Transmitter package for a photoacoustic sensor

Abstract

The present disclosure relates to a transmitter package (100) for a photoacoustic sensor. The transmitter package comprises a MEMS infrared radiation source (11) for emitting pulsed infrared radiation in a first wavelength range. The MEMS infrared radiation source (11) can be arranged on a base (10). The transmitter package (100) can further comprise a rigid wall structure (12) arranged on the base (10) and laterally surrounding an outer periphery of the MEMS infrared radiation source (11). The transmitter package (100) can further comprise a cover structure (13) attached to the rigid wall structure (12), the cover structure (13) comprising a filter structure (14) for filtering the infrared radiation emitted from the MEMS infrared radiation source (11) and for providing filtered infrared radiation in a second wavelength range that is reduced.

Description

Technical Field

[0001] Embodiments of this disclosure relate to a package including a transmitter device for a photoacoustic sensor. Some specific, but non-limiting, examples relate to a transmitter package including a rigid wall structure and a cover structure attached to the wall structure. In some embodiments, the cover structure may include an integrated filter structure. Background Technology

[0002] Photoacoustic (PA) effects are based on the conversion of pulsed radiation energy into sound. Different gases can be detected using the concept of photoacoustic spectroscopy (PAS) based on the photoacoustic effect. Pulsed radiation (e.g., infrared: IR) can be emitted into a measuring chamber containing a gas or gas mixture that includes at least one analyte, i.e., at least one gas to be detected, such as CO2. The gas may absorb the energy of the emitted pulsed radiation, causing alternating localized heating of the gas, resulting in thermal expansion inside the measuring chamber, thereby generating pressure waves that can be detected as sound. Some gases may absorb energy at different wavelengths, thus producing characteristic pressure waves or sound profiles.

[0003] In photoacoustic spectroscopy, hermetically sealed packages, such as ceramic and metal can packages, can be used to provide robust packaging architectures and high reliability, minimizing infrared signal leakage throughout the application's lifespan. Some photoacoustic sensors may include a MEMS film that acts as an infrared heat source and a cover that is transparent to the emitted radiation. Known PAS emitter packages require a deep cavity with a minimum distance of 1 mm between the MEMS film and the cover. However, the tooling cost of ceramic packages with such deep cavities is too high to meet the cost requirements of the overall gas sensor solution. Metal can packaging is typically expensive due to the materials and processes used in its manufacture. However, if precious metals, such as those with sapphire glass windows, are used, the cost of this gas sensor solution becomes uncompetitive.

[0004] Molded QFN packages (quadrilateral flat lead-free packages) offer cavity solutions at a reasonable cost. However, potential reliability issues exist due to cracking at the interface between the leadframe and the molding compound. Unfiltered broadband IR signals may leak through the cracks. Additionally, wire bonding to the pre-molded cavity requires space between the emitter die and the cavity sidewalls to accommodate the wire bonding head. This space is necessary to prevent the head from contacting the cavity wall during wire bonding and resulting in a large package coverage area. Furthermore, conventional solutions for monitoring gases (e.g., odorless and colorless CO2) are bulky, expensive, or simply not widely available.

[0005] Therefore, it is desirable to provide a photoacoustic sensor and a transmitter package for the photoacoustic sensor, wherein the transmitter package is small, robust, reliable, and can be manufactured at low cost. Summary of the Invention

[0006] Therefore, it is proposed to provide a transmitter package for a photoacoustic sensor according to the present invention.

[0007] According to one aspect of this disclosure, the transmitter package may include a MEMS infrared radiation source for emitting pulsed infrared radiation in a first wavelength range. The MEMS infrared radiation source may be disposed on a substrate. The transmitter package may further include a rigid wall structure disposed on the substrate. The rigid wall structure may laterally surround the outer periphery of the MEMS infrared radiation source. The transmitter package may further include a cover structure attached to the wall structure. The cover structure may include a filter structure for filtering the infrared radiation emitted from the MEMS infrared radiation source, thereby providing filtered infrared radiation in a reduced second wavelength range.

[0008] The following defines some non-limiting examples, variations, and embodiments. Attached Figure Description

[0009] In the following description, embodiments of the present disclosure will be described in more detail with reference to the accompanying drawings, wherein:

[0010] Figure 1A shows a schematic block diagram of a side cross-sectional view of a transmitter package according to one embodiment.

[0011] Figure 1B shows a schematic top view of a transmitter package according to one embodiment.

[0012] Figure 2 A schematic cross-sectional side view of a segment of a cover structure according to one embodiment is shown. The cover structure includes a filter structure and an optional anti-reflective coating.

[0013] Figure 3A shows a schematic perspective view of a rigid wall structure according to one embodiment.

[0014] Figure 3B shows a perspective view of a transmitter package according to one embodiment.

[0015] Figure 3C shows a schematic cross-sectional view of a transmitter package according to one embodiment.

[0016] Figure 4A shows a schematic top perspective view of a MEMS infrared emitter mounted on a substrate.

[0017] Figure 4B shows a bottom perspective view of a transmitter package according to one embodiment.

[0018] Figure 4C shows a top perspective view of a transmitter package according to one embodiment.

[0019] Figures 5A-5H illustrate a method for assembling a transmitter package according to one embodiment.

[0020] Figure 6 A schematic cross-sectional view of a transmitter package according to one embodiment is shown before the cover structure is mounted onto a rigid wall structure.

[0021] Figure 7 A schematic side view of a photoacoustic sensor according to one embodiment is shown. Detailed Implementation

[0022] In the following description, the same or equivalent reference numerals are used to denote the same or equivalent elements or elements having the same or equivalent functions.

[0023] The method steps depicted by means of the block diagrams and described with reference to the block diagrams can also be performed in an order different from the order depicted and / or described. Furthermore, method steps relating to a specific function of the apparatus can be replaced by the features of the apparatus, and vice versa.

[0024] Figure 1A shows a schematic side view of a transmitter package 100 according to one embodiment, and Figure 1B shows a schematic top view of the transmitter package 100.

[0025] The transmitter package 100 may include a MEMS infrared radiation source 11 for emitting pulsed infrared radiation 20 within a first wavelength range. Therefore, the infrared radiation source 11 may also be referred to as a MEMS infrared emitter 11. The MEMS infrared emitter 11 can be configured to emit a broadband infrared signal 20.

[0026] In this disclosure, broadband infrared radiation may include wavelengths above 780 nm, for example, between 780 nm and 1 mm, preferably between 780 nm and 100 μm, or between 780 nm and 10 μm. The MEMS infrared radiation source 11 may be configured to emit infrared radiation 20 within the aforementioned wavelength range.

[0027] MEMS infrared radiation source 11 can be disposed on substrate 10. For example, substrate 10 can be a laminated substrate, such as a printed circuit board (PCB). However, other types of substrates, such as substrates comprising conductive materials, semiconductor materials, or insulating materials, are also possible.

[0028] The transmitter package 100 may include a rigid wall structure 12 disposed on the substrate 10. The rigid wall structure 12 may be disposed around the outer periphery of the MEMS infrared radiation source 11, or in other words, the rigid wall structure 12 may laterally surround the MEMS infrared radiation source 11. The rigid wall structure 12 may be in direct contact with the MEMS infrared radiation source 11, or may be located away from, i.e., not in contact with, the MEMS infrared radiation source. For example, as shown in FIG1A, the rigid wall structure 12 may be laterally and / or vertically away from the MEMS infrared radiation source 11.

[0029] Therefore, the rigid wall structure 12 can provide at least a lateral surrounding portion for housing or encapsulating the MEMS infrared radiation source 11. The vertical distance can be 1 mm or greater, which will be referenced later. Figure 6 and 7 To explain in more detail.

[0030] The rigid wall structure 12 can be opaque or non-transparent (e.g., at least 95% or more) relative to the infrared radiation 20 emitted by the MEMS infrared radiation source 11. Therefore, the rigid wall structure 12 can substantially shield the emitted broadband infrared radiation 20 from radiating to the outside of the package. The rigid wall structure 12 can be arranged, for example, to be mounted on the substrate 10 in a gapless manner, i.e., there is no substantial gap between the rigid wall structure 12 and the substrate 10 through which the emitted broadband infrared radiation 20 would otherwise escape. Therefore, an optional sealant can be provided at the transition between the rigid wall structure 12 and the substrate 10. For example, a sealant or adhesive can be provided by a suitable adhesive device 53 (see Figure 5G).

[0031] The transmitter package 100 may also include a cover structure 13 attached to the rigid wall structure 12. The cover structure 13 may include a filter structure 14 for filtering the broadband infrared radiation 20 emitted from the MEMS infrared radiation source 11 to provide filtered infrared radiation 30 with a reduced second wavelength range.

[0032] The filter structure 14 can be embedded in the cover structure 13, as exemplarily shown in FIG1A. Alternatively, the filter structure 14 can be arranged on the bottom side of the cover structure 13, i.e., on the side of the cover structure 13 facing the MEMS infrared radiation source 11 (see Figure 1A). Figure 2 and Figure 6 Alternatively, the filter structure 14 can be arranged on the top side of the cover structure 13, i.e., on the side of the cover structure 13 facing away from the MEMS infrared radiation source 11.

[0033] As described above, the MEMS infrared radiation source 11 can emit a broadband infrared radiation signal 20 within a first broadband wavelength range. The filter structure 14 can allow only a portion of the emitted broadband infrared radiation 20 to be transmitted; that is, the filtered infrared radiation 30 is in a reduced second wavelength range compared to the first broadband wavelength range. For example, the filter structure 14 can allow only certain wavelengths that can be absorbed by the analyte to be detected to pass through. For example, if CO2 is to be detected, the filter structure 14 can allow only infrared radiation 30 with a wavelength of approximately ~4.2 μm to pass through, because this wavelength corresponds to an absorption peak different from that of the CO2 molecule. Therefore, more generally, the filter structure 14 can allow only a predetermined portion 30 of the emitted infrared radiation 20 to pass through, the predetermined portion 30 including a predetermined wavelength having a tolerance of ±10% (e.g., 4.2 μm ± 10%), or a tolerance of 15%, or a tolerance of 20%.

[0034] Figure 2 A cross-sectional view (not drawn to scale) of only one segment of a cap structure 13 according to a non-limiting example is shown. The cap structure 13 may include a cap substrate portion 13a, which may also be referred to as a die, such as bulk silicon. The thickness of the cap substrate portion (die) 13a may be between 100 μm and 1000 μm, for example, exemplarily depicted as 650 μm. The cap structure 13 may include a filter structure 14, which may be arranged on a first side of the cap substrate portion 13a (e.g., Figure 2 (the bottom side of the cover), for example, arranged on the first side of the cover structure 13 or the cover base portion 13a facing the MEMS infrared radiation source 11 in the assembled state.

[0035] The filter structure 14 may include or be configured as a filter stack comprising two or more layers 21, 22 alternately arranged on top of each other. As a non-limiting example, Figure 2 A first layer 21 comprising silicon oxide (SiO) and a second layer 22 comprising amorphous silicon (a-Si) are shown. Of course, other suitable materials are also feasible. However, this arrangement comprising a stack of layers (which includes multiple first layers 21 and multiple second layers 22 arranged alternately on top of each other) is suitable for use as a gas filter stack for transmitting only infrared radiation of a wavelength suitable for detecting a specific gas. In other words, the filter structure 14 can be configured to transmit only infrared radiation comprising wavelengths suitable for detecting a specific target gas. In the non-limiting case of CO2, for example, the filter structure 14 can be configured to transmit only infrared radiation comprising wavelengths of approximately ~4.2 μm ± 10%.

[0036] Optionally, the cover structure 13 may include an anti-reflective coating (ARC) 15. The anti-reflective coating 15 may be disposed on an opposite second side of the cover base portion 13a (e.g., Figure 2The antireflective coating 15 can be located on the top side of the cover structure 13, opposite to the filter structure 14, for example, on the side of the cover structure 13 facing away from the MEMS infrared radiation source 11. Similar to the filter structure 14 described above, the antireflective coating 15 can include a layer stack comprising two or more layers 21, 22 alternately arranged on top of each other. As a non-limiting example, Figure 2 A first layer 21 comprising silicon oxide (SiO) and a second layer 22 comprising amorphous silicon (a-Si) are shown. Of course, other suitable materials are also possible. Thus, the antireflective coating 15 may comprise a layer stack comprising a plurality of first layers 21 and a plurality of second layers 22 alternately arranged on top of each other.

[0037] The first and second layers 21, 22 in the filter stack 14 and / or anti-reflective coating 15 may comprise the same or different materials, depending on the desired properties, such as the desired wavelength to be filtered by the filter structure 14.

[0038] Therefore, the broadband infrared signal 20 emitted from the MEMS infrared radiation source 11 can pass through the cover structure 13, and more specifically, through the filter structure 14, the cover substrate portion 13a, and the optional anti-reflective coating 15. The filter structure 14 filters the broadband infrared signal 20 emitted from the MEMS infrared radiation source 11 to produce filtered infrared radiation 30 including a narrow second wavelength compared to the first broadband wavelength. The optional anti-reflective coating 15 can increase the infrared signal throughput to approximately 90%.

[0039] In summary, the cap structure 13 (e.g., comprising silicon) may include a filter stack 14 (e.g., suitable for CO2) on the front side of die 13a and an ARC (anti-reflective coating) 15 on the opposite back side of die 13a. The filter stack 14 may allow only infrared signals with a predetermined wavelength to be transmitted, for example, a wavelength of ~4.2 μm for carbon dioxide. Typically, the predetermined wavelength may correspond to a different absorption peak of an analyte molecule, such as a target gas to be detected, such as CO2.

[0040] Of course, CO2 is mentioned herein only as a non-limiting example. The cap structure 13 and filter structure 14 can be configured such that the corresponding filter response allows wavelengths suitable for excitation / detection of other analytes (e.g., gases) different from CO2 to pass through. These other analytes may include at least one of, for example, methane, CO, and volatile organic compounds (VOCs).

[0041] As described above, the transmitter package 100 may include a rigid wall structure 12, which will be described in more detail below with reference to Figures 3A to 3C. Figure 3A shows an example of the rigid wall structure 12 from a perspective view, Figure 3B shows an assembled transmitter package 100 including the rigid wall structure 12, and Figure 3C shows a cross-sectional side view of the assembled transmitter package 100 including the rigid wall structure 12.

[0042] The rigid wall structure 12 may include four adjacent wall portions 12a, 12b, 12c, 12d, which form a rectangular, particularly square, closed or cavity structure, internally including a cavity portion 31. The adjacent wall portions 12a, 12b, 12c, 12d may also be referred to as vertical or elongated wall portions. The wall structure 12 may include adjacent wall portions 12a, 12b, 12c, 12d with a different number and / or different geometry than the depicted wall portions. The rigid wall structure 12 may be topless and / or bottomless, i.e., the wall portions 12a, 12b, 12c, 12d may be independent or self-supporting.

[0043] The rigid wall structure 12 may further include a bottom wall portion 12f facing the substrate 10. Therefore, the rigid wall structure 12 can be mounted to the substrate 10 at its bottom wall portion 12f. The rigid wall structure 12 may also include a top wall portion 12e facing away from the substrate 10. A cover structure 13 may be disposed on said top wall portion 12e.

[0044] As can be seen in Figure 3B, the rigid wall structure 12 can be arranged on the substrate 10 at its bottom wall portion 12f, and can laterally surround the MEMS infrared radiation source 11, which can also be arranged on the same substrate 10. In other words, the rigid wall structure 12 can be arranged around the outer periphery of the MEMS infrared radiation source 11. Therefore, the MEMS infrared radiation source 11 can be placed within the cavity 31 formed by adjacent vertical wall portions 12a, 12b, 12c, 12d.

[0045] The rigid wall structure 12 may include a height h measured vertically from the substrate 10. The height h of the rigid wall structure 12 may be greater than the height of the MEMS infrared radiation source 11. Therefore, in the assembled state, the rigid wall structure 12 may protrude from the substrate 10 to a greater extent than the mounted MEMS infrared radiation source 11.

[0046] The cover structure 13, including the filter structure 14, can be mounted on the portion of the rigid wall structure 12 opposite to the substrate 10, that is, on the top wall portion 12e of the rigid wall structure 12 facing away from the substrate 10. Therefore, the rigid wall structure 12 and the cover structure 13, including the filter structure 14, arranged thereon can together provide a cavity 31 in which the MEMS infrared radiation source 11 is arranged.

[0047] During the assembly of the transmitter package 100, the cover structure 13, including the filter structure 14 (e.g., having a CO2 filter stack), can be directly bonded to the top of the rigid wall structure 12, i.e., on the plane of the top wall portion 12e (see Figures 4C and 5F). To ensure the thickness of the adhesive layer at the interface between the cover structure 13 and the rigid wall structure 12 and / or at the interface between the rigid wall structure 12 and the substrate 10, a recessed structure can be provided on at least one of the following: the top side and the bottom side of the rigid wall structure 12.

[0048] In the non-limiting example depicted in FIG3A, the rigid wall structure 12 may include a recessed structure 33 disposed in the top wall portion 12e facing away from the substrate 10. The recessed structure 33 may be arranged on top of each adjacent vertical wall portion 12a, 12b, 12c, 12d. The recessed structure 33 may be continuously disposed along the top wall portion 12e of adjacent wall portions 12a, 12b, 12c, 12d. As shown in FIG3A as a non-limiting example, the recessed structure 33 may be disposed along the lateral inner circumferential portion of the top wall portion 12e, which faces the cavity 31. As can be seen in FIG3B, the cover structure 13 can then be placed within the recessed structure 33. Therefore, the outer dimension of the recessed structure 33 may substantially correspond to the outer dimension of the cover structure 13, such that the cover structure 13 can fit into the recessed structure 33, preferably without any substantial gap.

[0049] As described above, the rigid wall structure 12 may be opaque (e.g., at least 95%) or non-transparent to the broadband infrared radiation 20 emitted by the MEMS infrared radiation source 11. However, as described above, the cover structure 13, including the filter structure 14, may be at least partially transparent to the infrared radiation 20 emitted by the MEMS infrared radiation source 11. Therefore, the rigid wall structure 12 and the cover structure 13 including the filter structure 14 can together provide an emitter package 100 that is laterally non-transparent to the emitted infrared radiation 20, except for the cover structure 13 mounted on the top wall portion 12e, which provides at least partial transmission of the emitted infrared radiation 20.

[0050] Figure 3C shows the assembled transmitter package 100 in a side cross-sectional view. The MEMS infrared radiation source 11 is arranged within a cavity 31 formed by adjacent vertical wall portions 12a, 12b, 12c, 12d of a rigid wall structure 12. Therefore, the adjacent vertical wall portions 12a, 12b, 12c, 12d can laterally surround the MEMS infrared radiation source 11, or in other words, the adjacent vertical wall portions 12a, 12b, 12c, 12d can be arranged around the outer periphery of the MEMS infrared radiation source 11. A cover structure 13, including a filter structure 14, can be mounted on the top wall portion 12f as explained above, for example, at least partially within the recessed structure 33.

[0051] As can be seen in detail here, the recessed structure 33 may optionally include a support feature 34. This support feature 34 (or support structure) may be disposed on the laterally inner peripheral portion of the recessed structure 33 (i.e., the inner peripheral portion facing the cavity 31). For example, the support feature 34 may be continuously disposed on the laterally inner peripheral portion of the recessed structure 33. The support structure 34 may be separate from the bottom portion 35 of the recessed structure 33. That is, the distance d1 between the top wall portion 12e and the support structure 34 is less than the distance d2 between the top wall portion 12e and the bottom portion 35 of the recessed structure 33. The support structure 34 is particularly useful for preventing the material, such as adhesive, used to bond the cover structure 13 to the wall structure 12 from flowing laterally inward into the MEMS infrared radiation source 11 inside the cavity 31 when the cover structure 13 is mounted.

[0052] Additional optional recessed structures 33 can be introduced within the rigid wall structure 12 to further enhance the reliability of the transmitter package 100. The recessed structure 33 enables a good mechanical seal around the sides of the cover structure 13, which includes the filter structure 14. The support feature 34 on the top wall portion 12e ensures the desired adhesive layer thickness for the cover / filter attachment adhesive and prevents the cover / filter attachment adhesive from flowing into the cavity 31 towards the MEMS infrared radiation source 11.

[0053] Additionally or alternatively, a support feature similar to that described above (not explicitly shown) may be provided on the bottom wall portion 12f of the rigid wall structure 12. Such a support feature may ensure the thickness of the adhesive layer when the rigid wall structure 12 is attached or bonded to the substrate 10.

[0054] In summary, the emitter package 100 should include a rigid wall structure 12 with sufficient rigidity to securely suspend the cover structure 13. Furthermore, the rigid wall structure should not be easily broken. Moreover, the rigid wall structure 12 should be opaque or non-opaque to the broadband infrared radiation 20 emitted by the MEMS infrared emitter 11. Additionally, the rigid wall structure 12 should include a material that allows for easy and secure attachment of the cover structure 13, as well as easy and secure attachment of the wall structure 12 itself to the substrate 10. To meet all these requirements, the rigid wall structure 12 may include or be made of a liquid crystal polymer (LCP).

[0055] Even though other molding materials with similar mechanical and optical properties to LCP can be used, LCP is still preferred because it will allow the transmitter package 100 to have a very small coverage area compared to conventional solutions (in addition to the advantages already mentioned above). As a non-limiting example, the coverage area or package outline dimensions of the transmitter package 100 may be 4 × 4 × 2.3 mm (with a tolerance of ±0.1 mm).

[0056] Figures 4A through 4C illustrate examples of assembling such a small transmitter package 100 according to the innovative principles described herein. Figure 4A shows a MEMS die 11 mounted on a substrate 10. The substrate 10 may include a first (top) side 10a on which the MEMS infrared radiation source 11 is mounted and a second (bottom) side 10b opposite to it. Figure 4A also shows wire bonding 42 connecting electrodes on the surface of the MEMS die to the substrate 10, for example, the first side 10a of the substrate 10.

[0057] The substrate 10 may be, for example, a laminated substrate having one or more electrical contact areas 41, such as a PCB. The MEMS infrared radiation source 11 may be connected to the electrical contact areas 41 via bonding wires 42 (e.g., gold wire bonding). Thus, the exemplary emitter package 100 may include a MEMS heater die or MEMS infrared radiation source 11, one or more gold wire bonds 42, a laminated substrate 10, a rigid wall structure 12 preferably comprising or made of LCP, and a combined filter cover 13, 14, for example comprising or made of silicon.

[0058] Figure 4B shows a bottom view of the emitter package 100. The substrate 10 includes one or more of the aforementioned electrical contact regions 41 on its bottom side 10b for attaching and contacting the emitter package 100 to a carrier substrate, such as a system or component board (not shown). The (LCP) wall structure 12 may be bonded to the solder resist of the laminated substrate 10 at its top side 10a.

[0059] Figure 4C shows a top view of the transmitter package 100. In this non-limiting example, the wall structure 12 may not include the aforementioned recessed structure 33. Instead, the top wall portion 12e of the rigid wall structure 12 may be substantially planar. The cover structure 13, including the filter structure 14, may be directly bonded to the planar top wall portion 12e.

[0060] Therefore, as described above, the (silicon) filter cover structures 13, 14 can be bonded to the top of the (LCP) wall structure 12 to form a sealed cavity 31. In operation, the MEMS infrared radiation source 11 can emit a broadband IR signal 30 propagating toward the (silicon) filter cover structures 13, 14, the broadband IR signal 30 including a first broadband wavelength. The filter cover structures 13, 14 can transmit only a portion of the broadband IR signal 20 with a reduced predetermined second wavelength (e.g., a second wavelength), for example, in a non-limiting example of CO2, the wavelength is approximately ~4.2 μm. As described above, the filter structure 14 can be modified to selectively allow other light wavelengths required for the detection of other gases to pass through.

[0061] Figures 5A-5H illustrate individual steps of an exemplary, but not limiting, process for manufacturing and assembling the transmitter package 100 according to the innovative principles described herein.

[0062] In Figure 5A, a substrate 10 may be provided, such as a laminated substrate like a PCB.

[0063] In Figure 5B, an adhesive device 51, such as an adhesive, may be provided on the first (top) side 10a of the substrate for bonding the MEMS infrared emitter 11 to the substrate 10.

[0064] In Figure 5C, the MEMS infrared emitter 11 can be bonded to the predetermined portion of the substrate 10, which includes the adhesive device 51.

[0065] In Figure 5D, the MEMS infrared emitter 11 can be electrically contacted with a dedicated electrical contact area 41 by means of an electrical conductor, such as a bonding wire 42.

[0066] In Figure 5E, a rigid wall structure 12 may be provided. An adhesive device 52, such as an adhesive, for bonding the cover structure 13, including the filter structure 14, may be provided at the top wall portion 12e of the rigid wall structure 12. In this non-limiting example, the top wall portion 12e may be planar, as described above with reference to Figures 4A-4C. Alternatively, the top wall portion 12e may include a recessed structure 33 (and optionally, a support structure 34) as described above with reference to Figures 3A-3C. In this case, the adhesive device 52 would be disposed within the recessed structure 33.

[0067] In Figure 5F, the cover structure 13, including the filter structure 14, can be bonded to the top wall portion 12e by the adhesive applicator 52.

[0068] In Figure 5G, an adhesive device 53 can be provided along the lateral outer periphery at the first (top) side 10a of the substrate 10 for bonding the rigid wall structure 12 to the substrate 10.

[0069] In Figure 5H, the rigid wall structure 12 has cover / filter structures 13, 14 disposed thereon. The rigid wall structure can be bonded to the substrate 10 by means of a suitable adhesive device 53, such as an adhesive or glue, disposed on the lateral peripheral portion of the substrate 10. In particular, the bottom wall portion 12f of the rigid wall structure 12 can be attached to the adhesive device 53 disposed on the first (top) side 10a of the substrate 10.

[0070] Figure 6 Another example of a transmitter package 100 based on the innovative principles described herein is shown. It illustrates a non-limiting example of how a cover structure 13, including a filter structure 14, is bonded to a rigid wall structure 12.

[0071] As described above, the cover structure 13 may include a cover substrate 13a, such as bulk silicon. In the case where the rigid wall structure 12 comprises or is made of LCP, silicon may have a good adhesion to LCP. However, alternatively or additionally, the cover substrate 13a may include other materials besides silicon, which may also have a good adhesion to LCP.

[0072] As described above, the filter structure 14 can be arranged on the cover substrate 13a, for example on the first side (e.g., the front side) facing the MEMS infrared emitter 11. An optional anti-reflective coating ARC 15 can be disposed on the other side (e.g., the rear side) of the cover substrate 13a, for example on the opposite second side away from the MEMS infrared emitter 11.

[0073] In a non-limiting example of the assembly process of the transmitter package 100, the front side of the cover structure 13 (e.g., with CO2 filter stack 14) can be directly bonded to the plane of the top wall portion 12e of the rigid wall structure 12, such as... Figure 6 As shown. However, as discussed above with reference to Figures 3A to 3C, this article refers to... Figure 6 All the contents described in the embodiments also apply to the rigid wall structure 12 with optional recessed structure 33.

[0074] To improve adhesion between the cover structure 13 and the rigid (LCP) wall structure 12 in a cost-sensitive package, a double-sided etched cover structure 13 can be used. For example, an anti-reflective coating 15 can be structured, for instance, by etching. In this regard, the anti-reflective coating 15 may include predetermined markings 57, such as a "Pin 1" mark, to aid in pattern identification during package assembly and testing. This avoids the use of expensive laser marking machines capable of marking on silicon. Therefore, the anti-reflective coating 15 may include indications for marking predetermined pin connection points.

[0075] Additionally or alternatively, the lateral peripheral portion of the antireflective coating 15 may be etched away along the lateral outer edge of the cover substrate 13a, so that the lateral outer edge of the cover substrate 13a remains uncovered. This provides a cutting path 56.

[0076] Additionally or alternatively, the filter structure 14 can be structured, for example, by etching. For instance, the filter structure 14 can be etched away from one or more laterally peripheral portions of the cover substrate 13a, leaving these laterally peripheral portions of the cover substrate 13a uncovered. Thus, the cover substrate 13a can include one or more uncovered portions 55 facing the top wall portion 12e. In a non-limiting embodiment, the uncovered portions 55 can be arranged, for example, along the laterally outer edge of the cover substrate 13a, i.e., the filter structure 14 can be completely removed along the entire laterally outer periphery of the cover substrate 13a facing the first side of the MEMS infrared emitter 11.

[0077] Therefore, in a non-limiting embodiment (not shown), the filter structure 14 may at least partially cover the first side of the cover base 13a, such that the lateral peripheral portion 55 of the cover structure 13 is not covered, wherein the cover structure 13 may be attached to the rigid wall structure 12 at least at the uncovered lateral peripheral portion 55.

[0078] In alternative embodiments, such as in Figure 6 In the non-limiting embodiment depicted, the filter structure 14 may laterally overlap with the top wall portion 12e and the adhesive device 52 disposed thereon. This is in Figure 6 The overlapping portion 58 is represented by a lateral overlap O1. The lateral overlap O1 can range from 20 μm to 200 μm, or from 50 μm to 150 μm. According to a particular embodiment, the lateral overlap O1 can be 100 μm ± 10%. In other words, the overlapping portion 58 can include a lateral extension O1 of 100 μm or less, at which the filter structure 14 laterally overlaps the rigid wall structure 12. The lateral overlap O1 of 100 μm or less and the adhesive region 52 between the cover structure 13 and the filter structure 14 allow for desired arrangement tolerances during assembly.

[0079] In other words, the cover structure 13 may include an overlapping portion 58 at which the filter structure 14 may laterally overlap with the rigid wall structure 12, wherein the cover structure 13 may be attached to the rigid wall structure 12 using at least one of its uncovered portion 55 and its overlapping portion 58. In some embodiments, the cover structure 13 may be attached to the rigid wall structure 12 using both its uncovered portion 55 and its overlapping portion 58.

[0080] Therefore, an adhesive device 52, such as an adhesive or glue, can be provided at the top wall portion 12e of the rigid wall structure 12. For example, the adhesive device 52 may include an adhesive layer thickness BLT ranging from 10 μm to 50 μm, or from 20 μm to 40 μm. In some non-limiting examples, the BLT may be approximately 30 μm ± 10%. The cover structure 13 can then be attached to the rigid wall structure 12 by the adhesive device 52. Preferably, the aforementioned uncovered portion 55 of the cover substrate 13a may be in contact with the adhesive device 52.

[0081] In summary, the cover structure 13 can be configured as a double-sided etched die, for example, comprising or made of silicon. For example, on the front side of the cover structure 13 facing the MEMS infrared emitter 11, those portions of the filter structure 14 directly facing the adhesive area (i.e., adhesive fixture 52) can be etched away to improve adhesion to the wall structure 12.

[0082] The advantages of this filter design are:

[0083] • By minimizing the risk of delamination in the filter structure 14 at the interface between the filter structure 14 and the filter attachment adhesive (i.e., the adhesive device 52), the reliability of the transmitter package 100 is improved.

[0084] • No need to use new laser marking equipment in the production line.

[0085] In other words, this non-limiting exemplary embodiment can provide for reducing the size of the overlapping region 55, thereby providing improved filter placement accuracy.

[0086] Further reference Figure 6 Each adjacent vertical sidewall portion 12a-12d of the rigid wall structure 12 may include a lateral thickness D1 between 0.1 mm and 1.0 mm or between 0.25 mm and 0.75 mm. In some non-limiting examples, the lateral thickness D1 may be 0.5 mm ± 10%.

[0087] The cavity 31 formed by the adjacent vertical sidewall portions 12a-12d of the rigid wall structure 12 may include a lateral internal dimension D2 between 1 mm and 5 mm or between 2 mm and 4 mm. In some non-limiting examples, the lateral internal dimension D2 may be 3 mm ± 10%.

[0088] When the cover structure 13 is attached to the rigid wall structure 12, i.e. when the transmitter package 100 is assembled, a predetermined minimum vertical distance H1 between the MEMS infrared transmitter 11 and the cover structure 13 or the filter structure 14 can be reliably provided.

[0089] As described above, in a preferred embodiment, the rigid wall structure 12 may comprise or be made of LCP. This is advantageous because it allows for the provision of an encapsulation or housing (i.e., rigid wall structure 12) with rigid sidewall portions 12a-12d. The rigidity of the sidewall portions 12a-12d is sufficient to provide a self-supporting wall structure 12 or encapsulation at which the cover structure 13, including the filter structure 14, can be securely attached. In some non-limiting examples, the predetermined minimum vertical distance H1 is 1 mm or greater.

[0090] As described above, the requirements for the PAS sensor dictate a minimum distance of at least 1 mm between the infrared emitter 11 and the cover structure 13. In conventional ceramic packages, a cavity of at least 1 mm must be created by patterning individual ceramic sheets and bonding them together. However, the manufacturing cost of such a ceramic package with a deep cavity is too high to meet the cost requirements of the overall sensor solution. As an alternative, a QFN molded package can be provided. However, such a QFN package may crack at the interface between the lead frame and the molding material.

[0091] The innovative principle described herein proposes a rigid wall structure 12 comprising or made of a liquid crystal polymer (LCP). This allows for the resolution of the aforementioned existing problems. In particular, the LCP wall structure 12 can provide sufficient rigidity for the suspension cap structure 13 to be spaced apart from the MEMS infrared emitter 11 by a vertical distance H1 of at least 1 mm. Compared to conventional packaging, the tooling costs for the innovative LCP wall structure 12 are significantly reduced. Furthermore, the LCP wall structure 12 can be easily mounted on the substrate 10, for example, by means of a suitable adhesive tool 53, such as an adhesive or glue.

[0092] Therefore, some embodiments may provide a photoacoustic emitter package 100 constructed using a liquid crystal polymer (LCP) wall structure 12 and sealed by a double-sided etched cap structure 13. Some advantages of the emitter package 100 described herein are:

[0093] • The LCP cavity can be arranged after the standard die attachment and wire bonding process. By completing the die bonding and wire bonding of the MEMS device before forming the cavity, a smaller coverage area device can be achieved. This avoids the need to create additional space within the cavity to provide passage for the wire bonding heads;

[0094] • Allows the use of an LCP product platform with minimal additional tooling costs (compared to molded QFN solutions and ceramic package solutions) and a cavity depth of 1.25mm;

[0095] • Allows the use of low-cost standard FR4 laminates;

[0096] • The stepped structure surrounding the top outer periphery of the LCP sidewalls prevents the dicing blade from damaging the silicon filter during package cutting.

[0097] Therefore, this invention describes a combination of an LCP (liquid crystal polymer) sidewall structure 12 and a double-sided etched (silicon) filter cover 13, 14. The LCP material meets the requirements of the rigid sidewall structure 12, which also blocks the unfiltered IR signal 30. The LCP package thus has low mounting / tooling costs.

[0098] Figure 7 A non-limiting example of a photoacoustic sensor 1000 according to an embodiment is shown. Based on the innovative principles described herein, the photoacoustic sensor 1000 may include a transmitter package 100. The transmitter package 100 may be disposed on a substrate 110. The photoacoustic sensor 1000 may further include a pressure transducer 1020, such as a microphone, disposed adjacent to the transmitter package 100 on the same substrate 110.

[0099] The photoacoustic sensor 1000 may further include a housing 1010, such as a non-transparent metal cover welded to a substrate 110. The housing 1010 may define a measurement chamber 1030. An emitter package 100 and a pressure transducer 1020 may be arranged within the measurement chamber 1030. The emitter package 100 may emit filtered infrared radiation 30 at a reduced second wavelength after passing through the cover structure 13, which includes a filter structure 14, into the measurement chamber 1030. As a non-limiting example, if the photoacoustic sensor 1000 is used to detect CO2, the filter structure 14 may be configured to allow infrared radiation to pass through at a reduced second wavelength of approximately ~4.2 μm ± 10%.

[0100] CO2 molecules can absorb the emitted (pulsed) infrared radiation 30, and the molecules begin to alternately heat and cool, thereby generating pressure or sound waves that can be detected by pressure transducer 1020 within the measuring chamber 1030.

[0101] Therefore, the exemplary photoacoustic gas sensor 1000 depicted herein includes the innovative transmitter package 100 described herein, which includes a MEMS die 11 that emits a pulsed broadband infrared (IR) signal 20. The transmitter package 100 can generate a wavelength-reduced, filtered pulsed infrared signal 30, for example, approximately ~4.2 μm ± 10%, which is absorbed by specific gas molecules, such as carbon dioxide gas molecules. The absorbed energy causes localized heating and pressure changes in the gas sensor 1000, which can be detected by a sensor such as a silicon microphone 1020. According to this non-limiting example, the signal generated by the microphone 1020 is correlated with the CO2 concentration in ambient air.

[0102] To accurately detect pressure changes in the photoacoustic gas sensor 1000, the transmitter package 100 may include:

[0103] • Rigid sidewall structure 12, which maintains constant package profile dimensions during operation of gas sensor 1000 (no expansion or contraction for transmitter package 100);

[0104] • Except for the predetermined reduced second wavelength range (for CO2, for example, about ~4.2 μm), no IR signal is leaked at other wavelengths;

[0105] • The vertical distance H1 between the MEMS infrared emitter 11 and the combined filter cover structure 13, 14 is greater than 1 mm (deep cavity);

[0106] • Small package coverage area to save space in the customer's final product;

[0107] • Low packaging cost to achieve cost-effective gas sensor solutions.

[0108] Therefore, as described above, the use of the emitter package 100 with the LCP sidewall structure 12 enables a new concept of a cost-sensitive optical package 100 for the gas sensor 1000. The double-sided etched (silicon) filter cover structures 13, 14 improve package reliability, minimizing interference during assembly operations. The use of the LCP sidewall structure 12 also allows for a smaller coverage area compared to conventional molded cavity solutions. A fast-response infrared light source can be achieved using the MEMS infrared emitter 11 within the emitter package 100.

[0109] This fast-response infrared light source can be used in various sensor applications. For example, the transmitter package 100 described herein enables significant miniaturization of gas sensors 1000, such as CO2 sensors. The photoacoustic sensor 1000 described herein is ideal for smart home applications and building automation, as well as IoT devices for indoor air quality.

[0110] Compared to commercially available gas sensors, the gas sensor 1000 described herein saves over 75% of space in the customer's final product. This disclosure also achieves unprecedentedly low manufacturing costs compared to solutions that include LEDs and lasers, typically used as mid-infrared light sources for photoacoustic systems. The gas sensor 1000 described herein not only enables high-capacity and cost-sensitive CO2 gas sensing applications but also allows for the sensing of other gases and volatile organic compounds.

[0111] Furthermore, a method for manufacturing a transmitter package 100 for a photoacoustic sensor 1000 is proposed, the method comprising:

[0112] A MEMS infrared radiation source 11 is provided for emitting pulsed infrared radiation 20 in a first wavelength range, and the MEMS infrared radiation source 11 is arranged on the substrate 10.

[0113] A rigid wall structure 12 is arranged on the substrate 10 such that it laterally surrounds the outer periphery of the MEMS infrared radiation source 11.

[0114] A cover structure 13 including a filter structure 14 is provided for filtering infrared radiation 20 emitted from the MEMS infrared radiation source 11 and providing filtered infrared radiation 30 in a reduced second wavelength range, and the cover structure 13 is attached to the rigid wall structure 12.

[0115] According to another embodiment, the step of arranging the MEMS infrared radiation source 11 on the substrate 10 can be performed before the step of arranging the wall structure 12 on the substrate 10. As described above, the rigid wall structure 12 can laterally surround the MEMS infrared radiation source 11. Therefore, the rigid wall structure 12 can form a cavity 31 in which the MEMS infrared radiation source 11 is located. Providing such a cavity 31 makes the solution smaller because no additional space is required inside the cavity 31 to accommodate the bonding head.

[0116] Although some aspects have been described in the context of the device, it is clear that these aspects also represent a description of the corresponding method, where a module or apparatus corresponds to a method step or a feature of a method step. Similarly, aspects described in the context of a method step also represent a description of a corresponding module, item, or feature of the corresponding device.

[0117] Although this disclosure has been described with reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. Various modifications and combinations of exemplary embodiments, as well as other embodiments of this disclosure, will be apparent to those skilled in the art upon reference to the specification. Therefore, it is intended that the appended claims cover any such modifications or implementations.

Claims

1. A transmitter package (100) for a photoacoustic sensor, the transmitter package (100) comprising: A MEMS infrared radiation source (11) is used to emit pulsed infrared radiation in a first wavelength range. The MEMS infrared radiation source (11) is directly disposed on the top surface of the substrate (10). A rigid wall structure (12) is directly disposed on the top surface of the substrate (10) and laterally surrounds the outer periphery of the MEMS infrared radiation source (11). A cover structure (13) is attached to the rigid wall structure (12), the cover structure (13) including a filter structure (14) for filtering infrared radiation emitted from the MEMS infrared radiation source (11) and providing filtered infrared radiation in a reduced second wavelength range. The top surface of the substrate (10) is located in the measurement cavity of the photoacoustic sensor, and a measurement component for the photoacoustic sensor is also provided on the top surface.

2. The transmitter package (100) according to claim 1, The rigid wall structure (12) therein comprises a liquid crystal polymer (LCP) or is made of a liquid crystal polymer.

3. The transmitter package (100) according to claim 1 or 2, The cover structure (13) is mounted on the portion of the rigid wall structure (12) opposite to the substrate (10), such that the cover structure (13) and the rigid wall structure (12) together form an emitter cavity (31), and the MEMS infrared heating source (11) is arranged in the emitter cavity.

4. The transmitter package (100) according to any one of claims 1 to 3, The rigid wall structure (12) is non-transparent to the infrared radiation (20) emitted from the MEMS infrared radiation source (11).

5. The transmitter package according to any one of claims 1 to 4, The rigid wall structure (12) includes a recessed structure (33) disposed on a top wall portion (12f) opposite to the base (10), and the cover structure (13) is installed within the recessed structure (33).

6. The transmitter package (100) according to claim 5, The recessed structure (33) includes a support structure (34) for preventing the cover attachment adhesive from flowing laterally inward toward the MEMS infrared radiation source (11), wherein the distance (d1) between the top wall portion (12e) and the support structure (34) is less than the distance (d2) between the top wall portion (12e) and the bottom portion (35) of the recessed structure (33).

7. The transmitter package (100) according to any one of claims 1 to 6, The cover structure (13) includes a cover base (13a), The filter structure (14) is disposed on the first side of the cover substrate (13a) facing the MEMS infrared radiation source (11), and / or An anti-reflective coating (15) is disposed on the second side of the cover substrate (13a) opposite to the MEMS infrared radiation source (11).

8. The transmitter package (100) according to claim 7, The cover substrate (13a) therein comprises silicon or is made of silicon.

9. The transmitter package (100) according to claim 7 or 8, The filter structure (14) partially covers the first side of the cover base (13a) such that the lateral peripheral portion (55) of the cover structure is not covered, wherein the cover structure (13) is attached to the rigid wall structure (12) at least at the uncovered peripheral portion (55).

10. The transmitter package (100) according to any one of claims 7 to 9, The cover structure (13) includes an overlapping portion (58), the filter structure (14) laterally overlaps the rigid wall structure (12) at the overlapping portion, and the cover structure (13) is attached to the rigid wall structure (12) using at least one uncovered portion (55) of the cover structure and at the overlapping portion (58) of the cover structure.

11. The transmitter package (100) according to claim 10, The overlapping portion (58) includes a lateral extension (O1) of 100 μm or less, wherein the filter structure (14) and the rigid wall structure (12) laterally overlap at the lateral extension.

12. The transmitter package (100) according to any one of claims 7 to 11, The filter structure (14) includes multiple layers (21, 22) deposited on a first side of the cap substrate (13a), and / or The anti-reflective coating (15) comprises multiple layers (21, 22) deposited on the second side of the cover substrate (13a).

13. The transmitter package (100) according to any one of claims 1 to 12, The distance (H1) between the cover structure (13) and the MEMS infrared radiation source (11) is at least 1 mm.

14. The transmitter package (100) according to any one of claims 1 to 13, The second wavelength range corresponds to the absorption spectrum of the target medium to be detected by photoacoustic principles.

15. A photoacoustic sensor (1000) comprising a transmitter package (100) according to any one of the preceding claims.

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