WAFER AND METHOD OF MANUFACTURING THE SAME
The present invention relates to a wafer (100) being subdivided and separable into a plurality of dies. Each die (110) comprises an array of capacitive micro-machined transducer cells (1). Each cell comprises a substrate (10) comprising a first electrode (11), a membrane (13) comprising a second electrode (14), and a cavity (12) between the substrate (10) and the membrane (13). Each cell (1) of at least a part of the dies (110) comprises a compensating plate (15) on the membrane (13), each compensating plate (15) having a configuration for influencing a bow (h) of the membrane (13). The configurations of the compensating plates (13) vary across the wafer (100). The present invention further relates to a method of manufacturing such a wafer and a method of manufacturing such a die.
The present application is a continuation of U.S. patent application Ser. No. 14/401,995 filed Nov. 18, 2014, which is the U.S. National Phase application under 35 U.S.C. § 371 of International Application No. PCT/IB2013/054455, filed May 30, 2013, which claims the benefit of U.S. Provisional Application Ser. No. 61/653,675 filed May 31, 2012. These applications are hereby incorporated by reference herein.
FIELD OF THE INVENTIONThe present invention relates to a wafer being subdivided and separable into a plurality of dies, each die comprising an array of capacitive micro-machined transducer cells, in particular capacitive micro-machined ultrasound transducer (CMUT) cells or capacitive micro-machined pressure sensor cells. The present invention further relates to a method of manufacturing such a wafer. The present invention further relates to a method of manufacturing such a die, in particular a die used to form an ultrasound transducer or pressure sensor.
BACKGROUND OF THE INVENTIONThe heart of any ultrasound (imaging) system is the ultrasound transducer which converts electrical energy in acoustic energy and back. Traditionally these ultrasound transducers are made from piezoelectric crystals arranged in linear (1-D) transducer arrays, and operating at frequencies up to 10 MHz. However, the trend towards matrix (2-D) transducer arrays and the drive towards miniaturization to integrate ultrasound (imaging) functionality into catheters and guide wires has resulted in the development of so-called capacitive micro-machined ultrasound transducer (CMUT) cells. A CMUT cell comprises a cavity underneath the cell membrane. For receiving ultrasound waves, ultrasound waves cause the cell membrane to move or vibrate and the variation in the capacitance between the electrodes can be detected. Thereby the ultrasound waves are transformed into a corresponding electrical signal. Conversely, an electrical signal applied to the electrodes causes the cell membrane to move or vibrate and thereby transmitting ultrasound waves.
The membrane of the CMUT cell in general consists of several materials or layers, for example a metal electrode embedded in oxides or silicone nitride. Residual stress in these layers causes the cell membrane to bend upwards or downwards, depending on the sign or direction of the stress. Therefore, the cell membrane has a bow or bending of a specific amount and in a direction (upwards or downwards). This bow or bending causes a shift in electrical and acoustical properties of the cell. For example, it influences the collapse voltage and, assuming constant bias voltage, also the centre frequency. Efforts have been made to solve this problem. For example, the paper “Fabrication of CMUT Cells with Gold Center Mass for Higher Output Pressure”, Hyo-Seon Yoon et al., 10th International Symposium on Therapeutic Ultrasound (ISTU 2010) AIP Conf. Proc. 1359, 183-188 (2011) discloses a way to improve the output pressure of a single CMUT cell by a modification to the basic CMUT cell structure, namely by adding a gold mass over the center of the top CMUT plate.
Furthermore, there are generally strict specifications on such an ultrasound transducer or CMUT device. Its manufacturing involves quite complex processes. In general, first a larger wafer is manufactured which is then separated into multiple dies each comprising an array of CMUT cells. A particular challenge in this respect is the yield loss in manufacturing when trying to meet the strict specifications for the ultrasound transducers or CMUT devices.
SUMMARY OF THE INVENTIONIt is an object of the present invention to provide a wafer with improved and/or cheaper manufacturing, in particular reduced yield loss. It is a further object of the present invention to provide an improved method of manufacturing a wafer and an improved method of manufacturing a die, in particular with reduced yield loss.
In a first aspect of the present invention a wafer being subdivided and separable into a plurality of dies is presented, each die comprising an array of capacitive micro-machined transducer cells, each cell comprising a substrate comprising a first electrode, a membrane comprising a second electrode, and a cavity between the substrate and the membrane, wherein each cell of at least a part of the dies comprises a compensating plate on the membrane, each compensating plate having a configuration for influencing a bow of the membrane, wherein the configurations of the compensating plates vary across the wafer.
In a further aspect of the present invention a method of manufacturing a wafer is presented, the method comprising: providing a wafer being subdivided and separable into a plurality of dies, each die comprising an array of capacitive micro-machined transducer cells, each cell comprising a substrate comprising a first electrode, a membrane comprising a second electrode, and a cavity between the substrate and the membrane, and providing a compensating plate on the membrane of each cell of at least a part of the dies, each compensating plate having a configuration for influencing a bow of the membrane, wherein the configurations of the compensating plates are varied across the wafer.
In a further aspect of the present invention a method of manufacturing a die comprising an array of capacitive micro-machine transducer cells is presented, the method comprising the steps of the method of manufacturing the wafer, and further comprising separating the die from the wafer.
The basic idea of the invention is to vary the configurations of the compensating plates across the wafer, in particular across the diameter of the wafer. Each compensating plate has a configuration for influencing (i.e. reducing or increasing) the bow of the membrane. Thus, each compensating plate provides compensation or a compensating effect for the bow of the membrane. It has been found that the bow of the membrane usually depends on the specific location on the wafer, in particular the location of the membrane or its corresponding die. For example, a centre die or centre dies has or have a different bow compared to an edge die or edge dies. This non-uniformity of the membrane bow is quite unwanted, as the electrical characteristics and acoustical characteristics such as the acoustical output pressure and the centre frequency, depend on the bow and are therefore also non-uniform. As there are strict specifications on the transducers, in particular CMUT devices, the non uniformity translates into yield loss. The present invention solves this problem by varying the configurations of the compensating plates across the wafer, in particular across its diameter. In this way the amount and/or direction of compensation is varied across the wafer. In this way the variation in membrane bow across the wafer is substantially reduced. Thus, there is compensation or compensating effect for the variation in membrane bow across the wafer. Thus, yield loss in manufacturing is reduced. It will be understood that the term diameter in this context in particular refers to the maximum dimension of the wafer across its surface (in a plane orthogonal to its thickness). The term configuration of a compensating plate in particular can refer to the shape, size and/or thickness of the compensating plate. In this way, the shape, size and/or thickness of the compensating plate is used to control the compensation. The shape, size and/or thickness determine the amount of compensation and also the direction (or sign) of compensation (upwards or downwards).
Preferably, the configurations of the compensating plates vary across the wafer such that the membrane bows of the cells are substantially uniform. In other words, the membrane bows of the cells across the wafer are made substantially uniform or the same. The membrane bows do not necessarily need to be zero, but to be substantially uniform across the wafer. It will be understood that the goal is to make the membrane bows of the cells exactly uniform or the same. However, in practice there still might be some minor or neglectable variation in membrane bow. For example, the variation or compensation can be performed in a number of steps, preferably in a small number of steps (e.g. two or three). Thus, in practice some variation in membrane bow might remain, which is however minor or neglectable. The variation of the configurations of the compensating plates across the wafer provides a significant improvement of the variation in membrane bow.
Preferably, the configurations of the compensating plates of the cells of each die are substantially uniform. In other words, the membrane bows of the cells within one die can be assumed to be (substantially) uniform or the same. This assumption is true due to the small size of the die compared to the overall size of the wafer. In practice there still might be some variation in membrane bow within one die, which is however minor or neglectable due to the small size of the die compared to the diameter of the wafer. In other words, the variation in bow within a die may be very small or neglectable. If there is still some minor variation within the die, this is much smaller compared to the variation across the wafer. Preferably, both the cell membrane bows within one die are substantially uniform as well as the bows of the different dies being substantially uniform.
Preferred embodiments of the invention are defined in the dependent claims. It shall be understood that the claimed method of manufacturing a wafer or method of manufacturing a die has similar and/or identical preferred embodiments as the claimed wafer and as defined in the dependent claims.
In a first embodiment the shapes of the compensating plates vary across the wafer. This provides for a particularly easy way of varying the configurations of the compensating plates. In this embodiment the configuration of a compensating plate refers to the shape of the compensating plate. For example, the shapes can vary between a circular shape and a ring shape, in particular for a circular shaped cell or cell membrane. A circular shaped and a ring shaped compensating plate can have opposite compensation effects (e.g. a circular shape can bend the membrane downwards and a ring shaped compensating plate can bend the membrane upwards).
In a second embodiment the sizes of the compensating plates vary across the wafer. This provides for a particularly easy way of varying the configurations of the compensating plates. In this embodiment the configuration of a compensating plate refers to the size of the compensating plate. For example, in manufacturing, the sizes of the compensating plates can easily be varied by using lithography mask with varying sizes and/or shapes. In this way only one additional lithography step for providing the compensating plates is needed.
In a variant of this embodiment each compensating plate has a circular shape with a plate diameter, and wherein the plate diameters vary across the wafer. This (continuous) circular shape or disk provides for a particularly easy and/or effective way of varying the sizes of the compensating plates, in particular for a circular shaped cell or cell membrane. When varying (i.e. increasing or decreasing) the plate diameter, the effect of the compensation is increased.
In another variant of this embodiment each compensating plate has a ring shape with an inner plate diameter, and wherein the inner plate diameters vary across the wafer. This ring shape provides for another particularly easy and/or effective way of varying the sizes of the compensating plates, in particular for a circular shaped cell or cell membrane. When varying (i.e. increasing or decreasing) the inner plate diameter, the effect of the compensation is increased.
In a third embodiment the thicknesses of the compensating plates vary across the wafer. This provides for a particularly effective way of varying the configurations of the compensating plates. In this embodiment the configuration of a compensating plate refers to the thickness of the compensating plate. For example, in manufacturing, the thicknesses of the compensating plates can effectively be varied by applying/depositing at least two material layers on each cell of at least a part of the dies. Multiple additional deposition steps for providing the compensating plates are then needed, but also the compensation effect that can be achieved is very good. Thus, the thicknesses can be varied in multiple steps, in particular multiple (metal) deposition steps. When increasing the thickness of the (metal) compensating plate, the effect of the compensation is increased.
In a variant of this embodiment at least part of the compensating plates comprise more layers than other compensating plates. This provides for a particular effective and/or easy way of varying the thicknesses of the compensating plates. For example, in manufacturing, the thicknesses of the compensating plates can effectively be varied by applying/depositing a first layer and applying/depositing at least a second layer such that at least part of the compensating plates comprise more layers than other compensating plates.
In another embodiment the configurations of the compensating plates vary stepwise from a first region of the wafer to a second region of the wafer. In this way only a limited number of variations or steps are needed to provide for a sufficient compensation. In particular, the first region and second region each comprises multiple dies. For example, the configurations of the compensating plates may vary stepwise from the first region to the second region as well as from the second region to a third region. For example, three or less regions may be sufficient to provide for a sufficient compensation.
In another embodiment the compensation plate is made of metal, in particular Aluminium. A metal enables to provide the compensating plate in a particularly easy manner. In particular Aluminium provides for a particularly predictable manufacturing process. Even though Aluminium may be preferred, any other material may be used as long as the stress of the compensating plate is well controlled.
In a further embodiment each cell comprises a protective coating over the compensating plate. This provides for protection of the compensating plate against its environment, e.g. against corrosion. This protective coating or passivation layer can be thin, for example below 200 nm or below 100 nm. For example, the protective coating can be made of silicone nitride (Si3N4).
In another embodiment the method comprises the step of determining the membrane bow of the cells of each die before providing the compensating plates. In this way the variation pattern or distribution of the membrane bows across the wafers can be determined. The configurations of the compensating plates can then be varied according to this variation pattern or distribution. In particular, this determination step can be performed only once for a first wafer when starting the manufacturing of wafers. The variation in the membrane bows can then be assumed to be the same for the subsequent wafers. Of course, it is also possible to perform this determination step before manufacturing each single wafer. This will provide for a more accurate compensation at the cost of a more time-consuming manufacturing. For example, the determination step can comprise determining if the amount of compensation across the wafer is sufficient, and adjusting the amount of compensation if it is determined that the amount of compensation across the wafer is not sufficient. Determining if the amount of compensation across the wafer is sufficient can for example be performed by an electrical measurement, for example the measurement of the collapse voltage.
In a further embodiment the step of providing the compensating plates comprises using a lithography mask with varying sizes and/or shapes. In this way the sizes and/or shapes of the compensating plates can be varied across the wafer. This provides for a particularly easy way of varying the configurations of the compensating plates. Only one additional lithography step for providing the compensating plates is needed. In other words, the size and/or shape of the compensating plate is applied via lithography, in particular by imaging and patterning of a lithography mask in photo resist.
In another embodiment the step of providing the compensating plates comprises applying a first layer and applying at least a second layer such that at least part of the compensating plates comprise more layers than other compensating plates. In this way the thicknesses of the compensating plates can be varied across the wafer. This provides for a particularly effective way of varying the configurations of the compensating plates. In other words, the thicknesses of the compensating plates can effectively be varied by applying/depositing at least two material layers on each cell of at least a part of the dies. Multiple additional deposition steps for providing the compensating plates are then needed, but also the compensation effect that can be achieved is very good. The thickness of the compensating plate is a deposition parameter. This deposition parameter can be chosen, in particular in such a way that the membrane bows of the cells are made substantially uniform.
These and other aspects of the invention will be apparent from and elucidated with reference to the embodiment(s) described hereinafter. In the following drawings
From a technology point of view, the pre-collapsed capacitive micro-machined transducer cell (in particular cMUT) can in principle be manufactured in any conventional way, which is for example described in detail in WO 2010/032156 A2, which is incorporated by reference herein.
Even though
In order to influence the bow, a compensating plate 15 on (e.g. on top of) the membrane or membrane base layer 13 can be used.
In general, stress in the membrane 13 may cause the membrane bow. In one example, a temperature change or thermally induced stress may be the root cause of membrane bow (or also called deflection). It generally stems from the design and characteristics of the materials making up the membrane 13. The second (top) electrode 14 is made from an electrically conductive material which is different from the material of the membrane or membrane base layer itself. Under the influence of temperature change, the two materials expand or contract at different rates and with different expansion characteristics. This creates a thermally induced stress and momentum within the membrane. This thermally induced stress and momentum may trigger movement in the membrane, thereby stimulating a change in capacitance. By applying the compensating plate 15 on the membrane 13, the bow or deflection of the membrane can be influenced.
The bow or deflection h of the membrane 13 can be modelled according to:
where
h is the deflection of the membrane towards the substrate at a centre point of the cavity, in particular the central (lateral) axis A, due to stress (e.g. thermally induced stress),
M is the momentum of the membrane due to stress (e.g. thermally induced stress),
D is the flexural rigidity of the membrane 13,
rm is a radius of the membrane (as defined from the centre point of the cavity),
rb-bot is a radius of the second (top) electrode 14 (as defined from the centre point of the cavity),
rb-top is a radius of the compensating plate 15 (as defined from the centre point of the cavity),
h1, h2 and h3 are the distances of a first side of the second (top) electrode 14, a second side of the second (top) electrode 14 and the thickness of the membrane 13, respectively (as measured from the inner surface of the membrane forming the side of the cavity),
ν is the Poisson ratio,
S is the thermal stress in the membrane,
E is Young's modulus for the material of the membrane 13, E1 and E2 relating to the second (top) electrode 14 and the membrane 13, respectively,
Δ T is a temperature change, and
α is a expansion coefficient of a material, α1 and α2 relating to the second (top) electrode 14 and the membrane 13, respectively.
It will be understood that thermally induced stress is just one example, but there may be other or additional reasons that result in stress in the membrane. Therefore, in the above formula, S may generally or also be referred to as the “stress value”. Furthermore, it will be understood that the above formula for the membrane bow or deflection h is derived for the particular membrane construction indicated in
The size (or diameter d, wherein only the radius d/2 or rb-top is shown in
A wafer used for manufacturing such a transducer device comprises a plurality of such dies 110, in particular as explained with reference to
It has been found that the bow h of a cell membrane 13 usually depends on the specific location on the wafer, in particular the location of the membrane 13 or its corresponding die. A centre die has a different bow h compared to an edge die. This will further be explained with reference to
This non-uniformity of the membrane bow, as for example shown in
With reference to
Now, the corresponding method of manufacturing such a wafer 100 will be explained. A method of manufacturing such a die 110 additionally comprises the (final) step of separating the die 110 from the wafer 100. The method of manufacturing such a wafer comprises first the step providing a wafer 100 being subdivided and separable into a plurality of dies 110, in particular a wafer with non-uniform membrane bows, such as for example shown in
With reference to
In each of the embodiments of
In each of the embodiments of
For each of
In the following a specific non-limiting example will be given for a better understanding. The compensating plates in region R1 can be ring shaped, such that the membranes bend upwards to reduce downward bow. In region R2 there can be no compensating plates. The compensating plates in region R3 can be circular shaped, such that the membranes bend downwards to reduce upwards bow. The compensating plates in region R4 can also be circular shaped, but having at least two layers to increase the amount of bowing. It will be understood that this is just an arbitrary non-limiting example and that the configurations of the compensating plates depend on the given variation pattern in membrane bows, for example determined by measuring the collapse voltages.
The effect of the compensation of the variation in membrane bows will now be explained with respect to
While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive; the invention is not limited to the disclosed embodiments. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims.
In the claims, the word “comprising” does not exclude other elements or steps, and the indefinite article “a” or “an” does not exclude a plurality. A single element or other unit may fulfill the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.
Any reference signs in the claims should not be construed as limiting the scope.
Claims
1. A plurality of dies separable from a wafer, each die comprising an array of capacitive micro-machined transducer (CMUT) cells, each cell comprising a substrate comprising a first electrode, a membrane comprising a second electrode, and a cavity between the substrate and the membrane,
- wherein each cell of at least a part of the dies comprises a compensating plate on the membrane, each compensating plate having a configuration for influencing a bow of the membrane,
- wherein the configurations of the compensating plates of the CMUT cells vary,
- wherein the sizes of the compensating plates vary, and
- wherein each compensating plate has a circular shape with a plate diameter, and wherein the plate diameters vary.
2. The plurality of dies of claim 1, wherein the configurations of the compensating plates vary such that the membrane bows of the cells are substantially uniform.
3. The plurality of dies of claim 1, wherein the shapes of the compensating plates vary.
4. The plurality of dies of claim 1, wherein the configurations of the compensating plates of the cells within one die are substantially uniform.
5. The plurality of dies of claim 1, wherein the compensating plates vary over at least some of the CMUT cells in the array of each die.
6. The plurality of dies of claim 1, comprising a first plurality of dies and a second plurality of dies, wherein the sizes of the compensating plates vary between the first plurality and the second plurality of dies.
7. An ultrasound device comprising one die from the plurality of dies of claim 1.
8. A plurality of dies separable from a wafer, each die comprising an array of capacitive micro-machined transducer (CMUT) cells, each cell comprising a substrate comprising a first electrode, a membrane comprising a second electrode, and a cavity between the substrate and the membrane,
- wherein each cell of at least a part of the dies comprises a compensating plate on the membrane, each compensating plate having a configuration for influencing a bow of the membrane,
- wherein the configurations of the compensating plates of the CMUT cells vary,
- wherein the sizes of the compensating plates vary, and
- wherein each compensating plate has a ring shape with an inner plate diameter, and wherein the inner plate diameters vary.
9. The plurality of dies of claim 8, wherein the configurations of the compensating plates vary such that the membrane bows of the cells are substantially uniform.
10. The plurality of dies of claim 8, wherein the shapes of the compensating plates vary.
11. The plurality of dies of claim 8, wherein the configurations of the compensating plates of the cells within one die are substantially uniform.
12. The plurality of dies of claim 8, wherein the compensating plates vary over at least some of the CMUT cells in the array of each die.
13. The plurality of dies of claim 8, comprising a first plurality of dies and a second plurality of dies, wherein the sizes of the compensating plates vary between the first plurality and the second plurality of dies.
14. An ultrasound device comprising one die from the plurality of dies of claim 8.
15. A plurality of dies separable from a wafer, each die comprising an array of capacitive micro-machined transducer (CMUT) cells, each cell comprising a substrate comprising a first electrode, a membrane comprising a second electrode, and a cavity between the substrate and the membrane,
- wherein each cell of at least a part of the dies comprises a compensating plate on the membrane, each compensating plate having a configuration for influencing a bow of the membrane,
- wherein the configurations of the compensating plates vary across the wafer,
- wherein the thicknesses of the compensating plates vary across the wafer, and
- wherein at least part of the compensating plates comprise more layers than other compensating plates.
16. The plurality of dies of claim 15, wherein the configurations of the compensating plates vary such that the membrane bows of the cells are substantially uniform.
17. The plurality of dies of claim 15, wherein the shapes of the compensating plates vary.
18. The plurality of dies of claim 15, wherein the configurations of the compensating plates of the cells within one die are substantially uniform.
19. The plurality of dies of claim 15, wherein the compensating plates vary over at least some of the CMUT cells in the array of each die.
20. An ultrasound device comprising one die from the plurality of dies of claim 15.
Type: Application
Filed: Feb 6, 2018
Publication Date: Jun 7, 2018
Inventor: Peter Dirksen (Eindhoven)
Application Number: 15/889,255