JP2009128362A - Sensor element for spectroscopic or optical measurement and method for manufacturing the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To improve the cost and convenience of a sensor element for spectroscopic or optical measurement and a method for manufacturing the same. <P>SOLUTION: A sensor element for spectroscopic/optical measurement includes: a sensor chip 2 having a measuring device 3 for sensing a beam 10; a cap chip 6 fixed to a sensor chip via a vacuum-sealing connector 5; a free space 7 formed between the cap chip and the measuring device and sealed with the connector 5; and a Fresnel zone structure 12 that focuses the incident beam 10 on the measuring device. A Fresnel lens is not realized as a three-dimensional structure on an upper surface of the chip but realized inside of the cap chip. That is, the Fresnel zone structure is formed inside of the cap chip. Therefore, the present invention makes it possible to realize a Fresnel lens, integrated on a cap wafer, which has a flat surface. It is possible to avoid a Fresnel lens taking the form of a three-dimensional structure and to realize the Fresnel lens in a cap wafer or a cap chip. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The invention relates to a sensor element for spectroscopic or optical measurement, the sensor element being fixed in a sensor chip with a measuring device sensitive to the beam and at least one vacuum-tight connection to the sensor chip. A cap tip, a free space formed between the cap tip and the measuring device and sealed by at least one vacuum-tight connector, and for focusing an incident beam on the measuring device It relates to a type having a Fresnel zone structure. The invention further relates to a method for producing such a sensor element.

  MEMS (Mikro-Elektro-Mechanisches-System) based infrared sensors are used inter alia for spectroscopic measurements for determining gas concentrations. For this, they are used in particular in evacuated metal casings, in particular in TO casings. In this case, the chip is placed at the bottom of the casing. The casing is sealed with a hermetically sealed metal cap, and an optical window is integrated in the cap. In this case, the optical window can also be realized as a lens for beam focusing.

  This type of form is relatively large and costly. Therefore, integrated sensors are known in which the sensor chip is sealed by a cap chip mounted hermetically sealed. Sensors covered in this way are generally formed with additional optical elements, with a defined distance between the sensors and the optical elements, so that suitable optical elements for beam focusing are realized. It has become so.

  Furthermore, it is known to form the beam focus with a Fresnel lens, in which case a three-dimensional structure is formed on the upper surface of the chip. A rapid change with a high refractive index is realized by the continuous silicon and air, and an effective refractive index depending on the location is set. A focusing action and an additional diffractive action are thus realized, so that the passing electromagnetic beam is decomposed into a spectrum. In this case, the lens structure projects only the relevant or important measurement wavelengths onto the active surface of the measuring device and therefore no additional filter elements are required.

  In this case, this type of three-dimensional structure is realized in a region smaller than μm, depending on the wavelength concerned. When the liquid reaches the lens surface, lens action and filter action no longer occur. In this case, the three-dimensional structure realized in this way prevents the lens surface from being washed or from completely evaporating the liquid thereon. When the structure size is small as in this type, significant dew formation may occur based on the adhesive force. In this case, contamination can occur with various substances such as water and oil droplets, especially in the area of automobiles.

  The object of the present invention is to provide a sensor element of the type mentioned at the outset and a method for its production which overcomes the drawbacks of the prior art.

  According to the present invention, the Fresnel lens is not realized on the upper surface of the chip as a three-dimensional structure, but is realized inside the cap chip. The structure can be realized in particular by doping the relevant regions. Therefore, according to the present invention, a Fresnel lens having a flat surface integrated on a cap wafer can be realized.

  The present invention is based on the recognition that very high refractive index differences can be realized by doping. Since the refractive index is proportional to the dielectric constant, a large drastic change in the refractive index can be realized by high doping of the material, so that a three-dimensional structure can be avoided for the Fresnel lens and the cap wafer. Or realization in a cap chip is possible. Thus, the three-dimensional structured surface problem that occurred in the prior art does not occur. According to the present invention, it is possible to realize a flat surface in which the material coming from the outside is difficult to fix and whose optical properties are independent or only slightly dependent on the externally attached material such as water and oil stains. can do. Furthermore, cleaning by complete evaporation of the contamination is possible, and in some cases simple mechanical cleaning is possible without the risk of mechanical damage.

  The differently doped regions as continuous, concentric ring structures according to the invention, in particular weakly doped regions and heavily doped regions, provide an effective refractive index depending on the location across the structure. . As a result, a beam that is incident on an outer region viewed laterally having a relatively high effective refractive index or a relatively highly doped region having a relatively large radial distance from each other. Is refracted more strongly than the beam incident on the center of the ring structure, thereby realizing the focusing action and thus the Fresnel lens action of the present invention that can concentrate the incident beam on the measuring device or the sensitive structure.

  According to the invention, the doping of the region is set in particular, but alternatively, with a different refractive index, regions with different refractive indices are basically formed on the substrate of the cap chip, for example by oxidation of Si material. It can be produced by chemical changes such as nitriding.

  According to the invention, the design for radiation is particularly important in the infrared region, in particular in the region of the absorption wavelength of gases such as CO2, nitrogen oxides and the like. In this case, the structure can be structured in an appropriate order for this purpose. Thus, according to the invention, it is possible in particular to determine the accumulation of CO2 in a CO2-containing refrigerant or in the interior air of a motor vehicle using a spectroscopic sensor for measuring the absorption of CO2 IR radiation.

  The sensor element of the invention can thus be realized without additional optical means for focusing the beam, i.e. without an additional lens in the optical path.

  Furthermore, since the Fresnel zone structure of the present invention advantageously focuses only the incident beam in the wavelength region around the central wavelength in the sensitive measuring device, no additional wavelength selective filter element is required.

  According to the present invention, the sensor wafer and the cap wafer are separately structured and can be joined later, so that wafer level fabrication is possible where individual sensor elements can be fabricated by individualization. .

  Next, the present invention will be described in detail based on the illustrated embodiment.

  The sensor element 1 of the present invention has a sensor chip 2 having a measuring device 3 sensitive to an IR beam. A cap chip 6 is fixed to the sensor chip 2 via a vacuum-tight seal glass connector 5 made of a glass material that melts at a low temperature. The cap chip 6 has a cavity 7 formed on the lower surface thereof by KOH etching. That is, the cavity forms a free space 7 in which a vacuum is formed between the measuring device 3 and the cap chip 6.

  The measuring device 3 is known as such and can have a thermopile structure, for example, so that a cavity 4 is formed for thermal isolation in the sensor chip 2. be able to. Furthermore, the measuring device 3 can have a photodiode, a photoresistor or another element sensitive to the associated beam, in which case the cavity 4 is not necessary for this type of element.

According to the present invention, the upper surface 8 of the cap chip 6 is realized flat or flat. The lower surface 9 is also advantageously flat in the region above the cavity 7. The cap chip 6 is formed with a Fresnel zone lens 12 for focusing the incident beam 10. The Fresnel zone lens 12 is formed in particular from a doped region 14 and an undoped region 15 with a ring-shaped structure as can be seen in FIG. These regions are realized on the upper surface 8 of the cap chip 6. In this case, for example, a low n-doped region 14 can be formed in the p-doped silicon of the cap chip 6. Advantageously, the region 14 is highly doped, i.e. n ⁺ doped, resulting in a high doping difference between the region 14 of the chip 6 and the region 15 located therebetween. Due to the proportional relationship between the dielectric constant ε and the refractive index n, a significant difference in refractive index is realized based on a high density difference.

が成り立ち、その際にｉ＝１，２，３，…である。 The exact ring structure of FIG. 3 is defined by the principle of a Fresnel zone lens 12 or Fresnel zone plate having a diffractive action in the ring structure. Based on the large object distance compared to the focal length, i.e. the very large distance of the IR light source from the Fresnel zone lens 12 (especially the parallel beam incidence, i.e. the object distance is infinite), the respective radius ri and alternating For a ring Ri having an index of refraction ni where the associated measurement wavelength is λ ₀ and the vertical distance between the top surface 8 and the measuring device 3 is H

Where i = 1, 2, 3,...

  Accordingly, the ring Ri having the radius ri is the alternating region 14 and region 15. Accordingly, the radius ri, that is, the distance in the lateral direction of the regions 14 and 15 with respect to the optical axis A (where i = 1, 2, 3,...) Increases outward. The width ai of the ring Ri here decreases toward the outside. The vertical height of the lateral region 14 can advantageously be constant.

The beam 10 is refracted more strongly in the laterally outer region having a relatively high effective refractive index than in the center of the ring structure. Fresnel zone plate known convergent or focusing action occurs from the illustrated in the case 2 and 3, structures of the doped region 14, the measuring device 3 is very related measurement wavelength lambda _0, for example, the absorption wavelength of CO2 It is selected to come to the focal point of the Fresnel zone lens 12 only. The other wavelengths are preferably suppressed or only the incident beam in the wavelength region Δλ around the measuring wavelength λ ₀ is focused on the sensitive measuring device 3, so that no separate wavelength-specific filter is required.

Accordingly, the vertical distance between the upper surface 8 and the measuring device 3 is such that the measurement wavelength λ ₀ , the refractive index of the regions 14 and 15 taking into account the geometric form i of the ring structure of the doped region 14 and the cap chip 6. It is appropriately selected depending on the light refraction at the lower surface 8.

The illustrated sensor element 1 therefore does not have additional lenses, mirrors or other optical means for focusing. The focusing of the incident IR beam 10 is carried out exclusively by refraction, diffraction and interference according to the principle of a Fresnel zone lens in the relevant measurement wavelength λ ₀ or the wavelength region Δλ around the measurement wavelength λ ₀ as the central wavelength.

  Instead of the IR beam 10, the Fresnel zone lens 12 may basically be designed for visible light.

  The sensor element 1 according to the invention may be designed in particular as a surface temperature sensor, which is relevant for broadband applications, for example for wavelengths in the region 8 to 14 μm.

  Fabrication can be done at the wafer level according to the present invention. The sensor wafer 2 and the cap wafer 6 are structured, in which the cavity 14 is formed from the back surface by KOH etching and the doped region 14 from the top surface 8 is doped by a doping method, for example ion implantation or other doping methods. In some cases, it is formed by an appropriate mask technique. Next, the cap wafer is fixed to the sensor wafer using the seal glass connector 5 and the individual sensor element 1 is produced by individualizing or sawing the wafer stack.

1 is a schematic longitudinal sectional view showing a sensor element of the present invention attached to a circuit support. Schematic cross-sectional view of the cap chip of the present invention Plan view of cap chip

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 Sensor element, 2 Sensor chip, 3 Measuring apparatus, 5 Connection body, 6 Cap chip, 7 Cavity, 8 Upper surface, 9 Lower surface, 10 Beam, 12 Fresnel zone structure or lens, 14 Doped area, 15 Undoped region

Claims (15)

Sensor element for spectroscopic or optical measurement, the sensor element comprising:
A sensor chip (2) with a measuring device (3) sensitive to the beam (10);
A cap chip (6) fixed to the sensor chip (2) by at least one vacuum-tight connector (5);
A free space (7) formed between the cap tip (6) and the measuring device (3) and sealed by at least one vacuum-tight connector (5);
In the form of a Fresnel zone structure (12) for focusing an incident beam (10) onto the measuring device (3),
Sensor element characterized in that the Fresnel zone structure (12) is formed in the cap chip (6). 2. A sensor element according to claim 1, wherein the upper surface (8) of the cap chip (6) is formed flat. 2. A sensor element according to claim 1, wherein the lower surface (9) of the cap chip (6) is formed flat. The sensor element according to any one of claims 1 to 3, wherein the Fresnel zone structure (12) is formed in contact with the upper surface (8) and / or the lower surface (9) of the cap chip (6). Sensor element according to any one of the preceding claims, wherein the Fresnel zone structure (12) has a region (14) doped in the cap chip (6). Sensor element according to claim 5, characterized in that the Fresnel zone structure (12) is formed of alternating rings (Ri) consisting of differently doped regions (14, 15). A ring-shaped doped region (14) is formed on the upper surface (8) and / or the lower surface (9), and a region (15) of the substrate material of the cap chip (6) is formed between the regions. The sensor element according to claim 6 remaining. The doped region (14) is highly doped, preferably doped with a conductivity type opposite to the substrate material of the cap chip (6). Sensor element. Sensor element according to any one of claims 5 to 8, wherein the doped region (14) is formed by substantially the same vertical height (h). Sensor element according to any one of claims 5 to 9, wherein the width (ai) of the doped region (14) decreases outward in the radial direction. The Fresnel zone lens (12) focuses an incident beam (10) in a wavelength region (Δλ) around a central measurement wavelength λ ₀ onto the measurement device (3). Sensor element. The sensitive measuring device (3) is sensitive to an IR beam (10) and the Fresnel zone lens (12) focuses the incident IR beam (10). Sensor element. The sensor element according to any one of claims 1 to 12, wherein the sensor element is formed as a surface temperature sensor, for example, with respect to a wavelength in a region of 8 to 14 µm. 14. The sensor element has only a Fresnel zone lens (12) as an optical device and a spectral filter and does not have another optical device that focuses the beam (10) and is wavelength selective. The sensor element according to any one of the above. 15. The method for producing a sensor element according to any one of claims 1 to 14, wherein at least the following steps:
A sensor wafer (2) having a plurality of sensitive measuring devices (3);
Cap wafer (6)
Etching the back surface (9) of the cap chip (6) to form a plurality of cavities (7);
Structuring, in particular doping, the upper surface (8) of the cap chip (6) as a Fresnel zone lens (12) with radial, concentric rings (14) with different radii (ri). And the cap wafer (6) is mounted on the sensor wafer (2) and vacuum-tightly connected so that each Fresnel lens structure (12) is arranged above the sensitive measuring device (3). A method comprising vacuum-tightly connecting in the body (5) and individualizing the wafer stack body thus formed into a plurality of sensor elements (1).

Images