METHOD FOR FABRICATING STRAIN SENSING FILM, STRAIN SENSING FILM, AND PRESSURE SENSOR

TECHNICAL FIELD

The present application relates to the technical field of semiconductor packaging, and in particular, to a method for preparing the strain sensing film, the strain sensing film and the pressure sensor.

BACKGROUND

With the further development of the consumer electronics industry and the increasing demand for thinning, traditional packaging solutions have been unable to meet market demands. High-performance logic chip packaging has adopted packaging solutions like the system on chip (SoC) and the system in a package (SIP), which have significant improvement in reducing product volume. However, such packaging solutions have limited application scope due to their complicated process flow and high cost. For application scenarios having relatively simple performance requirements, it is desired to develop a general solution for reducing the volume. According to the developing tend in the industry, die thinning is judged to be an effective solution to reduce the package volume.

In order to ensure that the die is not or less affected by the external environment, the die must be packaged before use. However, the existing chip on board (COB) packaging process is mainly aimed at the LED industry but not suitable for the packaging of sensor films, and is disadvantageous of being unable to package double sides and not supporting the attachment process of die attach film.

Technical Problems

It is one of objectives of embodiments of the present application to provide a strain sensing film, a method for preparing the same, and the pressure sensor including the strain sensing film, which aims at solving the technical problems, including but not limited to, that the existing COB packaging, when being applied to package the sensor film, has defects during both the production and use processes.

Technical Solutions

In order to solve the above technical problems, embodiments of the present application provide the following technical solutions:

In a first aspect, a method for fabricating a strain sensing film is provided, the method comprises:

thinning a semiconductor wafer to form a semiconductor film,

attaching a die attach film onto the semiconductor film;

dicing a resulting semiconductor film to form a plurality of independent strain films;

transferring the plurality of independent strain films to a substrate, and completely attaching the plurality of independent strain films to the substrate;

electrically connecting a metal pad of each of the plurality of the independent strain films with a corresponding metal pad of the substrate; and

packaging the plurality of the independent strain films.

In an embodiment, the fabrication method further comprises: performing signal test on binding wires after wire bonding.

In an embodiment, the fabrication method further comprises: performing function test on the plurality of the independent strain films after the packaging.

In an embodiment, the metal pads of the independent strain films and the corresponding metal pads of the substrate are connected respectively by any one method of wire bonding, tap bonding, silver paste bonding, aerogel bonding, and flip chip bonding.

In an embodiment, before the thinning treatment, the fabrication method further comprises: etching the semiconductor wafer to form an integrated circuit within the semiconductor wafer.

In an embodiment, the semiconductor wafer comprises at least one strain sensing resistor.

In an embodiment, the semiconductor wafer further comprises a signal processing circuit, and the signal processing circuit is in connection with the strain sensing resistor.

In an embodiment, the semiconductor wafer comprises at least one temperature sensor.

In an embodiment, the semiconductor wafer comprises at least two strain sensing resistors. One of the at least two strain sensing resistors is arranged in a direction different from at least one other strain sensing resistor.

In an embodiment, the semiconductor wafer comprises at least two strain sensing resistors. One of the at least two strain sensing resistors has a sensitivity coefficient different from that of at least one other strain sensing resistor.

In an embodiment, the semiconductor wafer comprises at least two strain sensing resistors. One of the at least two strain sensing resistors is perpendicular to at least one other strain sensing resistor.

In an embodiment, the semiconductor film has a thickness of smaller than or equal to 70 μm.

In an embodiment, the semiconductor film has a thickness of smaller than or equal to 50 μm.

In an embodiment, the semiconductor film has a thickness of smaller than or equal to 30 μm.

In an embodiment, the semiconductor film has a thickness of smaller than or equal to 25 μm.

In an embodiment, the semiconductor film has a thickness of smaller than or equal to 20 μm.

In an embodiment, the semiconductor film has a thickness of smaller than or equal to 15 μm.

In a second aspect, a strain sensing film is provided. The strain sensing film is packaged by adopting the fabrication method according to any one of the above embodiments.

In a third aspect, a pressure sensor is provided. The pressure sensor comprises the strain sensing film according to any one of the above embodiment.

BENEFICIAL EFFECTS

Advantages of the method for fabricating a strain sensing film, the strain sensing film, and a pressure sensor according to embodiments of the present application are summarized as follows: a semiconductor wafer is thinned to form a semiconductor film. A die attach film is attached onto the semiconductor film. A resulting semiconductor film is diced to form a plurality of independent strain films. The plurality of independent strain films are transferred to a substrate, and the plurality of independent strain films are completely attached to the substrate. A metal pad of each of the plurality of independent strain films is electrically connected with a corresponding metal pad of the substrate. The plurality of independent strain films are packaged. In this way, the package process of the strain sensing film is completed, which tackles the problem that the existing COB packaging has defects when being applied to package the sensor film.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to more clearly illustrate the technical solutions in the embodiments of the present application, the drawings that need to be used in the description of the embodiments or the exemplary art will be briefly described hereinbelow. Obviously, the accompanying drawings in the following description are only some embodiments of the present application. For those skilled in the art, other drawings can be obtained based on these drawings without creative work.

FIG. 1 is a schematic flowchart of a fabrication method provided by an embodiment of the present application;

FIG. 2 is a cross sectional schematic structural diagram of a semiconductor film after being diced provided by an embodiment of the present application;

FIG. 3 is a cross sectional schematic structural diagram of independent strain films attached to a substrate according to an embodiment of the present application;

FIG. 4 is a top view schematic structural diagram of independent strain films attached to a substrate according to an embodiment of the present application;

FIG. 5 is a cross sectional schematic structural diagram after wire bonding according to an embodiment of the present application;

FIG. 6 is a cross sectional schematic structural diagram after the packaging process according to an embodiment of the present application;

FIG. 7 is a top view schematic structural diagram of a substrate after dicing according to an embodiment of the present application;

FIG. 8a is a schematic structural diagram of a strain sensing film provided by an embodiment of the present application;

FIG. 8b is a schematic structural diagram of a strain sensing film provided by another embodiment of the present application;

FIG. 9a is a schematic structural diagram of a packaging process of a strain sensing film provided by an embodiment of the present application;

FIG. 9b is a schematic structural diagram of a packaging process of a strain sensing film provided by another embodiment of the present application;

FIG. 10 provides eight exemplary illustrative arrangements of strain sensing resistors on a strain sensing film according to an embodiment of the present application;

FIG. 11 is a schematic diagram of a strain sensing film provided by an embodiment of the present application; and

FIG. 12 is a schematic flowchart of a fabrication method provided by another embodiment of the present application.

DETAILED DESCRIPTION OF THE EMBODIMENTS

In order to make the purposes, technical solutions, and advantages of the present application clearer and more understandable, the present application will be further described in detail hereinafter with reference to the accompanying drawings and embodiments. It should be understood that the embodiments described herein are only intended to illustrate but not to limit the present application.

It should be noted that when an element is described as “fixed” or “arranged” on/at another element, it means that the element can be directly or indirectly fixed or arranged on/at another element. When an element is described as “connected” to/with another element, it means that the element can be directly or indirectly connected to/with another element. It should be understood that terms “upper”, “lower”, “left”, “right” and the like indicating orientation or positional relationship are based on the orientation or the positional relationship shown in the drawings, and are merely for facilitating and simplifying the description of the present application, rather than indicating or implying that a device or component must have a particular orientation, or be configured or operated in a particular orientation, and thus should not be construed as limiting the application. For those skilled in the art, the specific meanings of the above terms in the present application can be understood according to specific conditions. Moreover, terms like “first” and “second” are only used for the purpose of description, and will in no way be interpreted as indication or hint of relative importance or implicitly indicate the number of the referred technical features. “Multiple”/“a plurality of” refers to the number of two or more than two, unless otherwise clearly and specifically defined.

The current mainstream wafers used in mass production mostly adopt sizes of 6 inches, 8 inches, and 12 inches. Some new semiconductor materials are also fabricated into 4-inch wafers. In specific applications, one side of the wafer is fabricated into a circuit layer. For some simple dies applied in force sensors, the thickness of the circuit layer can be controlled within 10 μm. A side of a non-circuit layer can be called a base, a thickness of which is usually several hundred microns. Generally speaking, the larger the wafer size is, the greater the thickness of the base is. The greater thickness of the base ensures that the wafer can maintain a certain mechanical strength during the manufacturing, testing, and transfer process.

In order to ensure that the die is not or less affected by the external environment, the die must be packaged before use. The existing conventional die has a thickness of greater than 200 μm, and after adding the protection of the package, the height of the resulting die is always at least 500 μm higher than the substrate. In the COB packaging process, a plurality of LED dies are directly attached to a carrier, in order to improve the heat dissipation capability of the LED components. Since the main purpose is to dissipate heat, the substrate usually adopts an aluminum substrate. The semiconductor force sensor converts the force signal into the strain of the die itself, and then generates a responsive electrical signal change. Therefore, under the action of the same external force signal, the thinner the die is, the greater the strain is generated, and the greater the variation of the corresponding electrical signal is, that is, the higher the sensitivity of the sensor is. Therefore, it is a trend of future development in the force sensor to make the sensing die in the sensor thinner.

The main type of the pressure sensor is based on the piezoresistive strain gauge or its variants. In case that the piezoresistive material is used in the piezoresistive strain gauge, the conductivity or the resistivity of the strain gauge changes when the material is under stress. In one common form of such a strain gauge, a thin strip of the piezoresistive material is deposited, or attached, or bonded, onto a substrate to form a variable resistor. In order to solve the problem that the COB packaging process cannot be applied to the packaging of the pressure sensor die, embodiments of the present application provide a method for fabricating a strain sensing film. As shown in FIG. 1, the fabrication method in an embodiment comprises steps S10-S60.

In step S10, a semiconductor wafer is thinned to form a semiconductor film.

Specifically, in this embodiment, the first step of the packaging process is to thin the semiconductor wafer (that is, the wafer), which can be thinned to 15-70 μm according to user requirements.

In an embodiment, a protective film can be attached to the surface of the semiconductor wafer, and then a grinding machine is used with a suitable grinding wheel to perform the thinning processing from the base side of the semiconductor wafer to obtain a thinned semiconductor film. After grinding and thinning, in order to reduce the stress applied to the surface and inside of the semiconductor film, processes such as polishing or plasma etching can be selected to reduce surface stress and reduce the risk of subsequent damage to the semiconductor film. Furthermore, according to requirements, it is also possible to choose not to perform the polishing process.

In an embodiment, the semiconductor film comprises at least one selected from the group consisting of a silicon film, a gallium arsenide film, a gallium nitride film, a silicon carbide film, a zinc oxide film, and any combination thereof.

In step S20, a die attach film is attached onto the semiconductor film.

Specifically, after the semiconductor wafer is thinned, an attaching machine is used to attach a die attach film on the base side of the semiconductor film.

For the force sensor, the die attach film is often required to have a sufficiently high Young's modulus to fully transmit the strain of a main structure to the force sensor die. The smaller the thickness of the die attach film is, the greater the Young's modulus is, and the greater the sensitivity of the obtained sensor is on the sensor composed of the same force sensor die.

For general die packaging applications, no special requirements are imposed on the die attach film, as long as the process requirements are met.

In step S30, a resulting semiconductor film is diced to form a plurality of independent strain films.

Specifically, after the die attach film is attached to the surface of the semiconductor film, independent film dies can be formed by dicing the semiconductor film.

In an embodiment, a blade dicing method can be adopted, and the dicing can be achieved by high-speed rotation of blades of a diamond grinding wheel.

In an embodiment, for the film die of a mechanical sensor, in order to further reduce the stress caused by dicing, a laser dicing process can be used. Specifically, as shown in FIG. 2, a dicing depth satisfies that the die attach film 102 may be completely diced, or alternatively, a part of the viscosity reducing film 103 may be diced, so as to ensure complete dicing of the die attach film.

Furthermore, the die attach film can also be incompletely diced, and the final separation of the die attach film can be completed by stretching, and the film stack after the completion of the dicing is shown in FIG. 2.

In an embodiment, after attaching the die attach film, a layer of a viscosity reducing film 103 and a layer of a base film 104 can be added to ensure that the semiconductor film is not broken during the subsequent splitting process.

In step S40, the plurality of independent strain films are transferred to a substrate, and the plurality of independent strain films are completely attached to the substrate.

Specifically, in this embodiment, a die attach machine can be used to transfer the independent strain films. For example, the die attach machine can be a Panasonic's MD-P200. Under certain temperature and pressure conditions, the independent strain films are transferred from the wafer to the substrate, and then heated for a certain period of time to make the die attach film sticky, until the independent strain films are completely attached to the substrate.

In an embodiment, the viscosity reducing film in the wafer is firstly performed with viscosity reduction by using UV or heating equipment to make it easier to separate the dies and the independent strain films from the base film. The wafer and substrate are mounted to specified positions of the die attach machine. The substrate is heated to a specified temperature, such as 60-120° C. Thereafter, by using a visional positioning system, a suction nozzle is moved to a position right above a certain independent strain film, the suction nozzle is pressed down until touching the die, and picks up and lifts the independent strain film and the corresponding die attach film under the action of vacuum suction. By using the visional positioning system, the independent strain film together with the die attach film are transferred by the suction nozzle to above a designated position of the substrate. Then the suction nozzle moves down until the independent strain film and the die attach film contact the substrate.

By controlling the force and duration of the down pressing of the suction nozzle, it can be ensured that the die does not leave the substrate along with the movement of the suction nozzle, when the suction nozzle moves upward after turning off the vacuum suction. Then, the suction nozzle continues to move to the wafer to pick up a next die and start a next operation cycle until the bonding of all dies onto the substrate is completed. The die attach machine often has the function of automatic loading and unloading. Through the feeding box and the receiving box, the supply of empty substrates and the recycling of the die attached substrates can be automatically completed.

The die attach process in the above-mentioned embodiment is also applicable to the substrate having the independent strain films on both sides. Generally, after the die attachment on one side is completed, the die attachment on the other side can be completed by inverting the substrate in batches. After the die attach machine has been running for a period of time, the receiving box can be removed and put into the oven together with the substrate therein for baking. The baking condition is controlled between 100 and 200° C., and the baking time is controlled 1-5 hrs to ensure that the die attach film is fully cured, for example, the stack-up diagrams in FIG. 3 are cross-sections of a sample after die attachment on one side and two sides, respectively, and FIG. 4 is a top view of the sample after die attachment is completed. Referring to FIGS. 3-4, the independent strain films 101 can be attached to one side or both sides through the die attach film 102, and the independent strain films 101 are arranged on the substrate 100 in arrays.

In step S50, a metal pad of each of the plurality of independent strain films is electrically connected with a corresponding metal pad of the substrate.

In this embodiment, metal pads for circuit connection are provided on the substrate and the independent strain films, respectively, and an automatic wire bonding machine is used for wire bonding. The automatic wire bonding machine can be an Iccon plus from K&S.

Specifically, as shown in FIG. 5, under a certain temperature and pressure, through ultrasonic welding, metal wires 105 are used to connect the corresponding metal pads on independent strain films 101 and the corresponding metal pads on the substrate 100, respectively, to obtain the structure as shown in FIG. 5.

In an embodiment, the specific flow of the wire bonding process in the above step S50 may be as follows: an assembly of the substrate and the independent strain films after die attachment is mounted at a designated position of the wire bonding machine, and the assembly of the substrate and the independent strain films is heated to a certain temperature (for example, to a temperature of between 100 and 200° C.). The automatic wire bonding machine is provided with a ceramic nozzle in a hollow structure, and a gold wire passes through a middle channel of the ceramic nozzle. Under the guidance of the vision system, the ceramic nozzle of the automatic wire bonding machine is moved to a position right above a designated metal pad of the substrate, and then the ceramic nozzle moves downward until the gold wire in the ceramic nozzle touches the corresponding metal pad of the substrate. By controlling a power (which can be between 1 and 5 W) and the duration (which can be between 5 and 150 ms) of the ultrasonic wave, and a first point of the gold wire is soldered to the metal pad of the substrate. Thereafter, the ceramic nozzle moves with the gold wire to right above a corresponding metal pad of the die under a movement track control, the ceramic nozzle then moves downward until the gold wire in the ceramic nozzle contacts the metal pad of the independent strain film, and under the action of ultrasonic waves, the second point of the gold wire is soldered at the metal pad of the independent strain film. By controlling the ultrasonic power and duration of the soldering of the second point, the gold wire in the ceramic nozzle can be interrupted at the second point after the welding at the second point is completed. The ceramic nozzle and the gold wire continue to move to a position right above a next metal pad, and start a next operation cycle until all the metal pads on the substrate are soldered. The automatic wire bonding machines often have the function of automatic loading and unloading. Through the feeding box and the receiving box, the supply of empty substrates and the recycling of the substrates after the wire bonding process can be automatically completed. For the substrate having the independent strain films arranged on both sides, the substrate after being performed with the wire bonding process on one side is turned over, and then performed with wire bonding process on a second side again.

In step S60, the plurality of independent strain films are packaged.

Specifically, in this embodiment, the substrate, the independent strain films, and the metal wires soldered on the substrate after the wire bonding process are very fragile, and need to be protected by packaging processing in time.

In an embodiment, in the packaging process in step S60, a surface of the assembly after the wire-bonding and stitching process can be covered by spraying or coating a flexible protective glue, and a covering area and thickness can ensure complete covering of the die and the metal wire. The protective layer 106 is formed by curing, and its structure is shown in FIG. 6.

In an embodiment, in the packaging process in step S60, a glue dispenser may be used to package the independent strain films. The glue dispenser may be an A77s automatic glue dispenser from Axxon. Specifically, a specified thermosetting glue is loaded in the glue dispenser, the substrate performed with the wire bonding process is loaded to a designated position of the glue dispenser, the substrate and the glue dispensing nozzle are heated to a certain temperature, and under the guidance of the vision system, the glue dispensing nozzle moves to the designated position above the substrate. By controlling the opening and closing of the glue dispensing nozzle, combined with the control of the movement track of the glue dispensing nozzle, the shape of the glue spray can be controlled. By adjusting the nozzle movement speed and spray pressure, the thickness of the glue spray can be controlled. After the glue spraying is completed, the substrate can be cured.

According to the type of protective glue, the curing of the substrate can be selected from heating curing and UV curing. For samples having the independent strain films arranged on both sides, it is generally necessary to complete the application of the protective adhesive on one side first, and then to apply the protective adhesive on the second side after curing.

In an embodiment, the fabrication method further comprises: performing signal test on binding wires after wire bonding.

Specifically, in this embodiment, a pre-test, that is, a signal test on the binding wire, may be performed after the wire bonding is completed. Specifically, a special machine can be used to extract electrical signals from the substrate by means of probes, etc., to test whether product defects exist in this step. If a bad connection is detected, the product can be repaired before the packaging process.

Furthermore, if the product yield in this step of the production process is high, the pre-test step can be omitted, from the consideration of the production efficiency.

In an embodiment, the fabrication method further comprises: performing function test on the plurality of the independent strain films after the packaging.

Specifically, in this embodiment, after the packaging process is completed, a post-test process may be performed on the independent strain films, and the post-test process includes function test. The function test generally refers to the power-on test. Through the automatic test machine, the test head corresponding to a position of a substrate pin is used to draw out a substrate signal, and the substrate signal is transmitted to a test board for testing. The test results are combined to determine whether the sample is a good product.

In an embodiment, the post-test procedure may further include an appearance test. The appearance test may use an automated optical inspection (AOI) device to perform appearance inspection on the packaging sample, for example, to test whether the package has problems such as missing/overflowing/foreign matter.

After the test is completed, the substrate needs to be divided into individual samples. A structural schematic diagram of samples after dicing is shown in FIG. 7. The dicing process can be carried out by a laser dicing machine. By controlling the laser dicing power/speed and the number of dicing cycles, the dicing of the substrate is completed. Usually a substrate can contain dozens to hundreds of individual independent die modules.

After the laser dicing is completed, a single independent die module is packaged. The packaging is usually carried out by using a material tray or a material tape. After the packaging is completed, the packaged die modules can be delivered for use.

In an embodiment, the metal pads on the substrate are conductive pastes formed by any one of screen printing, inkjet printing, and roll-to-roll printing.

In this embodiment, the pads on the substrate may be made of metal material, and in addition to adopting the wire bonding, the electrical connection between the metal pad of each the independent strain film and the corresponding metal pad on the substrate may be achieved by tap bonding, silver paste bonding, aerogel bonding, and flip chip bonding.

In an embodiment, the substrate may include conventional substrates for electrical circuits, such as printed circuit boards, flexible printed circuit (FPC) boards, fiberglass boards, and the like; and may also include common plastic substrates used in printable electronics, such as polyimide (PI) sheets, polyethylene terephthalate (PET) sheets, polyurethane (PU) sheets, polycarbonate (PC) sheets, epoxy sheet, or thermoplastic polyurethane (TPU) sheet, and the like. The substrate may also include glass sheets, metal sheets, paper sheets, composite sheets, wooden boards, ceramic sheets, and the like.

In an embodiment, before the thinning treatment, the fabrication method further comprises: etching the semiconductor wafer to form an integrated circuit within the semiconductor wafer.

In this embodiment, the integrated circuit may be formed by doping, etching, filling, and other processes on one side of the semiconductor wafer. For example, a strain sensing resistor, a signal processing circuit, and the like may be fabricated in the semiconductor wafer by doping, etching, filling, and other processes.

In an embodiment, at least one strain sensing resistor is provided in the semiconductor wafer.

In an embodiment, the semiconductor wafer may include at least one strain sensing resistor, including a doped silicon region, a doped polysilicon region, or a doped amorphous silicon region.

Dopants can participate in the doping process by ion implantation, diffusion, or any other common doping manner. The dopant may be an n-type dopant compound, and the dopant may include phosphorus, arsenic, arsenic, nitrogen, lithium, or any combination. The dopant can be a p-type dopant compound, and the dopant can include boron, indium, gallium, copper, aluminum, helium, titanium, or any combination thereof.

The dopant may be or may not be uniform throughout the doping amounts. The concentration level of the dopant can be greater than 10¹⁵cm⁻³, greater than 10¹⁶cm⁻³, greater than 10¹⁷cm⁻³, greater than 10¹⁸cm⁻³, or greater than 10¹⁹cm⁻³. In case that the semiconductor wafer is made from a single crystal silicon, the lower the doping level of the strain sensing resistor is, the higher the sensitivity is. However, at a lower doping level, sensitivity may be more temperature-dependent, and an optimal doping level may exist for each specific system design and application.

In an embodiment, the semiconductor wafer further comprises a signal processing circuit; and the signal processing circuit is in connection with the strain sensing resistor.

In an embodiment, at least one of the strain sensing resistors in the semiconductor wafer can form a pressure sensor, for example, a plurality of the strain sensing resistors can form a Wheatstone bridge or a half Wheatstone bridge, which is configured to generate a corresponding electrical signal according to a stress or a strain applied onto the semiconductor wafer.

In an embodiment, as shown in FIG. 8a, the strain sensing film in this embodiment includes: a substrate 301a, and a semiconductor film 302a deposited on a top of the substrate. The semiconductor film includes: a signal processing circuit 303a and at least one pressure sensor 304a. The signal processing circuit 303a and the at least one pressure sensor 304a are electrically connected, and at least one electrical contact 305a deposited on a top of the semiconductor film is electrically connected to a metal pad on the substrate 301a. Furthermore, a cover layer 306a may also be provided on the strain sensing film for protecting the strain sensing film.

In an embodiment, as shown in FIG. 8b, the strain sensing film in this embodiment includes: a substrate 301b, and a semiconductor film 302b deposited on a top of the substrate. The semiconductor film includes: a signal processing circuit 303b and at least one pressure sensor 304b. The signal processing circuit 303b and the at least one pressure sensor 304b are electrically connected, and at least one electrical contact 305b is provided between the semiconductor film and the substrate to provide electrical connection. Furthermore, a space between the semiconductor film and the substrate is filled with an adhesive 306b.

In an embodiment, as shown in FIG. 9a, in the fabrication process flow of the strain sensing film, the signal processing circuit 401 and at least one strain sensing resistor 402 are formed on a top surface of the semiconductor wafer 403, and a protective layer 404 at least partially protects the top surface of the semiconductor wafer 403. A backside of the semiconductor wafer is thinned to reduce a thickness, the semiconductor wafer is diced into at least one independent strain film. A backside surface of the die is connected to the substrate 405, the substrate 405 may include a previously deposited metal pad 406, at least one bonding wire 407 is provided on the die, so that the pre-deposited metal pad 406 on the substrate is connected to the signal processing circuit, and finally a protective layer 408 is used for packaging.

In an embodiment, as shown in FIG. 9b, in the fabrication process flow of the strain sensing film, the signal processing circuit 501 and at least one strain sensing resistor 502 are formed on a top surface of the semiconductor wafer 503, and a protective layer 504 at least partially protects the top surface of the semiconductor wafer 503. A backside of the semiconductor wafer is thinned to reduce a thickness, a second protective layer 505 is provided on the backside of the semiconductor wafer, and the semiconductor wafer is diced into at least one independent strain film, the signal processing circuit on the top surface of the die is connected to the metal pad 507 on the substrate 506 through bonding wires 508, and finally the gap between the substrate 506 and the strain sensing film is filled with a filler 509.

In an embodiment, the semiconductor wafer comprises at least one temperature sensor.

In this embodiment, by building a temperature sensor in the semiconductor wafer, the temperature change in a deformation area can be accurately measured, thereby compensating for a resistance change caused by a temperature change in the deformation area, and preventing the problem that an external temperature sensor is unable to measure an exact temperature of the semiconductor film and therefore may result in measurement error.

In an embodiment, the signal processing circuit can be integrated in the semiconductor film, and the signal processing circuit is connected with the Wheatstone bridge, which is composed of the strain sensing resistors, and the temperature sensor. By applying the temperature value detected by the temperature sensor to a preset sensitivity calibration algorithm, the temperature effect of the strain sensing film is corrected. The sensitivity calibration algorithm can be obtained based on theoretical calculations or data measured under controlled conditions, or based on a combination of the theoretical calculations and data measured under controlled conditions.

In an embodiment, a plurality of temperature sensors may be built in the semiconductor film, and the signal processing circuit may obtain a sensor temperature by performing weighted calculation on a plurality of temperature detection signals output by the plurality of temperature sensors, and then acquire a corresponding effective sensitivity coefficient based on the sensor temperature obtained from the weighted calculation.

In an embodiment, the semiconductor wafer comprises at least two strain sensing resistors. One of the at least two strain sensing resistors responds differently to a same strain with respect to at least one other strain sensing resistor.

Furthermore, one of the resistors has a different sensitivity coefficient than at least one other resistor.

Since the sensitivity coefficient of one of the resistors on the semiconductor film is different from that of the at least one other resistor, during the strain sensing process, at least two different electrical signals are generated in these resistors, or at least two different resistance values are generated at the same time, thereby increasing the sensitivity of the semiconductor film, and accurate strain signals can still be detected in a small strain environment.

In an embodiment, the arrangement directions of the at least two resistors may be different, or the at least two resistors may be prepared by using different piezoresistive materials, or the thicknesses of the at least two resistors may be different, etc., such that the sensitivity coefficient of one of the resistors on the semiconductor film is different from that of the at least one other resistor.

In an embodiment, the semiconductor film includes at least one of a silicon (Si) film, a germanium (Ge) film, a gallium arsenide (GaAs) film, a gallium nitride (GaN) film, a silicon carbide (SiC) film, a zinc sulfide (ZnS) film, a zinc oxide (ZnO) film.

In an embodiment, at least two strain sensing resistors are provided in the semiconductor wafer. One of the at least two strain sensing resistors is arranged in a direction different from at least one other strain sensing resistor.

In an embodiment, one of the resistors is oriented perpendicular to at least one other resistor.

For a p-type doped (100) crystalline silicon material, two mutually perpendicular directions exist, with sensitivity coefficients thereof basically the same in magnitude and opposite in sign, and the Temperature Coefficient of Resistance (TCR) has little correlation with the direction. Therefore, two mutually perpendicular resistors may be provided on the same semiconductor film, so as to enhance the signal quantity output by the strain sensing film and reduce the influence of the ambient temperature on the signal quantity, under the same deformation.

Referring to FIG. 10, 201 represents the semiconductor film on which the resistors are built, but is not drawn to the scale of the actual semiconductor film, and 202 represents the resistors deposited on the semiconductor film. The rectangular shape of the resistor shown in FIG. 10 does not represent the actual shape of the resistor, but is used to indicate the direction of current flow, which flows parallel to long sides of the rectangle. Actual resistors may contain different aspect ratios, and may have a combination of different parts, with each part having its own aspect ratio. Similarly, the location of each resistor is illustrative and may not be the actual location, and this is the case for all configurations in FIG. 10. FIGS. 10a-10h only show the relationship of the orientation of the resistors, but not their locations.

The semiconductor film 201 in FIG. 10a is provided with four resistors, and each resistor is perpendicular to the other two resistors.

FIG. 10b can represent the same configuration, since both of FIG. 10a and FIG. 10b include four resistors, each resistor is perpendicular to the other two resistors, and each resistor is parallel to at least one side of the semiconductor film 201.

The semiconductor film 201 in FIG. 10c is provided with four resistors, each resistor is perpendicular to the other two resistors, and each resistor is arranged at an angle of 45 degrees with respect to at least one side of the semiconductor film 201.

The semiconductor film 201 in FIG. 10d is provided with four resistors, and the four resistors are arranged in parallel with each other.

Furthermore, the four resistors in FIG. 10d are electrically connected in parallel.

The semiconductor film 201 in FIG. 10e is provided with two resistors, and the two resistors are arranged perpendicular to each other.

The semiconductor film 201 in FIG. 10f is provided with two resistors, and the two resistors are arranged in parallel with each other.

Furthermore, the two resistors in FIG. 10f are electrically connected in parallel.

The semiconductor film 201 in FIG. 10g is provided with three resistors, and an angle between two adjacent resistors is 45 degrees.

The semiconductor film 201 in FIG. 10h is provided with three resistors, and an angle between two adjacent resistors is 60 degrees, or 120 degrees.

Furthermore, the circuit in FIG. 10c may further include four resistors, each resistor is perpendicular to two other resistors, each resistor is arranged at an angle of 45 degrees with respect to at least one side of the semiconductor film 201.

In this embodiment, one of the resistors is arranged in a different direction than at least one other resistor. During the strain sensing process, due to the anisotropy of the semiconductor material, the sensitivity coefficients in the two directions are different, or the electrical signal produced in the two resistors are different, or two different resistance values are generated at the same time. Specifically, in the embodiments of the present application, the “direction” of the resistor refers to the direction of the current flowing through the resistor, rather than the geometric shape of the resistor.

In an embodiment, at least two strain sensing resistors are provided in the semiconductor wafer. One of the at least two strain sensing resistors has a sensitivity coefficient different from that of at least one other strain sensing resistor.

In an embodiment, at least two strain sensing resistors are provided in the semiconductor wafer. One of the at least two strain sensing resistors is perpendicular to at least one other strain sensing resistor.

In this embodiment, two mutually perpendicular resistors can be arranged on the same semiconductor film to enhance signal quantity output by the strain sensing film under the same deformation.

In an embodiment, the semiconductor film 201 may comprise a half-Wheatstone bridge composed of two resistors. A strain level of one resistor may be different from a strain level of the other resistor during strain sensing in the sensing device.

In this embodiment, when the strain sensing film in this embodiment undergoes strain sensing, a gage factor of one of the resistors is different from the gage factor of at least one other resistor.

In an embodiment, a current flow direction in at least one resistor is perpendicular to a current flow direction in at least one other resistor.

In this embodiment, the semiconductor film 201 may include a half-Wheatstone bridge composed of two resistors. In the two resistors forming the half-Wheatstone bridge, the current flow direction in one resistor may be perpendicular to the current flow direction in the other resistor.

In an embodiment, a Wheatstone bridge is provided on the semiconductor film, and the Wheatstone bridge includes: a first resistor, a second resistor, a third resistor, and a fourth resistor. The second resistor and the third resistor have positive sensitivity coefficients, and the first resistor and the fourth resistor have negative sensitivity coefficients.

In this embodiment, as shown in FIG. 5, the strain sensing film is (100) the semiconductor film (201) with acrystallographic orientation, on which, a first resistor R1, a second resistor R2, and a third resistor R3, and a fourth resistor R4 are arranged. For a p-type doped (100) crystalline silicon material, two mutually perpendicular directions exist, with sensitivity coefficients thereof basically the same in magnitude and opposite in sign, and the TCR has little correlation with the direction. Therefore, by arranging the second resistor R2 to be perpendicular to the first resistor R1, and the third resistor R3 to be perpendicular to the fourth resistor R4, it can be realized that two resistors having positive sensitivity coefficients and two negative resistors having negative sensitivity coefficients are arranged on the semiconductor film, such that under the same deformation, the signal quantity output by the Wheatstone bridge can be enhanced, and the influence of the ambient temperature on the signal quantity can be reduced.

In an embodiment, dR₂/R₂=GF2×ε, dR₁/R₁=GF1×ε, Wheatstone bridge voltage signal dU=Vcc/2×(dR₂/R₂−dR₁/R₁)=Vcc×dR₂/R₂, in which, GF2 is the pressure inductance coefficient of a second resistor R2, GF1 is the pressure inductance coefficient of a first resistor R1, GF1=−GF2, ε is a strain at the Wheatstone bridge, and Vcc is a supply voltage of the Wheatstone bridge.

In this embodiment, four resistors are arranged on the semiconductor film 201, and the angles of the four resistors can be adjusted, for example, to make the second resistor R2 and the third resistor R3 have positive sensitivity coefficients, and the first resistor R1 and the fourth resistor R4 have negative sensitivity coefficients. In such condition, under the same deformation, dR₂/R₂=−dR₁/R₁, and the Wheatstone bridge voltage signal dU=Vcc/2×(dR₂/R₂−dR₁/R₁)=Vcc×dR₂/R₂, in such condition, the signal quantity of the Wheatstone bridge can be significantly increased.

In an embodiment, the semiconductor film has a thickness of smaller than or equal to 70 μm.

In an embodiment, the semiconductor film has a thickness of smaller than or equal to 50 μm.

In an embodiment, the semiconductor film has a thickness of smaller than or equal to 30 μm.

In an embodiment, the semiconductor film has a thickness of smaller than or equal to 25 μm.

In an embodiment, the semiconductor film has a thickness of smaller than or equal to 20 μm.

In an embodiment, the semiconductor film has a thickness of smaller than or equal to 15 μm.

In an embodiment, the semiconductor film 201 in the above embodiment may be a silicon film, and the elastic modulus of the silicon material is equivalent to that of steel, about 160 GPa. The greater the thickness thereof is, the greater the difficulty of deformation is. Therefore, By reducing the thickness of the silicon wafer, the thickness of the silicon wafer is smaller than or equal to 70 μm, or smaller than or equal to 50 μm, or smaller than or equal to 30 μm, or smaller than or equal to 25 μm, or smaller than or equal to 20 μm, or smaller than or equal to 15 μm. In such condition, the silicon film will become soft and easy to deform. The efficiency of strain transferring from the substrate to the silicon film can be improved, so that the effective sensitivity coefficient of the silicon film as well as the signal quantity can be significantly improved.

In a specific application, the order of steps S10, S20, S30, and S40 in the above preparation method can be adjusted according to requirements.

In another embodiment, as shown in FIG. 12, a method for fabricating a strain sensing film may comprise steps S11, S21, S31, S41, S51, and S61.

In step S11, a semiconductor wafer is thinned to form a semiconductor film.

In this embodiment, the first step of the packaging process is to thin the semiconductor wafer (ie, wafer), which can be thinned to 15-70 μm according to user requirements.

In step S21, the semiconductor film is diced to form a plurality of independent strain films.

In this embodiment, the semiconductor film is diced to form a plurality of individual film dies, and the dicing process used may be the same as the dicing process in step S20 of the foregoing embodiment.

In step S31, a glue is applied onto a substrate.

In this embodiment, the dicing process can be performed without attaching a die attach film onto the semiconductor film. Furthermore, the semiconductor film still needs to be attached to the base film via a viscosity reducing film.

After the dicing process is performed on the semiconductor film, a layer of glue is applied on a designated position of the substrate by a certain manner.

In an embodiment, the Young's modulus after curing of the glue needs to be greater than 100 MPa.

In step S41, the plurality of independent strain films are transferred to the substrate, and the plurality of independent strain films are completely attached to the substrate.

In this embodiment, the die is transferred to the substrate, and a certain viscosity is produced by glue. Specifically, the glue is cured by a certain process, for example, by heating or other means, so that the independent strain film is completely attached to the substrate.

In step S51, a metal pad of each of the plurality of independent strain films is electrically connected with a corresponding metal pad of the substrate.

In this embodiment, the electrical connecting process in step S50 in the above-mentioned embodiment may be performed to realize the electrical connections between the metal pads on the independent strain films and the corresponding metal pads of the substrate, respectively.

In step S61, the plurality of independent strain films are packaged.

In this embodiment, the packaging process in step S60 in the above-mentioned embodiment may be performed to realize the package of the independent strain films.

Embodiments of the present application further provide a stain sensing film. The strain sensing film is obtained by the method for fabricating a strain sensing film according to any one of the above embodiments.

Embodiments of the present application further provide a pressure sensor. The pressure sensor comprises the strain sensing film according to any one of the above embodiments.

When an external stress is applied, the pressure sensor can generate an electrical signal that can represent a resistance change, a current change, a voltage change, a charge change, or a resonant frequency change within the strain sensing film.

In this embodiment, the strain sensing film together with the signal processing circuit together (in particular, the strain sensing film is attached to the substrate, or at least one strain sensing film and a signal processing circuit are arranged on the substrate together) form a hybrid strain sensing system, so as to provide high sensitivity and flexibility for various application scenarios.

In an embodiment, the attachment method may be gluing, mechanical fixing, surface mounting technology (SMT), and the like.

The field of use for such a strain sensing system includes but is not limited to strain sensing, or force sensing, or touch sensing, or tactile sensing, in any human machine interface, or machine-machine interactions, for smart phones, tablets, personal computers, touch screens, virtual reality (VR) systems, gaming systems, consumer electronics, vehicles, scientific instruments, toys, remote controls, industrial machinery, bio-medical sensors to monitor heart rate, blood pressure, and the movements and acceleration of skins, muscles, bones, joints and other body parts; robotic sensors to measure touch, local pressure, local tension, movements and acceleration of any parts of the robots; vibration sensors for buildings, bridges and any other man-made structures; sensors to monitor strain, pressure, movement, acceleration of any parts of vehicles that may be used in land, air, water, or space; movement, acceleration, and strain sensors that can be incorporated into smart fabrics; and any other applications where local static or dynamic deformation, displacement, or strain need to be measured.

The aforementioned embodiments are only preferred embodiments of the present application, and are not intended to limit the present application. Any modification, equivalent replacement, improvement, and so on, which are made within the spirit and the principle of the present application, should be included in the protection scope of the present application.

	Number	Date	Country
	63064086	Aug 2020	US
	62992000	Mar 2020	US

METHOD FOR FABRICATING STRAIN SENSING FILM, STRAIN SENSING FILM, AND PRESSURE SENSOR

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