Patent Application
20180190896
The present invention relates generally to acoustic devices. More, specifically, the present invention relates to ultrasonic transmitters and receivers containing a conductive layer printed with nanoparticle inks.
Acoustic devices such as ultrasonic transmitters and receivers have a broad range of applications, such as in medical imaging, fingerprint scanners, etc. Ultrasonic transmitters and receivers include a conductive layer which is often prepared with sputtered metal or printed with polymer thick film (PTF) conductive pastes. Although a sputtered metal conductive layer has a low surface roughness, sputtering is a slow and high-cost process. It involves the use of a vacuum and is not compatible with roll-to-roll manufacturing. Sputtering a conductive layer of a few microns (i.e. μm) to about 10 microns in thickness is time-consuming. Printing PTF conductive pastes is an additive and low-cost process. It can be made in a roll-to-roll manner. However, PTF conductive layers can exhibit large surface roughness and nano-sized to micron-sized voids, which can translate into poor electrical performance and poor image quality due to ultrasonic wave energy loss at the roughness interface or the interfaces between the conductive materials and the voids. While progress has been made in providing improved ultrasonic transmitters and receivers with PTF conductive pastes, there remains a need for improved acoustic devices having a dense and smooth conductive layer.
An ultrasonic transmitter includes a piezoelectric layer, a first conductive layer which is above the piezoelectric layer, and a second conductive layer which is below the piezoelectric layer. At least one of the first and the second conductive layers comprises metal nanoparticles. The metal nanoparticles may be a silver nanoparticle, copper nanoparticle, gold nanoparticle, palladium nanoparticle, nickel nanoparticle, and the mixture thereof.
An ultrasonic receiver includes a piezoelectric layer, and a conductive layer which is on one side of the piezoelectric layer, and a thin-film transistor (TFT) array which is on the other side of the piezoelectric layer. The conductive layer comprises metal nanoparticles, which may be a silver nanoparticle, copper nanoparticle, gold nanoparticle, palladium nanoparticle, nickel nanoparticle, and the mixture thereof.
Use of metal nanoparticles as a conductive layer provides for ultrasonic transmitters or receivers with smooth, dense, and highly conductive electrodes, thus resulting in reduced ultrasonic energy loss and better image quality.
The piezoelectric film 202, conductive layer 204 and overcoat/protection layer 206 may be the same as or different from the piezoelectric layer 102, conductive layer 104 and/or 106, and overcoat/protection layer 108 described with respect to
The transmitter or the receiver may have a plurality of transmitter or receiver elements described above. A transmitter adjacent to the receiver generates a transmit signal at an ultrasonic frequency. The transmit signal is reflected from a surface such as a finger to produce a reflected signal which will be detected by the receiver. The received signal can be the reflected signal itself, or the superposition of the transmit signal and the reflected signal. In general, the received signal represents the difference in acoustic impedances across the surface.
The piezoelectric layer 102 or 202 may include ceramic materials, for example, PZT (lead zirconate titanate), PST (lead strontium titanate), quartz, (Pb, Sm)TiO3, PMN(Pb(MgNb)O3)-PT(PbTiO3), or other like materials. Organic piezoelectric materials such as PVDF(polyvinylidene fluoride, or polyvinylidene difluoride) or PVDF copolymer, terpolymers such as PVDF-TrFE (P(VDF-trifluoroethylene)), P(VDF-tetrafluoroethylene), poly(vinylidene fluoride-hexafluoropropylene) (P(VDF-HFP), poly(vinylidene fluoride-chlorotrifluoroethylene) (P(VDF-CTFE), poly(vinylidene fluoride-trifluoroethylene-chlorofluoroethylene) (P(VDF-TrFE-CFE)) and the like, may be used for certain applications, such as large area ultrasonic image scanners. The piezoelectric layer may have a thickness from about 5 μm to about 500 μm, including from about 5 μm to about 200 μm, and from about 10 μm to about 150 μm. In embodiments, the organic piezoelectric material is a free-standing film, having a thickness from about 15 μm to about 200 μm, including from about 20 μm to about 100 μm, specifically, the organic piezoelectric film is a PVDF film having a thickness from about 25 to about 35 μm. In some embodiments, 102 and 202 are the same piezoelectric material, for example, they both are organic piezoelectric material. In other embodiments, 102 and 202 are different materials.
The conductive layers 104, 106, and 204 may include a metal nanoparticle ink, such as copper, silver, gold, palladium, or nickel nanoparticle, an alloy thereof, or a mixture thereof. In one example the metal nanoparticle ink may be silver nanoparticle ink. An exemplary silver nanoparticle ink may include PG-007 and or PS-004 (Paru Inc., Korea), GDP-NO ink (ANP, Korea), PSI-219 (Novacentrix, USA), and the like. The ink may comprise, for example, from about 40 wt % to about ˜85 wt % silver nanoparticles including from about 60 wt % to about 80 wt % silver nanoparticles dispersed in proper solvents, for example, diethylene glycol, ethylene glycol (EG), propylene, glycol monomethyl ether acetate, propylene glycol monomethyl ether, terpineol, 2-(2-Ethoxyethoxy)ethanol, and the like solvent. The metal nanoparticle ink may be patterned into the desired electrode structures using screen (flat bed or rotary), flexo, gravure, aerosol-jet, dispense jet, inkjet, stencil printing methods, or other additive printing techniques. Alternatively, coating methods such as spin coating, dip coating, doctor blade coating, or slot die coating may be used to deposit the metal nanoparticle ink structure. Furthermore, the conductive layers 104, 106, and 204 may be fully or incompletely sintered. In one screen printing example a screen with 280 mesh counts and an emulsion thickness of 0.015 mm (0.0006 inch) may be used, with an off contact set at 40-50 μm.
The silver nanoparticle ink comprises silver nanoparticles having an average particle diameter in the range from about 2 nm to about 950 nm, alternatively, from about 5 nm to about 800 nm including from about 50 nm to about 300 nm. In some embodiments, the silver nanoparticles may have a shell layer such as an organic compound physically or chemically attached to their surface to prevent the aggregation of the nanoparticles in the ink. The particle size refers to the silver metal itself, and does not include the organic shell layer. The particle size can be determined using for example Transmission Electron Microscopy (TEM) or Scanning Electron Microscopy (SEM). In some embodiments, the silver nanoparticles are at least partially stabilized with a hygroscopic or water-soluble compound. Exemplary hygroscopic compound includes polyvinylpyrrolidone (PVP), polyvinyl alcohol (PVA), polyethyleneimine, hydroxyl cellulose, polyethylene glycol (PEG), polyethylene oxide (PEO), poly(acrylic acid), and the like. Other commonly used organic compounds such as organoamine or thiol compounds can be used as well.
In some embodiments, the metal nanoparticles are completed fused together in the conductive layer. Namely, individual metal nanoparticle cannot be detected using common tools such as SEM. In other embodiments, the metal nanoparticles are not completely fused together and individual metal nanoparticle can be clearly seen using common characterization tools. In particular, the average particle diameter of the metal nanoparticles in the conductive layer after drying and annealing is substantially the same as that in the metal nanoparticle ink.
The metal nanoparticle ink provides a smooth electrode film due to the small particle size and spherical shape. Silver nanoparticle ink, for example, may have a particle size from about 10 nm to about 800 nm, such as from about 50 nm to about 800 nm, or from about 80 nm to about 300 nm. As such, a transmitter and receiver using metal nanoparticle ink electrodes benefit from reduced ultrasonic energy loss and thus provide a highly conductive electrode for ultrasonic transmitter/receiver applications.
Conductive layers 104, 106, and 204 including, for example, silver nanoparticle ink, may have a thickness of about 1 to 20 μm, including 1 to 12 μm or 5 to 12 μm, and a low surface roughness. The surface roughness can be characterized using a profile surface roughness for example the parameter Ra by a surface profilometer. In some embodiments, the Ra is less than 0.4 μm, including less than 0.2 μm, or less than 0.1 μm. The surface roughness can also be characterized using areal roughness for example the parameter Sz or Sa by a white light interferometer. The conductive layer has an Sz, which is the distance from the highest peak to the lowest valley, of less than 5 μm, including less than 3 μm, or less than 2 μm, as determined by for example white light interferometry at a scan area of for example 3×3 mm2. The conductive layer has an Sa, for example, less than 0.4 μm, including less than 0.2 μm, or less than 0.1 μm as determined by for example white light interferometry at a scan area of for example 3×3 mm2. In this example, silver nanoparticle ink conductive layers 104, 106 and 204 may exhibit a gloss greater than about 50 gloss units (GU), including greater than about 80 GU, or greater than about 100 GU.
The metal nanoparticle ink conductive layers 104, 106, and 204 may be processed and dried and/or annealed at any temperature. The preferred temperature will have no adverse effect on the piezoelectric layer or other pre-deposited component. In some embodiments, the metal nanoparticle ink is dried and annealed at a temperature no more than 200° C., including no more than 170° C., or no more than 150° C., or no more than 100° C. In specific embodiments, the metal nanoparticles are processed (dried and annealed) at a temperature of 80° C. or less when PVDF, for example, is used as the piezoelectric layer 102 or 202. Furthermore, the metal nanoparticles are processed at a temperature of 60° C. or less when PVDF, for example, is used as the piezoelectric layer 102 or 202. PVDF film having a high d33 (a high content of beta-phase) is often obtained though dedicated mechanical stretching processes. Annealed and poled PVDF film has a crystal relaxation temperature of about 75° C. Therefore, processing the PVDF film above this relaxation temperature will cause reduction of the piezo-electrical properties such as the reduction of d33. In addition, processing the PVDF film at a high temperature (e.g. >80° C.) also causes a large shrinkage of the film due to the crystal relaxation. When PVDF is used as the piezoelectric layer, low-temperature processing of the metal nanoparticle layer is critical. This is significantly different from other piezoelectric materials such as inorganic piezoelectric materials and PVDF-TrFE copolymers, which have a stable piezoelectric phase at a relatively higher temperature and may, therefore, be processed at a relatively higher temperature. The metal nanoparticle conductive layer, for example, has a resistivity of less than 1.0×104 ohm-cm, including less than 8.0×10−5 ohm-cm and less than 5.0×10−5 ohm-cm. In specific embodiments, the conductive layer 104 and 106 in the transmitter may have a resistivity lower than 8.0×10−5 ohm-cm, lower than 5.0×10−5 ohm-cm, and even lower than 2.0×10−5 ohm-cm. The low resistivity results in minimal overall resistive losses, which are known to reduce sensitivity. The resistivity of conductive layer 204 may be the same or different from that of conductive layers 104 and 106. The metal nanoparticle conductive layer also exhibits optimal adhesion to the piezoelectric material. For example, the conductive layers 104, 106, and 204 may have an adhesion force to the piezoelectric layer 102 greater than 1.0 N/cm, including greater than 1.5 N/cm, and greater than 2.0 N/cm, as measured by the 90 degree peel method.
Due to the small particle size, the conductive layer made from the nanoparticle ink not only provides a smooth surface, but also exhibits a dense layer. Few, if any, voids or pinholes can be found in the conductive layer. On the other hand, due to the large particle size and the presence of polymer binders, the conductive layer prepared from the PTF paste has nano to micron sized voids/pinholes or nano to micron sized areas with polymer binder only.
The overcoat/protection layers 108, 110 may include a dielectric, insulating material, such as polyacrylate, epoxy resin, polyester, styrene polymer, polyamide, polyurethane, and the like. The overcoat layer can be processed in a similar manner to the conductive layer. The overcoat layer can be either thermally cured or UV cured.
It should be noted that the current acoustic device is different from other passive electronic devices involving a piezoelectric material and a metal nanoparticle conductive layer. In the present embodiments, an ultrasonic wave will pass through the conductive layer such that the conductive layer is considered an active component of the final integrated device. The layer will absorb, reflect, and scatter the ultrasonic wave. The metal nanoparticle conductive layer in conventional passive electronic devices may provide the function of conducting current only.
In certain embodiments, the transmitter and the receiver may include a PVDF piezoelectric layer and a metal nanoparticle conductive layer. The metal nanoparticle conductive layer may be dried and annealed at a temperature of up to about 80° C. and may have a surface roughness less than about 0.2 microns and a resistivity less than about 5.0×10−5 ohm-cm. In other embodiments, the metal nanoparticle conductive layer may be correspond to a silver nanoparticle conductive layer containing incompletely fused silver nanoparticles. The PVDF piezoelectric layer may have a beta-crystal phase more than 40 wt % or more than 50 wt % as determined by the differential scanning calorimetry method. The PVDF layer may have a d33 greater than 14×10−12 Coulombs/Newton (C/N), including greater than 16×10−12 C/N, or greater than 17×10−12 C/N.
When a finger is pressed on the platen, ultrasonic energy is generated and transmitted from the transmitter layer 604 up through the TFT array 606, receiver layer 602 and platen to the ridges of the finger. This ultrasonic energy is absorbed by the ridges and reflected by the valleys of the finger. The reflected energy is detected by the receiver layer 602 attached to the TFT array. The TFT array converts the received, reflected energy to a digital signal. External circuitry may translate that digital signal into a fingerprint image.
While various embodiments of the invention have been described, it will be apparent to those of ordinary skill in the art that many more embodiments and implementations are possible within the scope of the invention. Accordingly, the invention is not to be restricted except in light of the attached claims and their equivalents.
