In Claims 13 to 17, advantageous processes are proposed for producing a
sensor according to the invention. The sensor membrane can advantageously be produced by applying polysilicon films to structured silicon oxide auxiliary films and then removing the auxiliary films. An electrically insulating film to which the resistor arrangement is applied is then deposited on the polysilicon membrane. It is essential that this film structure is as stress-free as possible. The structured silicon oxide auxiliary films can advantageously be produced either by thermal oxidation of the masked surface of the silicon base or, alternatively, by depositing silicon oxide from a silane/oxygen atmosphere and then structuring the silicon oxide film. A further advantageous process for producing the sensor element is to etch a recess into the surface of a first silicon base and to deposit, on the surface of a second silicon base, a silicon oxide film,
24057 against which the first silicon base with its structured surface is bonded. In a subsequent process step, the entire second silicon base is then etched away except for a small residual thickness which corresponds to the resistor thickness, so that the silicon oxide film forms a film of the sensor membrane. The resistor arrangement is then formed from the thin silicon film. An insulating protective film is furthermore finally deposited over the membrane with the resistor arrangement. In this process it is also possible to first introduce a recess into the surface of the second silicon base, deposit a first electrically insulating film over this structured surface of the second silicon base and apply the resistor arrangement, for example in the form of polysilicon films to said film in the region of the recess and finally to deposit a silicon oxide film thereover. After bonding the first silicon base with the structured surface against the second silicon base via the silicon oxide film, the entire second silicon base is again etched away, the silicon oxide film, the resistor arrangement and the insulating film serving as protective film forming the membrane.
Drawing Exemplary embodiments of the invention are shown in the drawing and explained in greater detail in the description below.
Figures la to le show various process steps in the construction of a sensor element according to the invention, Figure 2 shows a section through a sensor element, Figures 3a to 3c show process steps for constructing a sensor element according to the invention by a second process, Figures 4a and 4b show different process steps corresponding to a third process and Figure 5 shows a section through a pressure-equalised sensor element.
is 24057 Description of the exemplary embodiments In Figure la, 1 denotes a silicon base to which a structured silicon oxide film 5 has been applied. The structured silicon oxide f ilm 5 can be produced either by suitable masking of the surf ace of the silicon base 1 and subsequent thermal oxidation of the points of the surf ace of the silicon base 1 not covered by the masking or by an oxidation of the entire surface and subsequent photolithographic structuring of the oxide. In the case of thermal oxidation, the silicon oxide film 5 is produced from the silicon of the silicon base 1 and atmospheric oxygen. Said silicon oxide film partially extends into the surface of the silicon base 1. A further possibility for producing a structured silicon oxide film is to deposit a silicon oxide film on the surface of the silicon base 1 from a silane/oxygen atmosphere and then to structure it in a photomasking process.
Figure lb shows the silicon base 1 with the structured silicon oxide film 5 which serves as auxiliary film and over which a polysilicon film 10 has been deposited. Etch channels 6 are introduced into the polysilicon film 10 in the region of the auxiliary film 5, and this is shown in Figure 1c. Figure ld shows the structure after the silicon oxide auxiliary film 5 has been etched out under the polysilicon film 10 through the etch channels 6, so that a cavity 15 has been produced between the polysilicon film 10 and the silicon base 1. An insulating film 11, preferably of silicon nitride, which also seals the etch channels 6 is deposited over this structure.
Figure le shows the final structure of the sensor element. Arranged on the insulating film 11 in the region of the cavity 15 are a heating resistor 21 and a temperature-dependent sensor resistor 22. The resistors 21 and 22 can advantageously be produced in the form of doped polysilicon films. As a protection against external effects and against contamination, a passivating film 12 has been deposited over the entire arrangement. Suitable - 6 24057 passivating films 12 are, in particular, silicon nitride or silicon oxynitride films.
Depending on the nature of the structured silicon oxide film, i.e. depending on whether it is a thermal oxide or an oxide film deposited from a silane/oxygen atmosphere, the process shown in Figures la to d produces membranes 20 corresponding to Figure le or Figure 2. The silicon base 1 in Figure le is not structured; the cavity 15 between the membrane 20 and the silicon base 1 is produced because the membrane 20 is formed into a dome in t he manner of a bubble on the silicon base 1, as it is produced if an oxide film deposited from a silane-oxygen atmosphere is used. The silicon base 1 in Figure 2 has a recess across which the membrane 20 is stretched, so that the cavity 15 is produced by sealing the recess in the silicon base 1. This structure can be produced if thermal oxide is used. In the sensor structure corresponding to Figure 2, it is advantageous that the sensor surface is relatively flat.
In Figure 3a, 1 denotes a.first silicon base into whose surface a recess 16 has been introduced. A silicon oxide film 18 has been deposited on the surface of a second silicon base 2. The arrow indicates that the second silicon base 2 is bonded to the structured surf ace of the f irst silicon base 1. This takes place via the silicon oxide film 18. After bonding, the entire silicon base 2 is removed in an etching process except for a thin silicon film 30. Into the latter a heating resistor 21 and a temperature-dependent sensor resistor 22 are then introduced by doping. as shown in Figure 3b. The silicon f ilm. 30 is then etched away with the exception of the resistors 21 and 22. Alternatively, the resistors 21 and 22 can also be introduced into the silicon base 2 for this purpose before bonding. Figure 3c shows the structure of the sensor element produced in this process. The recess in the first silicon base 1 is sealed by the silicon oxide film 18 which originates from the second silicon base 2, so that a cavity 15 is produced. The 24057 resistors 21 and 22 which are not removed during the etching process are arranged on the silicon oxide film 18. The entire sensor surface is covered with a passivating film 12 which serves as protective film against external effects, in particular contamination or attack by aggressive media. In this structure, the membrane 20 is essentially formed by the silicon oxide film 18. The resistors 21 and 22 are composed in this case of doped single-crystal silicon.
In Figures 4a and b, a process is described which is similar to the process for constructing the sensor element shown in Figures 3a to 3c. A recess 16 is again introduced into a first silicon base 1. Passivating film 12, preferably a silicon nitride, silicon oxide or even a silicon oxynitride film, is deposited on the surface of a second silicon base 2. Resistors 22 and 21 in the form of doped polysilicon films are then deposited on the second silicon base 2. Finally, a silicon oxide film 18 is applied to the prepared surface of the second silicon base 2. The second silicon base 2 is now bonded against the first silicon base 1 exactly as in the process described above, so that the recess 16 in the first silicon base 1 is sealed to form a cavity 15. In a subsequent etching step, the entire silicon base 2 is etched away. In this process it is no. longer necessary to apply a further passivating film to the sensor element surface since the passivating film 12 takes over the function of a protective film. In addition, this sensor has an unstructured surface, and this reduces the deposition of particles and the swirling of the medium.
Since the sensor elements shown in Figures 1 to 4 also act as pressure sensors, it is expedient to arrange next to the actual sensor element a further sensor element which is constructed in the same manner as the actual sensor element, but has only at least one temperature-independent sensor resistor so that it only determines the pressure but not any temperature effects. This correction parameter for the pressure can be 24057 subtracted from the signal of the actual sensor element whose resistors are temperature-dependent.
Figure 5 shows the structure of a sensor element whose membrane 20 does not adjoin a sealed cavity. The silicon base 1 has a rear-side etching 17 which completely penetrates the silicon base 1 so that a pressure equalisation exists between the two sides of the sensor element. This structure suppresses interfering effects due to pressure differences.
- 9 Claims 1. Sensor for measuring the velocity or flow rate of a f lowing medium, in particular a gas, having a sensor element, exposed to the flowing medium, with a membrane which has a resistor arrangement comprising at least one heating resistor and at least one temperature-dependent sensor resistor, the at least one sensor resistor being a component of an evaluation circuit, characterised in that the sensor element has a silicon base (1), in that a sequence of thin films (10. 11, 12; 18, 12) in which the membrane (20) is constructed is applied to the silicon base (1), and in that a cavity (15) exists between the mem brane (20) and the silicon base (1).
2. Sensor according to Claim 1, characterised in that the at least one heating resistor (21) and the at least one sensor resistor (22) are arranged next to each other on a film (11; 18) and in that said film (11; 18) is composed of electrically insulating material.
Sensor according to Claim 1. characterised in that the at least one heating resistor and the at least one sensor resistor are arranged above each other on two different films and in that between the at least one heating resistor and the at least one sensor resistor there is at least one film of electrically insulating material.
4. Sensor according to Claim 1, characterised in that the heating resistor and sensor resistor are identi- Sensor according to one of the preceding claims, - 10 24057 characterised in that at least one f ilm (12) of electrically insulating material which completely covers the resistors (21, 22) is applied to the films (11; 18) on which the at least one heating resistor (21) and/or the at least one sensor resistor (22) are arranged.
6. Sensor according to one of the preceding claims, characterised - in that the resistors (21, 22) are composed of doped single-crystal silicon or of doped polysilicon.
7. Sensor according to one of the preceding claims, characterised in that the films of electrically insulating material are preferably composed of silicon nitride, silicon oxynitride or silicon oxide and said films are deposited in a low-stress manner.
Sensor according to one of the preceding claims, characterised is in that a polysilicon film (10) is formed into a dome in bubble fashion on the silicon base (1), in that a first electrically insulating film (11) is applied to the polysilicon film (10) and in that the resistor arrangement is applied to 25 the first electrically insulating film (11).
10.
terised 0. Sensor according to one of Claims 1 to 7, charac- terised in that the silicon base (1) has a recess (16), in that a polysilicon film (10) which seals off the recess (16) is applied to the silicon base (1), in that a first electrically insulating film (11) is applied to the silicon film (10), and in that the resistor arrangement is applied to the first electrically insulating film (11). g-nR-- accordinn to one of Claims 1 to 7 charac- in that the silicon base (1) has a recess (16), in that a first electrically insulating film (18), 11 - 24057 preferably a silicon oxide film, which seals off the recess (16) is applied to the silicon base (1), and in that the resistor arrangement is applied to the first electrically insulating film (18).
11. Sensor according to one of the preceding claims, characterised in that the silicon base (1) has a rear-side opening (17) so that the silicon base (1) is completely pierced in the region of the membrane (20).
12. Sensor according to one of the preceding claims, characterised in that a further sensor element having a membrane is present which is exposed to the flowing medium, the membrane of the further sensor element having at least one temperature- independent sensor resistor for determining pressure.
Process for producing a sensor according to Claim 8 or 9, characterised in that an S'02 film (5) structured in plinth fashion is introduced into a surface andlor applied to a surface of the silicon base (1), 13.
in that a polysilicon film (10) is deposited on the surface of the silicon base (1) over the S'02 film (5) structured in plinth fashion.
in that etch channels (6) are introduced into the polysilicon film (10) in the region of the S'02 film (5) structured in plinth fashion, in that the polysilicon film (10) is underetched through the etch channels (6) by etching away the SiO, film (5) formed in plinth fashion, in that a first electrically insulating film (11) is deposited on the polysilicon film (10), in that the resistor arrangement is applied to the first electrically insulating film (11) and in that at least one electrically insulating passivating film (12) which completely covers the resistor arrangement is applied to the first elec trically insulating film (11).
- 12 is 24057 14. Process according to Claim 13, characterised in that the structured S'02 f "ra (5) is produced by depositing S'02 in a silane/oxygen atmosphere.
Process according to Claim 13, characterised in that the structured S'02 film (5) is produced by thermal oxidation of the masked surface of the silicon base (1).
16. Process for producing a sensor according to Claim or 11. characterised in that a recess (16, 17) is preferably etched into at least one surface of a first silicon base (1), in that an insulating film (18), preferably an S'02 film, is deposited on a surface of a second silicon base (2), in that the first silicon base (1) is bonded against the second silicon base (2) via the insulating film (18), in that the entire second silicon base (2) is then etched away with the exception of a thin silicon film (30) in that at least one heating resistor (21) and at least one sensor resistor (22) is [sic] introduced into the thin silicon film (30) by doping, in that the thin silicon f ilm (30) is etched away except for the at least one heating resistor (21) and the at least one sensor resistor (22) and in that at least one electrically insulating passivating film (12) which completely covers the resistors (21, 22) is applied to the S'02 film (18) 17. Process for producing a sensor according to Claim or 11, characterised in that a recess (16, 17) is preferably etched into at least one surface of a first silicon base (1), in that a first electrically insulating film (12) is deposited on a surface of a second silicon base (2), in that a resistor arrangement, preferably of polysilicon, is applied to the first electrically 24057 18.
insulating film (12), in that a further electrically insulating film (18), preferably an S'02 film, is deposited on the first electrically insulating film (12) and over the resistor arrangement, in that the first silicon base (1) with a structured surface is bonded against the second silicon base (2) via the 1 S'02 film (18) and in that the entire second silicon base (2) is then etched away.
Any of the sensors substantially as herein described with reference to the accompanying drawings.
19. Any of the processes for producing a sensor substantially as herein described with reference to the accompanying drawings.