized value of several volts.

An example of a specific design is a calorimetric type thermal gas flow sensor manufactured on a KEF 4.5 Ohm⋅cm silicon substrate with a diameter of 150 mm and orientation (100). The dimensions of the gas flow sensor crystal are 3×4 mm. The 1.26 µm thick sensor membrane consists of four alternating layers of oxide and silicon nitride - SiO ₂ (600 nm)/Si ₃ N ₄ (130 nm)/SiO ₂ (400 nm)/Si ₃ N ₄ (130 nm). The sensor membrane has dimensions - 500×750 µm ² . The temperature-sensitive elements of the membrane are made of platinum with a thickness of 200 nm, the line width of the temperature-sensitive element is 5 µm, the length of the temperature-sensitive element is 440 µm. The role of conducting contacts is performed by an aluminum layer 200 nm thick. To estimate the required amplification for the signal from the Wheatstone measuring bridge, we assume that with a diagonal current of 2 mA, a temperature difference between the heaters of 15 ° C, a resistance of a separate temperature-sensitive element of 600 Ohm and the inclusion of four thermistors with temperature coefficients of resistance a of about 0.003 ° C ^{- 1} into a full measuring Wheatstone thermal bridge, the signal on the diagonal of the thermal bridge will be 0.108 mV, i.e. up to a voltage level of 5 V, it will be enough to amplify the signal by only 46 times. Also taking into account the fact that in each group the temperature-sensitive elements are placed in close proximity to each other to ensure better thermal contact, and the groups are spaced apart to minimize heat transfer through the membrane and the predominance of heat transfer through the gas, it can be concluded that the proposed thermal sensor gas flow rate will have improved accuracy and measurement range in comparison with the mentioned prototype.

Sources of information

1. Japan Patent JP4505842 B2.

2. International patent WO 2020255788.

3. US patent US20050006235 A1 - prototype.

Claims (1)

Thermal gas flow sensor of the calorimetric type, containing a sensitive element, which is a thin dielectric membrane in a silicon crystal with temperature-sensitive elements on the membrane, characterized in that the temperature-sensitive temperature correction element is located on the crystal, and four temperature-sensitive elements are placed on the sensor membrane, which simultaneously serve as heaters and temperature sensors and placed two by two in two groups along the flow of the measured gas, and in each group the temperature-sensitive elements are placed in close proximity to each other to ensure better thermal contact, and the groups of temperature-sensitive elements are spaced apart to minimize heat transfer through the membrane and the predominance of heat transfer through the gas, and the temperature-sensitive elements form a Wheatstone measuring bridge, in the shoulders of which temperature-sensitive elements from different groups are placed.

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