WO2025201734A1

WO2025201734A1 - Method for producing a plurality of sensor elements and sensor element

Info

Publication number: WO2025201734A1
Application number: PCT/EP2025/054151
Authority: WO
Inventors: Kohei NAWAOKA; Christopher Soon; Jan Ihle
Original assignee: TDK Electronics AG
Current assignee: TDK Electronics AG
Priority date: 2024-03-25
Filing date: 2025-02-17
Publication date: 2025-10-02
Anticipated expiration: 2026-09-25
Also published as: DE102024108455A1

Abstract

A method for producing a plurality of sensor elements (100) is described, the method comprising the following steps: A) Determining resistance changes of a functional layer (7) over a wafer (200) and providing a wafer (200); B) Forming a plurality of electrode pairs (4a, 4b) on the wafer (200); C) Applying a functional material to a partial region of each electrode pair (4a, 4b) to form a plurality of functional layers (7); D) Sintering of the functional layers (7); E) Applying a protective layer (8) to a top side (11) of the wafer (200), the protective layer (8) completely covering the top side (11) except for partial regions to which contact pads (10a, 10b) are applied in the subsequent process step; F) Forming contact pads (10a, 10b) in the partial regions free of the protective layer (8); G) Singulation; wherein the resistance changes over the wafer (200) are compensated by a specific design of each electrode pair (4a, 4b) and / or by a specific design of each functional layer (7). Moreover, a sensor element (100) produced by the method and a plurality of sensor elements (100) as intermediate products are described.

Description

Method for producing a plurality of sensor elements and sensor element

The present invention relates to a method for producing a plurality of sensor elements , preferably a plurality of temperature sensors . The present invention further relates to a sensor element , preferably a temperature sensor, produced by the method and to a plurality of sensor elements as an intermediate product .

To integrate passive components , such as sensors , capacitors , heaters , etc . , the dimensions for modern packaging solutions must be adapted towards micrometer or even nanometer scale . To achieve such miniaturi zation thin film technologies must be used . For processing thin film NTC structures are deposited on a substrate , e . g . Si-wafers . The quality of the thin film material changes with the position on the substrate . These quality changes influence the resistance and create a resistance deviation over the substrate .

So far, in thin film technology, trimming electrodes are employed to influence the resistance . Those must be cut in additional measuring and cutting processes after the deposition . The disadvantages are the time needed for measuring and cutting, chances of damaging the dies or layers on the die by the prober needles .

The German patent application DE 10 2020 122 923 Al , the content of which is part of this application by reference , describes a sensor element for temperature measurement with a thin- film NTC thermistor . It is an obj ect of the present invention to describe a method and a sensor element with improved properties .

This obj ect is solved by a method for producing a plurality of sensor elements , a sensor element and a plurality of sensor elements as intermediate products according to the independent claims .

According to one aspect , a method for producing a plurality of sensor elements is described . The sensor elements are temperature sensors .

The method comprising the following steps :

A) In a first step, a substrate , e . g . a wafer, is provided . The wafer, in particular partial areas of the wafer, form a plurality of carriers of the final sensor elements after a singulation of the sensor elements ( see step G) ) .

The wafer comprises silicon, silicon carbide or glass ( silicate or borosilicate glass ) . Alternatively, the wafer may comprise AIN or AI2O3, for example . The wafer has a top side and a bottom side . The top side is that side of the wafer onto which the components of the sensor elements are applied in the following method steps .

In an optional step, an electrically insulating layer, preferably SiC>2 , is formed on the top side of the wafer .

B ) In a next step, a plurality of electrode pairs is formed on the wafer . One electrode pair comprises a first electrode and a second electrode . In the final , separated sensor elements one electrode pair is part of one respective sensor el- ement . In other words , the final sensor element has one pair of electrodes , i . e . a first electrode and a second electrode .

The first and second electrodes of each electrode pair are spatially and electrically insulated from each other . The electrodes may comprise electrode fingers . In this case , the first and second electrodes intertwine in the form of interdigital structures . Alternative the electrodes may comprise a spiral structure . In this case , they intertwine in the form of spiral structures .

The first and second electrodes each have a connection area for electrically connecting the electrodes to contact pads ( see step F) below) .

C ) In a further step, a functional material is applied at least to a partial region of each electrode pair to form a plurality of functional layers on the wafer . The respective final ( singulated) sensor element comprises at least one functional layer .

In an alternative embodiment , step C ) may take place before step B ) so that the electrode pairs are deposited at least partly directly onto the functional layers .

The functional material preferably comprises an NTC ceramic based on an oxidic material in the perovskite or spinel structure type . Alternatively, the functional material can be based on a carbide or a nitride material in the Wurtzite structure type . Alternatively, the functional material may comprise or constitute a thin film of vanadium oxide or SiC . The functional layer is deposited as a full-surface thin film and structured in a further process step, e.g. by means of lithography. After deposition, the NTC layer is not yet fully crystallized.

Before the wafer and the further components are provided, i.e. before the manufacturing process starts, resistance changes of the functional layer over the wafer are determined. This may take place during multiple previous process runs. Functional layers produced for example in sputtering processes have systematic, repeatable, and reproduceable variations of quality occurring over multiple process runs. Quality criteria include for example a thickness, a composition, a crystallinity, etc. These systematic variations / quality changes influence the resistance of the functional layer over the wafer and result in resistance changes over the wafer.

Therefore, to determine the resistance changes (of the functional layer) over the wafer the thickness, composition and/or quality of a material of the functional layer is monitored and evaluated at each position on the wafer during multiple process runs. From this evaluation a quality change and, thus, resistance changes of the functional layer over the wafer are determined in advance, i.e. before the actual manufacturing process begins.

During the steps B) and C) , the resistance changes of the functional layer over the wafer are compensated. In the following, the term "resistance changes" automatically implies resistance changes of the functional layer even if the term "functional layer" is not explicitly mentioned. As the changes in resistance are known in advance , they can be compensated by the design of the respective sensor element . Particularly, changes in resistance are compensated by a speci fic design of each electrode pair and / or by a speci fic design of each functional layer .

Accordingly, a speci fic design of each electrode pair depends on the predetermined resistance of the functional layer at the position on the wafer where the electrode pair is deposited . Additionally or alternatively, a speci fic design of each functional layer depends on the predetermined resistance of the functional layer at the position on the wafer where the functional layer is deposited . Therefore , multiple sensor element designs are present on one single wafer .

D) In a further step, the functional layer is subj ected to a heat treatment to form the structure or properties ( sintering of the functional material ) .

E ) In a next step, a protective layer is formed on the top side of the wafer . The protective layer completely covers the top side except for partial regions to which contact pads are applied in the subsequent process step .

The protective layer can comprise oxides , nitrides , ceramics , glasses or polymers and is produced using a PVD (physical vapour deposition) or a CVD ( chemical vapour deposition) process and is structured using wet chemical etching or dry etching . The protective layer has a thickness of < 10 pm, preferably < 5 pm, particularly preferably < 1 pm . Ideally, the protective layer has a thickness < 0 . 5 pm . F) In a next step, contact pads are formed in the partial regions free of the protective layer for electrical contacting the final sensor elements .

In each case , a contact pad is connected directly to a connection area of the first and second electrodes . The contact pads can comprise Cu, Au, Ni , Cr, Ag, Ti , W, Pd or Pt . Preferably, the contact pads comprise Au . Preferably, the contact pads have a layer structure with Au and Ti , whereby Ti is applied as an adhesive layer under the Au layer . Preferably, the contact pads have a thickness of < 5 pm .

G) As the method generates a large number of sensor elements in parallel (parallel processing) it further includes the step of singulation . This means that the ( intermediate product ) sensor elements are separated into the final sensor elements . In the following, the term " intermediate sensor elements" denotes the sensor elements before singulation .

Separation takes place in up to two steps :

( 1 ) Separation in x/y direction ( length & width) . This can be done , for example , by plasma etching or sawing and notching of the functional layer and wafer . The wafer from which the sensor elements are separated is not sawn through, but only cut to a defined thickness .

( 2 ) Separation in the z-direction (height ) . Grinding is carried out from the rear . A grinding process removes material from the bottom side of the wafer up to a defined final component thickness .

This is done by :

- Grinding of the sensor elements from the bottom side , whereby a grinding process removes material from the back of the wafer up to a defined final component thickness , whereby the sensor elements are separated .

- Optional plasma etching of the ground-down bottom side of the wafer to reduce microcracks , for example .

Alternatively, separation is done by cutting through the whole wafer . In this step no grinding is necessary . However, the resulting sensor elements may be thicker in this case . Thus , this kind of separating is not being limited to an element thickness of 100 pm to 50 pm but between 1000 pm and 50 pm .

The final shape of the respective sensor element is created during the separation step . For example , the final separated sensor element can have a rectangular, square , hexagonal or octagonal basic shape . A hexagonal basic shape (honeycomb shape ) proves to be particularly advantageous for spiral shaped electrodes , as the space on the wafer can be optimally utili zed .

A width and a length of the respective final sensor element is ideally between 1000 pm and 200 pm .

Overall , the method produces a plurality of sensor elements having a very compact design so that the respective sensor element can be embedded directly into an electrical system as a discrete component . In this context , the term "discrete" means that the sensor element can be integrated into electrical structures , for example a MEMS (Micro Electro Mechanical System) or a SESUB ( Semiconductor Embedded in Substrate ) structure , as a compact and sel f-contained system . The sensor elements are especially suited for temperature control of IC (Integrated Circuit) and embedded systems. Furthermore, the sensor elements are especially suited for temperature control of laser diodes and optical transceivers.

Moreover, due to the specific design of the electrodes / the functional layer in dependence of the predetermined resistance values of the functional layer over the wafer, the sensor elements produced by the method have a very narrow resistance tolerance. In other words, the respective final sensor element has only a very small deviation range from a nominal resistance (nominal value of the resistance) . Usually, the resistance deviation over the wafer can be up to 10%. Larger deviations can still be compensated with the method described above. However, the total size of the sensor element may be adapted / increased to allow for the necessary adjustment space. Adjusting the design of the sensor elements over the wafer allows to reduce resistance variation to below 1% .

Controlled manipulation of material properties in an NTC thin film by e.g. implanting doping elements, concentration gradients, etc. is, however, very difficult as the specific crystal structure such as spinel, perovskite, or wurtzite depend on many factors such as composition, heat treatment, lattice mismatch, stresses, etc. Manipulating of just one material characteristic is also difficult, meaning for example that changing only the resistivity without affecting the temperature sensitivity (B-value) by such a method may not be possible. As such, to reduce variation over the wafer structural solutions as described above are necessary to compensate for inhomogeneities . According to one embodiment, for compensating the resistance changes of the functional layer over the wafer a length, a thickness, and/or a width of the electrodes of the electrode pairs is varied. Additionally or alternatively, a distance between the electrodes of the electrode pairs can be varied.

This means, for compensating the resistance variations one can change the interdigital electrode length (i.e. the length of one / of multiple electrode fingers) for the electrode pairs over the wafer. Length can be varied in a range from 15 % to 1 % with a lower limit of 0.5 pm.

Alternatively or in addition, the interdigital electrode width (i.e. the width of one / of multiple of the electrode fingers and/or of the connection area of the electrodes) can be varied over the wafer. The width can be varied in a range from 15 % to 1 % with a lower limit of 0.5 pm.

Alternatively or in addition, the interdigital electrode thickness (i.e. the thickness of one / of multiple of the electrode fingers and/or of the connection area of the electrodes) can be varied over the wafer. The thickness can be varied in a range from 15 % to 1 %.

Alternatively or in addition, the distance between interdigital electrodes / electrode fingers of the respective electrode pair can be varied over the wafer. The distance between electrodes can be varied in a range from 15 % to 1 % with a lower limit of 0.5 pm.

The same can be applied for electrodes with spiral structure.

In particular, a distance between the two spiral electrodes of the electrode pairs can be varied, a length of the spiral electrodes can be varied, a width of the spiral electrodes can be varied and/or a thickness of the spiral-shaped electrodes can be varied . In this context , the same ranges in varying length, width, thickness and distance between electrodes apply here as for the interdigital structure .

By varying the structure of the electrode pairs over the wafer, the resistance changes can be ef fectively reduced without the need of an additional trimming step . Thus , the sensor elements are produced in a very cost-ef fective way .

According to one embodiment , for compensating the resistance changes of the functional layer over the wafer a length, a thickness , and/or a width of the functional layers is varied .

This means , for compensating the resistance variation one can change the NTC layer length over the wafer . The length can be varied in a range from 15 % to 1 % with a lower limit of 0 . 5 pm .

Alternatively or in addition, the NTC layer width can be varied over the wafer . The width can be varied in a range from 15 % to 1 % with a lower limit of 0 . 5 pm .

Alternatively or in addition, the NTC layer thickness can be varied over the wafer . The thickness can be varied in a range from 15 % to 1 % with a lower limit of 0 . 5 pm .

By varying the structure of the functional layers over the wafer, the resistance changes can be ef fectively reduced without the need of an additional trimming step . Moreover, a combination of any of the mentioned variations is possible ( e . g . vary length of interdigital electrodes and simultaneously change the distance between the electrodes ; vary the length of the interdigital electrodes and simultaneously change the NTC-layer width, etc . ) .

During the method the total si ze of the respective sensor element is adapted / increased to allow for the necessary adj ustment space . Accordingly, the intermediate product sensor elements before singulation and, thus , also the resulting final ( i . e . singulated) may be slightly larger as compared to the sensor elements according to the state of the art .

Due to the design variations of the sensor elements over the wafer, the resistance variation can be signi ficantly reduced and the singulated sensor elements have a very small deviation range from a nominal resistance .

According to a further aspect , a sensor element is described . Preferably, the sensor element is produced by the method described above . All features disclosed with respect to the sensor element or the method are also disclosed correspondingly with respect to the respective other aspect , and vice versa, even i f the respective feature is not explicitly mentioned in the context of the respective aspect .

The sensor element comprises at least one carrier having a top side and a bottom side . The top side of the carrier is adapted to be electrically insulating . Preferably, an insulating layer, for example SiCt or SiaN4 , is formed on the top side of the carrier . The insulating layer is formed directly on the top side of the carrier and may be composed of one or more layers . The carrier represents a partial region of the substrate ( i . e . the wafer ) described in connection with the method and, in particular, comprises the same material as the wafer . Accordingly, the carrier comprises silicon, silicon carbide , glass , AIN or AI2O3 as a carrier material . The carrier can have a rectangular, square , hexagonal or octagonal basic shape .

The sensor element further comprises at least one functional layer which has a material with a temperature-dependent electrical resistance . The functional layer is arranged on the carrier .

The sensor element further comprises a pair of electrodes . The electrode pair comprises a first electrode and a second electrode . The electrode pair is adapted and arranged for electrically contacting the functional layer . The ( first and second) electrodes are preferably thin- film electrodes .

The electrode pair I s formed directly on a top side of the functional layer . In other words , the functional layer is formed between the electrodes and the carrier .

Alternatively, the electrode pair can be arranged directly on a bottom side of the functional layer . In this case , the electrode pair is formed between the functional layer and the carrier .

For better adhesion of the electrode material an adhesion promoter such as a thin film < 10 nm of Ti or Cr can be used .

The sensor element further comprises at least two contact pads for electrically contacting the sensor element . One re- spective contact pad is connected directly with one of the first and second electrodes .

The sensor element is adapted to be integrated directly into an electrical system as a discrete component . In particular, the sensor element is designed for direct integration into a MEMS structure and/or into a SESUB structure .

Moreover, the sensor element is produced by the method described above . Therefore , it has a very narrow resistance tolerance . In other words , the sensor element has a very small deviation range from a nominal resistance (nominal value of the resistance ) .

According to one embodiment , the respective electrode ( first and second electrode ) of the electrode pair is formed in a spiral shape . In other words , the respective electrode has a curve that runs around a central point and moves away from or towards this center depending on the observer' s perspective .

The respective electrode can have a round spiral shape . Alternatively, the respective electrode can also have an angular, for example rectangular, spiral shape . The spiral-like design results in a very space-saving configuration of the electrodes .

The first and second electrode are spiraled or intertwined . In other words , the two electrodes form intertwined spirals that are spaced apart from each other on the carrier or on the functional layer . The two electrodes both run around the central point . According to one embodiment , the respective electrode ( first and second electrode ) has a plurality of electrode fingers . The electrode fingers of the two electrodes are arranged alternately with respect to each other .

According to an embodiment , the respective electrode ( first and second electrode ) has a connection area . The connection area is designed to connect the respective electrode to the respective contact pad . Each contact pad is directly connected to a connection area .

According to a further aspect , a plurality of sensor elements as intermediate products is described . Preferably, the sensor elements are an intermediate product of the method described above . All features disclosed with respect to the sensor elements as intermediate product or the method are also disclosed correspondingly with respect to the respective other aspect , and vice versa, even i f the respective feature is not explicitly mentioned in the context of the respective aspect .

This aspect refers to the sensor elements before singulation . The sensor elements are arranged on the wafer described in connection with the method for producing a plurality of sensor elements . As described above , the functional layer comprises a known / predetermined resistance variation . In other words , there are resistance changes of the functional layer over the wafer, wherein the resistance at each position at the wafer is known before the further components of the sensor elements are provided .

The respective sensor element comprises at least one functional layer which has a material with a temperaturedependent electrical resistance . The sensor elements each comprise a pair of electrodes for electrically contacting the functional layer and at least two contact pads for electrically contacting the respective sensor element after singulation .

The sensor elements arranged on the wafer comprise di f ferent designs . This means , the sensor elements structurally di f fer from one another . In particular, a design of the pair of electrodes of the respective sensor element and/or a design of the functional layer of the respective sensor element is dependent on a resistance of the functional layer at the position of the respective sensor element .

By means of the di f ferent designs , resistance deviations of the functional layer can be compensated . A further structuring or trimming of the sensor elements thus becomes redundant . Accordingly, the sensor elements can be produced in a very cost-ef ficient way .

According to one embodiment , the sensor elements di f fer from one another in a length, a thickness , and/or a width of the electrodes of each electrode pair . Additionally or alternatively, the sensor elements di f fer from one another in a distance between the electrodes of each electrode pair .

According to one embodiment , the sensor elements di f fer from one another in a length, a thickness , and/or a width of the functional layers .

The drawings described below are not intended to be to scale . Rather, individual dimensions may be enlarged, reduced or even distorted for better representation . Elements which are similar or which perform the same function are designated with the same reference signs .

It show :

Figure 1 an exploded view of a sensor element according to the state of the art ,

Figure 2 a sectional view of the sensor element according to Figure 1 ( state of the art ) ,

Figure 3 a top view of a sensor element with spiral shaped electrodes ,

Figure 4 a deposition state of a functional layer according to the state of the art ,

Figure 5a a top view of a plurality of sensor elements as an intermediate product according to the state of the art ,

Figure 5b a top view of a plurality of sensor elements as an intermediate product according to a first embodiment of the present invention,

Figure 5c a top view of the sensor elements according to Figure 5b arranged on a wafer,

Figure 6a a top view of a plurality of sensor elements as an intermediate product according to the state of the art , Figure 6b a top view of a plurality of sensor elements as an intermediate product according to a second embodiment of the present invention,

Figure 6c a top view of the sensor elements according to Figure 6b arranged on a wafer,

Figure 7a a top view of a plurality of sensor elements as an intermediate product according to the state of the art ,

Figure 7b a top view of a plurality of sensor elements as an intermediate product according to a third embodiment of the present invention,

Figure 7c a top view of the sensor elements according to Figure 7b arranged on a wafer .

Figures 1 and 2 show an illustration of a sensor element 1 according to the state of the art . The sensor element 1 serves to illustrate a basic structure of the sensor element 100 described below in connection with Figures 3 , 5b, 5c, 6b, 6c, 7b and 7c . Reference is made to the German patent application DE 10 2020 122 923 Al with regard to the detailed description of the essential features of the sensor element 1 according to Figures 1 and 2 .

The sensor element 1 is an NTC thin- film temperature sensor and has a carrier 2 with a top side 11 and a bottom side 12 . The top side 11 of the carrier 2 has an insulating layer 3 which comprises , for example , SiCt . The sensor element 1 also comprises a pair of electrodes 4a, 4b, i . e . a first electrode 4a and a second electrode 4b . The ( first and second) electrodes 4a and 4b of the electrode pair 4a, 4b are spaced apart from each other and are arranged on the insulating layer 3 of the carrier 2 . The electrodes 4a, 4b comprise thin metal films . The electrodes 4a, 4b comprise an interdigital structure . Alternatively, the electrodes 4a, 4b may comprise a spiral structure ( see Figure 3 and related description below) .

The sensor element 1 further comprises a functional layer 7 with a top side 14 and a bottom side 15 . The functional layer 7 is an NTC thin film . The functional layer 7 only partially covers the insulating layer 3 on the top side 11 of the carrier 2 . Preferably, the functional layer 7 is at least partially applied onto the electrodes 4a, 4b .

In the embodiment shown in Figures 1 and 2 , the electrodes 4a, 4b are formed between the carrier 2 and the functional layer 7 , in particular on the bottom side 15 of the functional layer 7 . However, in an alternative embodiment (not explicitly shown) , the electrodes 3a, 4b can be formed on a top side 14 of the functional layer 7 , as well .

The sensor element 1 also comprises at least two contact pads 10a, 10b for electrically contacting the sensor element 1 .

The contact pads 10a, 10b are connected with connecting areas 6 , 102a, 102b of the electrode pair 4a, 4b ( see Figures 1 and 3 ) .

The sensor element 1 may also comprise a protective layer 8 .

The protective layer 8 completely covers the top side of the sensor element 1 with the exception of the contact pads 10a, 10b . The protective layer 8 has recesses 9 from which the contact pads 10a, 10b protrude for electrical contacting the sensor element 1 .

Due to the compact design of the individual components of the sensor element 1 , the sensor element 1 is suitable for integration into MEMS or SESUB structures .

In contrast to sensor element 1 shown in Figures 1 and 2 , the resistance of the sensor element 100 described by the following Figures is adj usted without the need of trimming the electrodes 4a, 4b and/or the functional layer 7 . For this purpose , the design of the respective sensor element 100 is adj usted during its manufacturing process depending on preevaluated resistance changes of the functional layer over the wafer, which is described in detail in connection with the Figures 4 , 5a to 5c, 6a to 6c, and 7a to 7c .

It is to be noted that the resulting final sensor element 100 has essentially the same components as the sensor element 1 according to Figures 1 and 2 . In other words , the basic structure of the sensor element 100 corresponds to the structure of the sensor element 1 of Figures 1 and 2 , as already mentioned above . With regard to the details of the components and the mode of operation of the sensor element 100 , reference is therefore made to the above description or to document DE 10 2020 122 923 Al which is incorporated in this application by reference . Unless otherwise stated, the reference signs given in connection with Figures 1 and 2 are also used in the following .

It is further to be noted that the term " ( final ) sensor element 100" denotes the sensor element 100 after singulation from the wafer 200 . The term " intermediate (product ) sensor element 100" denotes the sensor element 100 before singulation, i . e . in a condition where the sensor element 100 is still arranged on the wafer 200 . I f in the following no speci fic distinction is made between final and intermediate sensor elements , the explanations may refer to both types of sensor elements ( i . e . before or after singulation) .

It is further to be noted that according to the embodiment shown in Figures 1 and 2 , the electrodes 4a, 4b of the sensor element 100 are designed as interdigital thin- film electrodes . In particular, the electrodes 4a, 4b each comprise a flat end area / connection area 6 and an area with electrode fingers 5 . The area with the electrode fingers 5 is formed in a center region of the carrier 2 . The flat end area 6 and the area with the electrode fingers 5 merge into one another . The two electrodes 4a, 4b form an interdigital structure in the area of the electrode fingers 5 .

However, as an alternative , the electrodes 4a, 4b of the sensor element 100 according to the present invention can also have a spiral structure ( see Figures 3 and 6a, 6b ) . In particular, the electrodes 4a, 4b can be spiraled interwoven . Also for the design with spiral shaped electrodes 4a, 4b, spatial separation of the two spiral electrodes 4a, 4b ensures electrical separation of the electrode pair 4a, 4b .

In the embodiment with spiral shaped electrodes , the electrodes 4a, 4b can have a round spiral shape as shown in Figure 3 or they can have an angular, e . g . rectangular, spiral shape (not explicitly shown in the Figures ) . I f the electrodes 4a, 4b have a round spiral shape , the sensor element 100 can have , for example , a rectangular, a square or honey- comb basic shape . In the case of an angular spiral shape of the electrodes 4a, 4b, the sensor element 100 can have a square or rectangular basic shape , for example . In other words , the carrier 2 is , for example , rectangular in this case .

In the following, a method for producing a plurality of sensor elements 100 is described . In particular, a plurality of temperature sensor elements 100 are produced by the method .

At first and in particular, before the actual manufacturing process begins , resistance changes over a functional layer 7 / functional material on a wafer 200 are determined . Generally, functional layers produced for example in sputtering or spin coating processes have systematic, repeatable , and reproduceable variations of quality occurring over multiple process runs .

Quality criteria include for example a thickness , a composition and a crystallinity of the functional layer . Moreover, with special regard to sputtering processes , deposition parameters , a target si ze , etc . play a role for the quality of the functional layer, as well . For example , the functional layer 7 formed by a sputtering process by means of a sputtering target 20 may be arched, i . e . it may be thicker in a center and thinner at edges , which results in resistance fluctuations of the functional layer over the wafer 200 ( Figure 4 ) .

As the quality variations are systematic, they can be evaluated over multiple process runs so that , ideally, the speci fic resistance of a functional layer 7 over a wafer 200 can be determined in advance . Next, for producing a plurality of sensor elements 100, a wafer 200 is provided. The provided wafer 200 comprises silicon, silicon carbide or glass (silicate or borosilicate glass) . Alternatively, the wafer 200 may comprise AIN or AI2O3, for example. The wafer 200 has a top side 11 and a bottom side 12 (Figure 4) . The top side 11 is that side of the wafer 200 onto which the components of the sensor elements 100 are applied in the following method steps.

Subsequently, in an optional step, an electrically insulating layer 2, preferably SiCt, is formed on the top side 11 of the wafer 200.

B) In a next step, a plurality of electrode pairs 4a, 4b is formed on the wafer 200 (see Figures 5b, 6b, 7b) . One electrode pair 4a, 4b comprises a first electrode 4a and a second electrode 4b.

The first and second electrodes 4a, 4b of each electrode pair are spatially and electrically insulated from each other. The electrodes 4a, 4b can comprise electrode fingers 5 (Figures 5b, 7b) . In this case, the first and second electrodes 4a, 4b intertwine in the form of interdigital structures. Alternative the electrodes 4a, 4b may comprise a spiral structure (Figure 6b) . In this case, they intertwine in the form of spiral structures.

As stated above, the first and second electrodes 4a, 4b each have a connection area 6, 102a, 102b for electrically connecting the electrodes 4a, 4b to contact pads 10a, 10b (see method step F) ) . In an alternative embodiment (not explicitly shown) , electrode material can be used as contact pads. A separate step for connecting the electrodes 4a, 4b and the contact pads 10a, 10b may thus be redundant .

In a further step C ) , a functional material is applied at least to a partial region of each electrode pair 4a, 4b to form a plurality of functional layers 7 on the wafer 200 . This step is illustrated in Figure 7b .

Step C ) may also take place before step B ) so that the electrode pairs 4a, 4b are deposited at least partly onto the functional layers 7 .

The functional material preferably comprises an NTC ceramic based on an oxidic material in the perovskite or spinel structure type . Alternatively, the functional material can be based on a carbide or a nitride material in the Wurtzite structure type . Alternatively, the functional material may comprise or constitute a thin film of vanadium oxide or SiC .

The functional layer 7 is deposited as a full-surface thin film and structured in a further process step, e . g . by means of lithography . After deposition, the NTC layer is not yet fully crystalli zed .

As the resistance changes of the functional layer over the wafer 200 are known ( see step A) , they can be compensated during the steps B ) and/or C ) by the speci fic design of the respective sensor element 100 .

In particular, based on the investigations of the resistance variation of the functional layer across a wafer 200 as described above , a speci fic design of the respective ( intermediate ) sensor element 100 on the wafer 200 is selected . Par- ticularly, changes in resistance are compensated by a specific design of each electrode pair 4a, 4b and / or by a specific design of each functional layer 7. Therefore, multiple sensor element designs 100 are present on one single wafer 200. In other words, the intermediate product sensor elements 100 differ from one another by their specific design.

For example, for compensating the resistance changes over the wafer 200 a length, a thickness, and/or a width of the electrodes 4a, 4b of the electrode pairs is varied (Figures 5b, 5c, 6b, 6c) . Additionally or alternatively, a distance between the electrodes 4a, 4b of the electrode pairs can be varied (Figure 5b) .

As can be gathered from Figure 5b, for compensating the predetermined resistance variations of the functional layer 7 over the wafer 200, the interdigital electrode width (i.e. the width of the electrode fingers 5) is varied over the wafer 200. Thus, the wafer 200 comprises electrode pairs 4a, 4b with very broad electrode fingers 5 (Figure 5b, bottom left - 1 -) up to electrodes pairs 4a, 4b having very thin electrode fingers 5 (Figure 5b bottom right - 4 -) depending on the resistance of functional layer 7 at the position where the respective electrode pair 4a, 4a is located on the wafer 200 (see Figure 5c) .

In contrast thereto, in the state of the art as shown in Figure 5a, the electrode fingers 5 of all electrode pairs 4a, 4b comprise the same width.

Additionally or as an alternative, the interdigital electrode length (i.e. the length of the electrode fingers 5) for the electrode pairs 4a, 4b is varied over the wafer 200 (not ex- plicitly shown) depending on the determined resistance changes .

Alternatively or in addition, the thickness of the electrode fingers 5 ( i . e . the extension of the electrode fingers 5 perpendicular to main direction of extension of the wafer 200 ) is varied over the wafer 200 .

Alternatively or in addition, the distance between the electrode fingers 5 can be varied over the wafer 200 ( see Figure 5b ) .

The same applies for electrodes 4a, 4b with spiral structure as can be gathered from Figures 6b and 6c, where the length of the spiral shaped electrodes 4a, 4b is varied over the wafer 200 depending on the resistance changes of the functional layer 7 over the wafer 200 . In contrast thereto , in the state of the art as shown by Figure 6a, the length of the spiral arms is equal for all electrode pairs 4a, 4b over the wafer 200 .

Moreover, also a distance between the two spiral electrodes 4a, 4b of the electrode pairs can be varied, a width of the spiral electrodes 4a, 4b can be varied and/or a thickness of the spiral-shaped electrodes 4a, 4b can be varied (not explicitly shown) to compensate the predetermined resistance changes over the wafer 200 .

By varying the structure of the electrode pairs 4a, 4b over the wafer 200 , the resistance changes can be ef fectively reduced without the need of an additional trimming step . As a result , the sensor elements 100 arranged on the wafer 200 ( intermediate product sensor elements 100 ) have a plurality of di f ferent designs which is in contrast to intermediate product sensor elements according to the state of the art which all have the same design .

Additionally or alternatively ( see Figures 7b and 7c ) , a design of each functional layer 7 can be varied depending on the resistance of the functional layer 7 at the position on the wafer 200 where the functional layer 7 is deposited . In particular, for compensating the resistance changes over the wafer 200 a length, a thickness , and/or a width of the functional layers 7 can be varied .

Figure 7b shows an example where the width of the functional layer 7 changes depending on the position of the intermediate sensor element 100 on the wafer 200 ( see Figure 7c ) and thus , depending on the resistance at the position of the respective sensor element 100 .

In this context , Figure 7b bottom left shows a sensor element 100 having a very broad functional layer 7 , Figure 7b top right shows a sensor element 100 having a very narrow functional layer 7 and Figure 7b top left and bottom right show functional layers 7 with intermediate widths . The final width of the respective functional layer 7 depends on the resistance of the functional layer 7 at the speci fic position of the respective functional layer / the respective sensor element 100 .

In contrast thereto , according to the state of the art ( Figure 7a ) all sensor elements 100 arranged on the wafer 200 ( intermediate product sensor elements ) comprise functional layers 7 having the same width . Alternatively or in addition, the functional layer length can be varied over the wafer (not explicitly shown) . Alternatively or in addition, the NTC layer thickness can be varied over the wafer (not explicitly shown) .

By varying the structure of the functional layers 7 over the wafer, the resistance changes can be ef fectively reduced without the need of an additional trimming step .

It is to be noted that also a combination of any of the mentioned variations is possible ( e . g . the length of the electrode fingers 5 can be varied and simultaneously the distance between the electrode fingers 5 can be changed and / or the length of the electrode fingers 5 can be varied and simultaneously the width of the functional layers 7 can be changed etc . ) to compensate resistance variations of the functional layer 7 over the wafer 200 .

It is further to be noted that a controlled manipulation of material properties in an NTC thin film by e . g . implanting doping elements , concentration gradients , etc . would be very di f ficult and also technically impractical as the speci fic crystal structure such as spinel , perovskite , or wurtzite depend on many factors such as composition, heat treatment , lattice mismatch, stresses , etc . Manipulating of j ust one material characteristic is also di f ficult , meaning for example that changing only the resistivity without af fecting the temperature sensitivity (B-value ) by such a method may not be possible . As such, to reduce variation over the wafer 200 structural solutions as described above are necessary to compensate resistance variations of the functional layer 7 over the wafer 200 . In a subsequent method step D) , the functional layers 7 are subj ected to a heat treatment to form the structure or properties ( sintering of the functional material ) and in a next step E ) , a protective layer 8 is formed on the top side 11 of the wafer 200 . The protective layer 8 completely covers the top side 11 except for partial regions to which contact pads 10a, 10b are applied in the subsequent process step .

In a further step E ) , contact pads 10a, 10b are formed in the partial regions free of the protective layer 8 for electrical contacting the sensor elements 100 . As mentioned above , this step may be redundant when electrode material is used as contact pads .

One contact pad 10a, 10b is connected directly to a connection area 6 , 102a, 102b of the first and second electrodes 4a, 4b . The contact pads 10a, 10b can comprise Cu, Au, Ni , Or, Ag, Ti , W, Pd or Pt . Preferably, the contact pads 10a, 10b comprise Au . Preferably, the contact pads 10a, 10b have a layer structure with Au and Ti , whereby Ti is applied as an adhesive layer under the Au layer .

Finally, in step G) , the sensor elements 100 are singulated . This means that the ( intermediate product ) sensor elements 100 are separated into the final sensor elements 100 .

Separation takes place in two steps :

( 1 ) Separation in x/y direction ( length & width) . This can be done , for example , by plasma etching or sawing and notching of the functional layer and wafer .

During step G) , the final shape of the respective sensor element 100 is created . For example , the final separated sensor element 100 can have a rectangular, square , hexagonal or octagonal basic shape . The final separated sensor elements 100 may have a thickness between 100 pm and 50 pm, for example . A width and a length of the respective final sensor element 100 is ideally between 1000 pm and 200 pm .

The final sensor elements 100 are especially suited for temperature control of IC ( Integrated Circuit ) and embedded systems . Furthermore , the final sensor elements 100 are especially suited for temperature control of laser diodes and optical transceivers .

Due to the speci fic design of the electrodes 4a, 4b / the functional layers 7 in dependence of the predetermined resistance values of the functional layer 7 over the wafer 200 , the sensor elements 100 produced by the method have a very narrow resistance tolerance . In other words , the respective sensor element 100 has only a very small deviation range from a nominal resistance (nominal value of the resistance ) . Adj usting the design of the sensor elements 100 over the wafer 200 allows to reduce resistance variation to below 1 % .

The description of the obj ects disclosed herein I s not limited to the individual speci fic embodiments . Rather, the features of the individual embodiments can be combined with each other in any way - as far as this makes technical sense . List of reference signs

1 Sensor element

100 Sensor element

200 Wafer

2 Carrier

3 Insulating layer

4a, b Electrode pair

4a ( First ) electrode

4b ( Second) electrode

5 Electrode finger

6 End region / connection area

102a Connection area

102b Connection area

7 Functional layer

8 Protective layer

9 Recess

10a, b Contact pad

11 Top side of the carrier / the wafer

12 Bottom side of the carrier / the wafer

13 Surface of the sensor element

14 Top side of the functional layer

15 Bottom side of the functional layer

16 Target

D Thickness of the sensor element

L Edge length of the carrier

Claims

1. Method for producing a plurality of sensor elements (100) , the method comprising the following steps:

A) Determining resistance changes of a functional layer (7) over a wafer (200) and providing a wafer (200) ;

B) Forming a plurality of electrode pairs (4a, 4b) on the wafer (200) ;

C) Applying a functional material to a partial region of each electrode pair (4a, 4b) to form a plurality of functional layers ( 7 ) ;

D) Sintering of the functional layers (7) ;

E) Applying a protective layer (8) to a top side (11) of the wafer (200) , the protective layer (8) completely covering the top side (11) except for partial regions to which contact pads (10a, 10b) are applied in the subsequent process step;

F) Forming contact pads (10a, 10b) in the partial regions free of the protective layer (8) ;

G) Singulation; wherein the resistance changes over the wafer (200) are compensated by a specific design of each electrode pair (4a, 4b) and / or by a specific design of each functional layer (7) .

2. The method according to claim 1, wherein a specific design of each electrode pair (4a, 4b) depends on the resistance of the functional layer (7) at the position on the wafer (200) where the electrode pair (4a, 4b) is deposited and / or a specific design of each functional layer (7) depends on the resistance of the functional layer (7) at the position on the wafer (200) where the functional layer (7) is deposited.

3. The method according to claim 1 or claim 2, wherein multiple designs of the sensor elements (100) are generated .

4. The method according to any one of the previous claims, wherein for compensating the resistance changes in the functional layer (7) over the wafer (200) a length, a thickness, and/or a width of the electrodes of the electrode pairs (4a, 4b) and/or a distance between the electrodes of the electrode pairs (4a, 4b) is varied.

5. The method according to any one of the previous claims, wherein for compensating the resistance changes in the functional layer (7) over the wafer (200) a length, a thickness, and/or a width of the functional layers (7) is varied.

6. The method according to any one of the previous claims, wherein for determining resistance changes of the functional layer (7) over the wafer (200) a thickness, a composition and/or quality of a material of the functional layer (7) is evaluated and compensated for.

7. The method according to any one of the previous claims, wherein the electrodes of each electrode pair (4a, 4b) are formed in a spiral shape.

8. The method according to any one of claims 1 to 6, wherein the electrodes of each electrode pair (4a, 4b) comprise electrode fingers (5) .

9. The method according to any one of the previous claims, wherein in step G) a rectangular, square, hexagonal or octagonal basic shape of the respective sensor element (100) is generated .

10. The method according to any one of the previous claims, wherein before step B) an electrically insulating layer (3) is formed on the top side (11) of the wafer (200) .

11. Sensor element (100) for measuring a temperature comprising

- at least one carrier (2) having a top side (11) and a bottom side (12) , the top side (11) being adapted to be electrically insulating,

- at least one functional layer (7) which has a material with a temperature-dependent electrical resistance, the functional layer (7) being arranged on the carrier (2) ,

- a pair of electrodes (4a, 4b) , for electrically contacting the functional layer (7) ,

- at least two contact pads (10a, 10b) for electrically contacting the sensor element (100) , wherein one respective contact pad (10a, 10b) is connected directly with one of the electrodes (4a, 4b) , wherein the sensor element (100) has only a very small deviation range from a nominal resistance and wherein the sensor element (100) is produced by a method according to any one of the previous claims.

12. Sensor element (100) according to claim 11, wherein the respective electrode (4a, 4b) of the electrode pair is formed in a spiral shape and wherein the electrodes (4a, 4b) are at least partly guided into one another in a spiral shape.

13. Sensor element (100) according to claim 11 or claim 12, wherein the respective electrode (4a, 4b) has a connection area (6, 102a, 102b) and wherein a respective contact pad (10a, 10b) is directly connected to the connection area (6, 102a, 102b) .

14. Sensor element (100) according to any one of claims 11 to

13, wherein an insulating layer (3) is formed directly on the top side (11) of the carrier (2) .

15. Sensor element (100) according to any one of claims 11 to

14, wherein the carrier (2) comprises silicon, silicon carbide, glass, AIN or AI2O3 as a carrier material and/or wherein the carrier (2) comprises a hexagonal or octagonal basic shape.

16. Sensor element (100) according to any one of claims 11 to

15, wherein the sensor element (100) is designed for direct integration into a MEMS structure and/or into a SESUB structure.

17. A plurality of sensor elements (100) as intermediate products , wherein the sensor elements (100) are arranged on a wafer (200) and wherein the respective sensor element (100) comprises :

- at least one functional layer (7) comprising a material with a temperature-dependent electrical resistance,

- a pair of electrodes (4a, 4b) for electrically contacting the functional layer (7) ,

- at least two contact pads (10a, 10b) for electrically contacting the respective sensor element (100) , wherein the sensor elements (100) comprise different designs.

18. Plurality of sensor elements (100) according to claim 17, wherein the sensor elements (100) differ from one another in a length, a thickness, and/or a width of the electrodes of each electrode pair (4a, 4b) and/or in a distance between the electrodes of each electrode pair (4a, 4b) .

19. Plurality of sensor elements (100) according to claim 17 or claim 18, wherein the sensor elements (100) differ from one another in a length, a thickness, and/or a width of the functional layers ( 7 ) .

20. Plurality of sensor elements (100) according to any one of claims 17 to 19, wherein a design of the pair of electrodes (4a, 4b) of the respective sensor element (100) and/or a design of the functional layer (7) of the respective sensor element (100) is dependent on a resistance of the functional layer (7) at the position of the respective sensor element (100) .