SENSOR ELEMENT FOR THERMAL ANEMOMETRY

Abstract

A sensor element for thermal anemometry includes a semiconductor substrate and a thin-film diaphragm attached to the semiconductor substrate and having a front side and a rear side. A resistive heating element and a temperature-dependent resistor are attached to the front side of the thin-film diaphragm. In the area of the rear side of the thin-film diaphragm, the semiconductor substrate has a first recess. A silicon layer including a recess which merges with the first recess of the semiconductor substrate is located between the thin-film diaphragm and the semiconductor substrate.

Claims

1. A sensor element for thermal anemometry, the sensor element comprising: a semiconductor substrate; a thin-film diaphragm attached to the semiconductor substrate; a resistive heating element on a front side of the thin-film diaphragm; a temperature-dependent resistor on the front side of the thin-film diaphragm; and a silicon layer between the thin-film diaphragm and the semiconductor substrate, wherein a first recess, which is in the silicon layer, merges with a second recess, which is in the semiconductor substrate at a rear side of the thin-film diaphragm.

2. The sensor element of claim 1, further comprising: a first silicon oxide layer between the silicon layer and the semiconductor substrate.

3. The sensor element of claim 2, wherein the thin-film diaphragm includes a second silicon oxide layer formed in one piece with the first silicon oxide layer.

4. The sensor element of claim 1, further comprising: in an area of the first recess, a first silicon oxide layer coating the silicon layer.

5. The sensor element of claim 4, wherein the thin-film diaphragm includes a second silicon oxide layer formed in one piece with the first silicon oxide layer.

6. An air mass flow meter comprising: a sensor element for thermal anemometry, the sensor element including: a semiconductor substrate; a thin-film diaphragm attached to the semiconductor substrate; a resistive heating element on a front side of the thin-film diaphragm; a temperature-dependent resistor on the front side of the thin-film diaphragm; and a silicon layer between the thin-film diaphragm and the semiconductor substrate, wherein a first recess, which is in the silicon layer, merges with a second recess, which is in the semiconductor substrate at a rear side of the thin-film diaphragm; an activation circuit for electrically heating the thin-film diaphragm using the heating element; and an evaluation circuit for determining a temperature of the thin-film diaphragm using the temperature-dependent resistor.

7. A method for manufacturing a sensor element for thermal anemometry, the method comprising: applying a first silicon oxide layer including a first recess on a surface of a semiconductor substrate; covering the first silicon oxide layer and the semiconductor substrate with a silicon layer; covering the silicon layer with a second silicon oxide layer; applying a resistive heating element and a temperature-dependent resistor on the second silicon oxide layer; and removing the semiconductor substrate and silicon layer in an area of the first recess of the first silicon oxide layer.

8. The method of claim 7, wherein the removal of the semiconductor substrate is performed using deep reactive ion etching.

9. The method of claim 7, wherein the silicon layer is provided with a vertical trench at a boundary of the first recess, and the second oxide layer is attached in such a way that it fills the trench.

10. The method of claim 9, wherein the trench terminates at the first silicon oxide layer so that the first and second silicon oxide layers connect to each other via the trench.

11. The method of claim 7, wherein, in the removing, a width of a section of the semiconductor substrate that is removed is greater than a width of the first recess.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0020] FIG. 1 shows a schematic representation of an air mass flow meter according to an example embodiment.

[0021] FIGS. 2A-2D show the sensor element at various stages of its manufacture and in its finished state, according to an example embodiment of the present invention.

DETAILED DESCRIPTION

[0022] FIG. 1 shows a schematic representation of an air mass flow meter 100 according to an example embodiment. Air mass flow meter 100 is generally designed for determining the flow velocity of a passing medium. In particular, air mass flow meter 100 is designed to be used in a motor vehicle for determining a volume flow of air which, for example, enters an internal combustion engine. For this purpose, air mass flow meter 100 is preferably situated in an area of an intake manifold 105 or a similarly defined passageway for the passage of fluid medium 110, in particular air.

[0023] Air mass flow meter 100 includes a diaphragm 115 to which a resistive heating element 120 and two temperature-dependent resistors 125 are attached. Heating element 120 is preferably located between first and second temperature-dependent resistor 125 with respect to a direction of movement of medium 110.

[0024] Furthermore, air mass flow meter 100 preferably includes a controller 130 which includes an activating circuit 135 for resistive heating element 120 and an evaluation circuit 140 for determining the temperature of diaphragm 115 in the areas of each of first and second temperature-dependent resistor 125.

[0025] If diaphragm 115 is heated in the area of heating element 120, the heat flows via diaphragm 115 to temperature-dependent resistors 125. A movement velocity of fluid medium 110 influences how strongly upstream resistor 125 and downstream resistor 125 are heated. This temperature difference can be determined by evaluation circuit 140 as a function of the heating caused by activating circuit 135 and resistive heating element 120. Preferably, controller 130 additionally includes a processing unit which is configured for determining the velocity of fluid medium 110 or the mass of fluid medium 110 passing through intake manifold 105 per unit of time based on the temperature difference and the activated heating effect. The processing unit can be integrated with evaluation circuit 140 and, in one specific embodiment, includes a programmable microcomputer. A result of the evaluation or processing can be provided externally via an interface 145.

[0026] FIGS. 2A-2d show a sensor element 205 for an air mass flow meter 100, such as that of FIG. 1, at various stages of a method 200 of its manufacture. Sensor element 205 includes diaphragm 115, resistive heating element 120, and a temperature-dependent resistor 125. A description of steps of method 200 initially ensues and, subsequently, a more detailed explanation of formed sensor element 205.

[0027] In a first step 210, a semiconductor substrate 235, in particular in the form of a silicon wafer, is provided. A first silicon oxide layer 240 is applied on a surface of semiconductor substrate 235. First silicon oxide layer 240 preferably includes silicon dioxide (SiO.sub.2). The application can include growing first silicon oxide layer 240 by thermal oxidation or deposition. First silicon oxide layer 240 includes a recess 245 which, in one specific embodiment, is formed by initially applying first silicon oxide layer 240 over a large area on the surface of semiconductor substrate 235 and subsequently removing it again in a predetermined area. The size of recess 245 later determines the shape, size, and position of diaphragm 115.

[0028] In a second step 215, first silicon oxide layer 240 and semiconductor substrate 235 are covered in the area of recess 245 with the aid of a silicon layer 250. Silicon layer 250 can in particular be grown epitaxially. The thickness of silicon layer 250 is selected in such a way that requirements for a mechanical stability and sufficient thermal conductivity are met.

[0029] Silicon layer 250 is preferably provided with a vertical trench 255 in the area of a boundary of recess 245 of first silicon oxide layer 240. Trench 255 terminates in the vertical direction preferably at first silicon oxide layer 240, immediately adjacent to a lateral boundary of recess 245. Trench 255 can in particular be etched anisotropically, the etching process terminating at first silicon oxide layer 240.

[0030] Trench 255 is preferably filled with silicon oxide. For this purpose, a thermal oxidation or, alternatively, a deposition of silicon dioxide can be carried out with the aid of LPCVD (low-pressure chemical vapor deposition). This preferably completely fills trench 255 with silicon oxide.

[0031] Optionally, a surface of silicon layer 250 can be planarized using a known CMP (chemical mechanical fabrication) step of semiconductor technology.

[0032] In a step 220, diaphragm 115 in the form of a second silicon oxide layer 260, a passivating layer 265, in particular in the form of a nitride layer, a third silicon oxide layer 270, and resistive heating element 120, preferably in the form of a platinum layer 275, are initially applied in succession on the same side of semiconductor substrate 235 using known process steps. Platinum layer 275 is preferably covered both on its upper side and on its underside by third silicon oxide layer 270. A connecting element 280, for example, in the form of an aluminum conducting element, is preferably provided in order to provide an electrical contact to platinum layer 275 through the sheathing of third silicon oxide layer 270. The provision of layers 260 through 275 and connecting element 280 is known in the related art and can be adopted in an arbitrary variant.

[0033] In a last step 225, parts of semiconductor substrate 235 and silicon layer 250 are removed in the area of recess 245 of first silicon oxide layer 240. For this purpose, it may be necessary to carry out lithographic steps which are calibrated in their position with respect to the steps carried out on the diametrically opposed side (front side-rear side adjustment). The removal is preferably carried out using reactive ion etching from a rear side of diaphragm 115, i.e., from the side of semiconductor substrate 235. The removal is preferably carried out using reactive ion etching in the Bosch process or one of its refinements. As a result, a cavity 290 is formed under first silicon oxide layer 240, so that second silicon oxide layer 260 forms diaphragm 115 in the area of recess 245 of first silicon oxide layer 240. Depth etching of cavity 290 terminates in the vertical direction at first silicon oxide layer 240 or second silicon oxide layer 260. When trench 255 has been introduced in second step 215 and filled with silicon oxide, the filled silicon oxide forms a lateral boundary of cavity 290 in the area of first recess 245.

[0034] Sensor element 205 shown in FIG. 2D includes a diaphragm 115 and a temperature-dependent resistor 125 whose positions in relation to each other and to a cavity 290 under diaphragm 115 are precisely defined. In one exemplary embodiment, diaphragm 115 is approximately 400 μm×400 μm in size. A transition between diaphragm 115 and a boundary of cavity 290 in the area of semiconductor substrate 235 can be precisely determined. A dissipation of heat from diaphragm 115 to semiconductor substrate 235 in an unforeseen or undesirable manner can be prevented. Method steps for defining diaphragm 115 and resistor 125 are carried out from the same side (upper side). A recess 285, which together with recess 245 forms a cavity 290, is introduced into semiconductor substrate 235. Cavity 290 is formed from the opposite side (rear side), but stops at a silicon oxide layer 240, 260 formed in process steps from the front side, and therefore can exactly match one another.

[0035] Provided sensor element 205 can be improvably used on air mass flow meter 100 of FIG. 1 in order to determine a temperature on a predetermined location of diaphragm 115 and subsequently, as explained above, based on this determination, to determine a flow velocity or a mass flow of fluid medium 110.