Patent Application
20220357213
The invention relates to a flexible passive electronic component, in particular to a sensor measuring the change in electric resistance caused by external impacts such as temperature, gas absorption, chemical reactions, particle flow, light input and force input. The flexible passive electronic component has an electrical structure in which the electric resistance of the electrical structure changes by the external input. The electrical structure is provided on a substrate. The invention also relates to a method for producing the flexible passive electronic component, especially the sensor.
A temperature sensor is an example of such a sensor. Currently used thin-film temperature sensors with a positive temperature coefficient are based on a flat metal layer applied to a substrate. With a temperature change ΔT, the electrical resistance of the metal sheet R(T) changes according to R(T)=R(0)·(1+ξ(T)·ΔT), with R(0) equal to the resistance at T=0. With many metals, the temperature coefficient ξ(T)—at least at temperatures 0<T<200° C.—is constant and independent from the temperature. A resistance coefficient independent from the temperature is advantageous as it facilitates the determination of the temperature from the resistance measured value.
An important prerequisite for a linear dependence of the resistance on the temperature or the like, besides the suitable choice of the metal, is the correct choice of the substrate. Ideally, the substrate should have a similar coefficient of thermal expansion (CTE) to that of the sensor metal deposited on it. Especially, if the CTE of the substrate is larger than that of the metal, cracks in the metal layer can occur, and this leads to an undesired drift of the sensor. Aluminium oxide (Al2O3) is a particularly suitable substrate for Pt thin-film sensors. The CTE of Al2O3 is 6.5-8.9E-6 (1/K) and is therefore similar to the CTE of the platinum with 8.8E-6 (1/K).
However, ceramic bodies as substrates for temperature sensors have the disadvantage of a lack of mechanical stability with a substrate thickness below 500 μm. In particular, the handleability of larger substrates becomes difficult even at the thickness of less than 1000 μm due to the high tendency to break.
U.S. Pat. No. 9,209,047 discloses a flexible semiconductor device in which a device layer is arranged on a polymer layer.
However, flat substrates such as polymer films, especially polyimide films, are not suitable for precise thin film temperature sensors because polymer films change in shape or physical property over time. In addition, the CTE of the polyimide film with 20E-6 (1/K) is too high for the platinum. The conventional technique of producing single encapsulated dies and thin substrates has been described only for ICs but not for tiny and flat sensor with high accuracy.
The object of the invention is to realize a precise and at the same time flexible passive electronic component, flat sensor.
This object is accomplished in accordance with a flexible passive electronic component, in particular a sensor, whose substrate is formed from an insulating layer and optionally a further inorganic layer with a total thickness of at most 50 μm, and whose height is at most 150 μm. The high precision and long-term stability of the flexible passive electronic component is ensured by using the inorganic layer. In contrast to polymeric substrates, the inorganic layer shows no temporal change and has a relatively low CTE. In addition, in contrast to ceramic substrates, the inorganic layer or the substrate consisting of the insulating layer and the inorganic layer can be thinned down to a thickness of 50 μm or less, while keeping the mechanical stability. The thinned substrate provides flexibility to the flexible passive electronic component. This flexibility can still be maintained by the limited overall height of the flexible passive electronic component with at most 150 μm. Furthermore, the flexible passive electronic component, especially the sensor, with small mass follows rapidly the surrounding environment, resulting in a quick response in measurements.
In particular, the flexible passive electronic component is not an integrated circuit (IC) or does not contain an integrated circuit (IC).
In a preferred embodiment, the substrate has a thickness of at most 35 μm, particularly preferably at most 20 μm. It proves to be especially advantageous because the thinner substrate provides more flexibility while maintaining its stability. The minimum thickness of the inorganic layer together with the insulating layer is exemplified by 10 μm.
In one embodiment of the invention, it is possible that the substrate consists only of the insulating layer.
In a preferred embodiment, the flexible passive electronic component has a height of at most 70 μm, particularly preferably at most 40 μm. The reduced height of the flexible passive electronic component proves to be advantageous in terms of more flexibility and quicker responsiveness.
In a preferred embodiment, the flexible passive electronic component has a flexibility that is bendable with a radius of bending curvature of at least 5 mm, preferably at least 2 mm, preferably at least 1 mm, wherein a relative difference of resistivity (d R/R(0), wherein d means delta or Δ) of the electrical structure before and after bending may not exceed 0.5%. The flexible passive electronic component with such a flexibility may be suitable for use under a bending stress due to a thermal expansion/shrink or external mechanical stress.
Such a flexibility of the flexible passive electronic component can be measured as follows. The flexible passive electronic component is glued on a Polyimide foil (Kapton®, 25 μm) to determine the flexible passive electronic component's flexibility. Prior to gluing, the surface of the Kapton film is activated by treatment with a corona discharge. A cyano-based glue (MINEA, superglue) is applied on the backside of the thin flexible passive electronic component and the flexible passive electronic component is pressed on Polyimide foil for the reaction time of the glue. The polyimide foil is wrapped around a cylindrical bar with the flexible passive electronic component directed outwards. The diameter of the bar is chosen to be in the range of 0.5 mm to 10 mm. When being wrapped, the polyimide foil is in close contact with the surface of the cylindrical bar. Therefore, the radius of curvature applied to the flexible passive electronic component is approximatively half of the bar's diameter. The duration at which maximal bending is applied is set to approximately one second. The relative difference of resistivity d R/R(0) (wherein d means delta or Δ) of the electrical structure is measured before and after each bending cycle. The flexible passive electronic component is meant to be flexible as long as the radius of curvature is not smaller than the minimal radius Rm. Rm is defined by the radius at which d R/R(0)=0.5%.
The inorganic layer is preferably made of an inorganic crystalline material, in particular silicon, silicon carbide, gallium arsenide or sapphire, or is made of an inorganic amorphous material, in particular quartz glass, borosilicate glass or glass, or preferably is made from a silicon-on-insulator (SOI) wafer.
In a preferable embodiment, the flexible passive electronic component has a length, a width and the height, wherein the cross-section (length×width) is at most 4 mm2, preferably at most 2 mm2, particularly preferably at most 1 mm2. Owing to the comparatively small length and width with the limited height, advantageously a low mass and consequently a rapid responsiveness are guaranteed.
In a preferable embodiment, the electrical structure includes at least one conductor track and at least two electrical contact pads, wherein the at least two electrical contact pads are electrically connected to the at least one conductor track. The conductor track may be constructed to extend in the shape of a meander.
In a preferable embodiment, the at least one conductor track has a uniform width, wherein the width is not more than 5 μm and the standard deviation of the width is not more than 5% of the width. The thin and uniform conductor track is advantageous for quick and stable response to the rapidly changing measurement parameters. The conductor track may be formed at least partially as an adjustment straining to allow in-line trimming of the conductor track's overall resistance.
In a preferable embodiment, the conductor track has a temperature coefficient of electrical resistance of at least 3.000 ppm/K, preferably at least 3.500 ppm/K, particularly preferably at least 3.800 ppm/K.
In a preferable embodiment, the flexible passive electronic component may further comprise at least one additional electrical structure. Each additional electrical structure may include at least one conductor track. The electrical structure and the at least one additional electrical structure may be arranged in a multilayer structure such that:
The increased sensing area with the multi-layered electrical structure is advantageous for the improved sensing accuracy and performance, while keeping the flexible passive electronic component small.
In a preferable embodiment, the insulating layer is made of metal oxides and/or metal nitrides, in particular silicon dioxide, silicon nitride, aluminium oxide, aluminium nitride, hafnium oxide or hafnium nitride. The insulating layer made of such a material exhibits stable electrical insulation on the inorganic layer. The material of the insulating layer, however, is not limited to these, as long as necessary electrical insulation can be obtained on the substrate surface.
In a further preferable embodiment, the flexible passive electronic component may further comprise a cover layer at least partially covering the electrical structure. The cover layer may be formed from an inorganic layer. The electrical structure is thereby protected, particularly in an aggressive environment, so that the long-term stability can be achieved.
In a further preferable embodiment, the flexible passive electronic component may further comprise a first protective layer. The first protective layer at least partially, in particular completely, covers the cover layer. The first protective layer may be made of polymeric material such as polystyrene (PS), polyethylene (PE) and polyimide (PI). Preferably, the material consists of photostructurable polyimide (PS-PI). The material is photostructured according to a mask of the cover layer.
In a preferable embodiment, the electrical structure is designed as a sensor element and/or heater element. The sensor may be designed as a temperature sensor, flow sensor, particle sensor or chemical sensor.
In an example of the flow sensor, one heater element may be arranged between two temperature sensor elements in the flow direction.
In an example of the temperature sensor, the electrical structure may have an electrical resistance of at least 100 Ohm, preferably at least 1.000 Ohm, particularly preferably at least 10.000 Ohm. Low self-heating is thereby achieved.
In an example of the electrical structure as the heater element, it may have an electrical resistance of at most 5 Ohm, preferably at most 2 Ohm, preferably at most 1 Ohm. The conductor track of the heater element is preferably designed as a square, a U-loop. Preferable the heater is designed as multiple parallel addressed conductor tracks, whereas the conductor tracks have uniform width, the width being not more than 5 μm and the standard deviation of the width being not more than 5% of the width.
In a further preferred embodiment, the electric structure is combined with a communications system. The communications system may include one or more wireless interfaces and/or one or more wireline interfaces, which allow the electric structure to communicate via one or more networks.
Such wireless interfaces may provide for communication under one or more wireless communication protocols, such as Bluetooth and/or WiFi (e.g., an IEEE 802.11 protocol) and/or Long-Term Evolution (LTE) and/or WiMAX (e.g., an IEEE 802.16 standard) and/or a radio-frequency ID (RFID) protocol and/or near-field communication (NFC) and/or other wireless communication protocols.
In a preferable embodiment, the flexible passive electronic component may further comprise a second protective layer on the lower side of the inorganic layer. The second protective layer may be made of polymeric material such as polystyrene (PS), polyethylene (PE) and polyimide (PI). The protective layer is not limited to these as long as it has appropriate mechanical stability, electrical insulation and heat transfer properties.
The above objective is accomplished according to an inventive flexible passive electronic component system. The flexible passive electronic component system comprises at least one flexible passive electronic component according to any of the above inventive aspects, and at least one electrical control. The electrical control may be electrically connected to the electrical structure. The electrical control is configured to control the flexible passive electronic component.
In an example of a temperature flexible passive electronic component, the electrical control is configured to supply a constant current to the electrical structure (resistance element), to measure the voltage across the electrical structure, calculate the resistance value based on Ohm's law, and derive a temperature corresponding to the resistance value.
The above objective is accomplished according to an inventive method for producing a flexible passive electronic component, in particular an aforementioned flexible passive electronic component according to the invention. The method comprises following process steps:
In a preferable embodiment, to apply the electrical structure, a metal layer may be first applied to an inorganic wafer such as silicon wafer, silicon-on-insulator (SOI) wafer or glass wafer, which is then photolithographically structured. The metal layer may be made of platinum (Pt), aluminium (Al), nickel (Ni) or alloys such as an aluminium-copper (Al—Cu) alloy with 99.5% Al and 0.5% Cu, or an alloy containing at least Pt, Al and/or Ni.
In a preferable embodiment, the method may further comprise steps of:
In an embodiment of the invention, it is possible that in step e), the inorganic wafer is completely removed so that the substrate comprises only the insulation layer.
It proves to be especially advantageous in that the electrical structure can be prevented from damage during the thinning process.
In the following description, a temperature sensor will be mainly described. However, the application of the present invention is not limited to the temperature sensor, and the invention can be advantageously applied to a flow sensor, a chemical (gas) sensor, particle sensor, or the like.
In accordance with
The substrate 10 has a thickness t10, which is at most 50 μm, preferably at most 35 μm, particularly preferably at most 20 μm. In other words, the thickness t10 of a substrate 10 is made of a thickness t2 of the inorganic layer 2 and a thickness t3 of the insulating layer 3. In the case where the substrate 10 has only an insulating layer 3, the thickness t10 is equivalent to the thickness t3.
The sensor 1 has a height h1, which is at most 150 μm, preferably at most 70 μm, particularly preferably at most 40 μm. The thinned substrate 10 provides flexibility to the sensor 1. This flexibility can still be maintained by the limited overall height h1 of the sensor 1 with at most 150 μm. Furthermore, the sensor 1 with small mass follows rapidly the surrounding environment, resulting in a quick response in measurements.
The inorganic layer 2 may be made of an inorganic crystalline material, in particular silicon, silicon carbide, gallium arsenide or sapphire, or made of an inorganic amorphous material, in particular quartz glass, borosilicate glass or glass, or made from a silicon-on-insulator (SOI) wafer.
The insulating layer 3 may made of metal oxides and/or metal nitrides, in particular silicon dioxide, silicon nitride, aluminium oxide, aluminium nitride, hafnium oxide or hafnium nitride.
The electrical structure 4 may be made of platinum, nickel, aluminium or alloys such as an alloy at least containing platinum, nickel and/or aluminium, or an aluminium-copper (Al—Cu) alloy with 99.5% Al and 0.5% Cu.
The sensor 1 may further comprise a cover layer 5 at least partially covers the electrical structure 4. The cover layer 5 may expose one or more parts of the electrical structure 4 for one or more electrical contact pads 4a. The cover layer 5 may be formed from an inorganic layer.
The sensor 1 may further comprise a protective layer 6 on the lower side 2b of the inorganic substrate 10. The protective layer 6 may be made of polymeric material such as polystyrene (PS), polyethylene (PE) or polyimide (PI). The protective layer 6 is not limited to these as long as it has appropriate electrical insulation and heat transfer properties. The specific shape of the electrical structure 4 can be seen from
Besides the contact pads 4a, the electrical structure 4 includes at least one conductor track 4b. The conductor track 4b is electrically connected to both of the electrical contact pads 4a. The conductor track 4b has a first end and second end. The first end of the conductor track 4b is connected to one contact pad 4a; the second end of the conductor track 4b is connected to the other contact pad 4a.
In the illustrated example, the conductor track has a meander shape that repeatedly bends in S-shape. The shape of the conductor track 4b is not limited to this as long as a necessary conductor length or resistance value is obtained. Preferably, the conductor track 4b has a uniform width, wherein the width is not more than 5 μm and the standard deviation of the width is not more than 5% of the width. The thin and uniform conductor track 4b is advantageous for quick and stable response to the rapidly changing measurement parameters. The conductor track 4b may be formed at least partially as an adjustment straining.
The conductor track 4b may be applied as a sensor element and/or heater element. In an application to a chemical sensor, a catalytic material may be provided on the conductor track 4b as a sensor element. Also, another conductor track (not shown) as a heater element may be placed adjacent to above or below the conductor track 4b as the sensor element. In an application to a flow sensor, one conductor track as a heater element may be arranged between two conductor tracks as sensor elements.
In an example of the temperature sensor, the electrical structure 4 may have an electrical resistance of at least 100 Ohm, preferably at least 1.000 Ohm, particularly preferably at least 10.000 Ohm. Low self-heating is thereby achieved.
In an example of the electrical structure 4 as the heater element, it may have an electrical resistance of at most 5 Ohm, preferably at most 2 Ohm, preferably at most 1 Ohm. The conductor track of the heater element is preferably designed as a square or a U-loop. Preferable the heater is designed as multiple parallel addressed conductor tracks, whereas the conductor tracks have uniform width, the width being not more than 5 μm and the standard deviation of the width being not more than 5% of the width.
Although not shown, the sensor 1 may further comprises at least one additional electrical structure. Each additional electrical structure may include at least one conductor track which has the same structure as the conductor track 4b described above. The electrical structure 4 and the at least one additional electrical structure may be arranged in a multilayer structure such that:
The increased sensing area with the multi-layered electrical structures 4 is advantageous for the improved sensing accuracy or performance, while keeping the sensor 1 small. The additional insulating layer may have the same configuration as the insulating layer 3 on the inorganic layer 2.
Design example of the sensor for the temperature sensor may be as follows:
With sensor area 1×1 mm2 (1 mm in length and 1 mm in width), half of the substrate is coved by the contact pads 4a and the conductor track 4b, whereby the area of the contact pads 4a is at least 2×0.15 mm2 and the conductor track is formed as meander. The width of conductor track (line) of platinum is 1 μm; the spacing between the adjacent metal lines is 1.5 μm. The number of parallel metal lines is 200 with a total length of 20 cm. The electric resistivity of platinum (ρPt) is 1.06 μOhm m. The electric resistance R of the conductor track is obtained by R=ρPt×L/(B×H)
With reference to
Optionally, the electrical contact pads 4a are passivated with galvanic precious metal layers (not shown) applied thereon. An electroplating such as electro-nickel immersion gold (ENIG) may be applied on the electrical contact pads 4a.
Further advantages of the invented sensors can be found as follows:
A 200 mm silicon wafer with a silicon oxide layer on one side and a layer thickness of 1.2 μm is used as the inorganic wafer for the flexible sensor according to the invention. A metal layer is deposited on the silicon oxide layer. The metal layer has a total thickness of 850 nm and consists of an Al—Cu alloy of 99.5% Al and 0.5% Cu. The metal layer is then lithographically divided into individual electrical structures optional isolated from each other. The electrical structures each consist of a conductor track and two contact pads at the two ends of the conductor track. Each line of the conductor track has a homogeneous width and is arranged in a meander shape between the two contact pads. The electrical structures, which are differently designed on the wafer, are defined in particular by the following parameters:
After structuring the metal layer, the entire surface of the metal layer is first coated with an inorganic cover layer consisting of oxide and nitride layers. The total thickness of the inorganic cover layer is 1.4 μm. To ensure later electrical addressing, this cover layer is opened at the contact pads.
Subsequently, the surface is coated with photostructurable polyimide (PS-PI) with a dry layer thickness of approximately 3 μm and photostructured according to the mask of the cover layer. The surfaces of the contact pads remain free from the inorganic cover layer and the polyimide layer. The uncoated contact pads are then electroplated with a nickel/gold layer approximately 12 μm thick (ENIG process).
The individual structures are characterized electrically one after the other. The two contact pads of an electrical structure are contacted with the two double contact tips of a wafer tester. By applying a voltage to the contact tips, R0, i.e. the electrical resistance of the electrical structure at a temperature of 0° C., is determined. The temperature coefficient of resistance (TCR) is determined by determining the resistance R at a second temperature.
Table 1 shows the mean values and the standard deviation of the electrical resistance R0 and the temperature coefficient TCR of the electrical structures of group Z1. For this purpose, individual fields distributed on the wafer are evaluated. Field 51 is located in the center of the wafer, fields 1 and 5 at its lower edge and fields 79 and 83 at its upper edge. 8 structures of group Z1 are evaluated on each field. The structures of group Z1 have a line width W=6.5 μm.
As can be seen from Table 1, the mean value of the resistance R0 of structure Z1 is 247.2 Ohm with a standard deviation of 2.1 Ohm relative to the five fields examined. Thus, the relative standard deviation of the resistance R0 is 0.85%. The mean value of the TCR is 4137.9 ppm/K with a standard deviation of 2.4 ppm/K. Thus, the relative standard deviation of the TCR is 0.058%.
In Table 2, the electrical resistances of the electrical structures from group Z18 are statistically evaluated. The electrical structures have a line width W=0.5 μm. There are 18 adjacent electrical structures on each field in the lowest row. Their electrical resistances are shown in Table 2 for one field each from the center, top and bottom of the wafer. The electrical resistance R of the electrical structures was determined at room temperature.
As Table 2 shows, the mean value of resistance R of structure Z2 is 9285.6 Ohm with a standard deviation of 131.7 Ohm relative to the three fields studied. Thus, the relative standard deviation of the resistance R is 1.4%.
Table 3 shows the mean values and standard deviations of the TCRs of the electrical structures of different groups from field 51.
As Table 3 shows, the TCR and the standard deviation of the TCR are largely independent from the width of the conductor track, at least for the conductor track width in the range 0.5 μm to 6.5 μm, although a slight but systematic increase of the mean TCR with broader line width is obvious.
The backside of the silicon wafer from the design example 1 is chemomechanically thinned to a wafer plus insulator thickness of approximately 10 μm. A protection layer of polyimide of approximately 13 μm is then applied to the thinned reverse side.
The electrical structures are then separated with a laser or by dicing. The individual structures are mechanically flexible and can be bent around a round bar with a radius of less than 1 mm. Repeated bending back and forth around the round bar does not significantly alter the R0 value and the TCR value of the electrical structure.
To determine the surface temperature of a test piece, the electrical structure from design example 3 is attached to the surface of the test piece and the two contact pads are connected via leads to a resistance measuring device. By determining the electrical resistance of the electrical structure and using a calibration curve, the surface temperature of the specimen can be deduced.
Sensors according to Execution example 3 with the difference that the silicon oxide layer on one side has a layer thickness of 20 μm. The backside of the silicon wafer is chemomechanically thinned until nothing is left. A protection layer of polyimide of approximately 15 μm is then applied to the thinned reverse side.
This sensor has a similar flexibility as the sensor according to Execution example 3.
In a further example the contact pads are both raised above the conductor track of the electronic structure. The advantage of this design is to reduce the thermal contact area between the sensor and its mount. The sensor structure is free-floating and the thermal leakage is reduced. The free-floating arrangement of the temperature sensor is realized by connecting the raised contact pads directly to electrical lines of a circuit board.
Sensors according to Execution example 3 with R0=9.285 Ohm are glued on a 25 μm Polyimide foil and wrapped around various cylindrical bars with different diameters and d R/R0 is measured. The inorganic layer-thickness in total is 20 μm (thinned silicon inorganic layer: 10 μm; thickness of the inorganic isolating layer and protective layer: 10 μm). The results are outlined in Table 4.
According to Table 4, the sensors are flexible if the radius of curvature does is not smaller than 1 mm, by means Rm<1 mm.
| Number | Date | Country | Kind |
|---|---|---|---|
| 19196138.2 | Sep 2019 | EP | regional |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2020/074315 | 9/1/2020 | WO |
