WO2014113032A1

WO2014113032A1 - Thin film stack

Info

Publication number: WO2014113032A1
Application number: PCT/US2013/022374
Authority: WO
Inventors: JR. James Elmer ABBOTT; Peter Mardilovich; Christopher Shelton
Original assignee: Hewlett-Packard Development Company, L.P.
Priority date: 2013-01-21
Filing date: 2013-01-21
Publication date: 2014-07-24

Abstract

A thin film stack has a first layer including silicon dioxide and a second layer directly on the first layer that includes a blend of zinc oxide and silicon dioxide. A third layer that includes zinc oxide is directly on the second layer.

Description

THIN FILM STACK

Background

[0001] Thin films are known in the micro and nanoelectronics industry for the fabrication of sensors, actuators, transistors, etc. In many applications, metal and non-metal films are layered to form a stack.

[0002] For example, piezoelectric devices, such as piezoelectric inkjet printheads or sensors, can be prepared by stacking various piezoelectric materials, other films, and metals, e.g., conductors and/or electrodes, in specific configurations for piezoelectric actuation or piezoelectric sensing. In the case of a piezoelectric printhead, piezoelectric actuation on or in an ink chamber can be used to eject or jet fluids therefrom. One such piezoelectric material is lead zirconate titanate, or "PZT," which can be grown or otherwise applied on the surface of a metal electrode.

Brief Description of the Drawings

[0003] Figures 1A and 1 B are block diagrams illustrating examples of thin film stacks.

[0004] Figure 2 is a flow diagram illustrating a method for creating thin film stacks.

[0005] Figures 3A-3G are charts illustrating X-ray photoelectron spectroscopy (XPS) analyses of thin film components mixing and segregation. [0006] Figure 4 is a chart illustrating results of an X-ray diffraction (XRD) characterization.

[0007] Figure 5 is a schematic view illustrating a portion of an example inkjet printhead.

[0008] Figure 6 is a block diagram illustrating example thin film stack layers of the inkjet printhead of Figure 5.

[0009] Figures 7A and 7B are charts illustrating XPS analysis depth profile data.

Detailed Description

[0010] In the following detailed description, reference is made to the

accompanying drawings which form a part hereof, and in which is shown by way of illustration specific examples. In this regard, directional terminology, such as "top," "bottom," "front," "back," etc., is used with reference to the orientation of the Figure(s) being described. Because the various components can be positioned in a number of different orientations, the directional terminology is used for purposes of illustration and is in no way limiting. It is to be understood that other versions may be utilized and structural or logical changes may be made without departing from the scope of the present invention. The following detailed description, therefore, is not to be taken in a limiting sense, and the scope of the present invention is defined by the appended claims. It is to be understood that features of the various examples described herein may be combined with each other, unless specifically noted otherwise.

[0011]Thin film structures are employed in many applications. Adhesion between various materials in thin film stacks, including the adherence of metals to non-metal films, present challenges, particularly in environments where high temperatures, piezoelectric vibration, and certain migrating elements or compounds may be present in nearby layers.

[0012] The piezoelectric printhead is an example of such a device that can be prepared or used under some of these conditions. In the case of a piezoelectric printhead, piezoelectric actuation on or in an ink chamber can be used to eject or jet fluids therefrom. One such piezoelectric material is lead zirconate titanate, or "PZT," which can be grown or otherwise applied on the surface of a metal electrode, such as platinum, ruthenium, palladium, and iridium, as well as some conductive and non-conductive oxides, such as lrO₂, SrRuO₃, ZrO₂, etc.

[0013] The appropriate adhesion of the metal electrode (which upon completion, can have PZT or another piezoelectric material applied to one side thereof) to an underlying layer can be provided by an adhesion layer including an annealed blend of zinc oxide and silicon dioxide. The adhesion of many noble metal electrodes and other metals (e.g., copper) that do not adhere well to other materials, such as especially non-metallic surfaces, oxide surfaces, or polymers, may not typically strong enough without the presence of an adhesive layer.

Furthermore, even if adhesion is acceptable by using other types of adhesive materials, there can be other drawbacks with some of these other known adhesive materials that are used in thin film stacks. In piezoelectric printheads, for example, various layers of metal and non-metal films are stacked and adhered together; and high temperatures, piezoelectric actuation, and migration of lead or other materials can be common from layer to layer. Lead ion migration through metal electrodes can undermine the function of the device over time. Known adhesive layers tend to underperform when exposed to very high manufacturing temperatures.

[0014] In accordance with aspects of the present disclosure, a thin film stack has a first layer including silicon dioxide, a second layer including a blend of zinc oxide and silicon dioxide directly on the first layer, and a third layer that includes zinc oxide directly on the second layer. In some examples, the second layer is formed by depositing zinc oxide on the first layer and heating the first layer and the zinc oxide at a first temperature for a predetermined first time period.

[0015] Figures 1A and 1 B are block diagrams illustrating examples of thin film structures, and Figure 2 is a flow diagram illustrating an example of a process for forming such structures. In each of the various embodiments described herein, whether discussing the thin film stack device or related methods, there may be some common features of each of these embodiments that further characterize options in accordance with principles discussed herein. Thus, any discussion of the thin film stack or method, either alone or in combination, is also applicable to the other embodiments not specifically mentioned. For example, a discussion of the various layers in the context of the thin film stack is also applicable to the related method, and vice versa.

[0016] In Figure 1A, a first structure 1 is illustrated, wherein a first layer 10 including a silicon dioxide substrate is provided (block 20 of Figure 2). Zinc oxide 12 is deposited on the substrate 10 in block 22 of Figure 2. Block 24 represents an annealing process in which the first layer 10 and the zinc oxide 12 are heated to form a second layer 14 (block 26) that includes a blend of zinc oxide and silicon dioxide. As shown in the thin film stack 2 of Figure 1 B, the second layer 14 is directly on the first layer 10, and a zinc oxide third layer 12 is directly on the second layer 14 after the annealing process 24.

[0017]The annealing process 24 drives mixing of the zinc and silicon dioxide, which improves adhesion of subsequent layers and stabilizes the system for later high temperature processing. For example, this stabilization creates a robust adhesion layer for a metal electrode deposition in a PZT driven piezoelectric inkjet device. The second and third layers 12, 14 not only facilitate secure adhesion of the metal electrode, but also act as a robust barrier to lead diffusion into the silicon dioxide substrate 10.

[0018] In some examples, the annealing 24 was performed in air or a

nitrogen/oxygen (N2/O2) mixture that mimics the air composition in a furnace or in a rapid thermal processing (RTP) system. The anneal duration varied from about 30 seconds when using RTP, to about 60 minutes in a furnace in some examples. Suitable ramp rates are about 5-200°C/s for RT and about 1- 20°C/min in a furnace.

[0019] Figures 3A - 3G illustrate atomic concentration (%) for 50 nm of zinc oxide deposited on a silicon dioxide substrate using X-ray photoelectron spectroscopy (XPS) for anneal temperatures from 400°C to 1000°C.

Specimens were analyzed using a PHI Quantera Scanning ESCA. The spectrometer uses a monochromatic aluminum x-ray source with a photon energy of 1486.6eV. The samples were cleaved from larger samples so as to avoid fingerprints and analyzed without further preparation. The areas of interest were determined from a secondary electron image generated by the scanning photon beam and the analysis was performed using a 20 pm photon beam placed on the area of interest.

[0020] A low spectral resolution survey spectrum was acquired from each area of interest on the sample. Higher spectral resolution data were acquired from the detected elements to determine the surface chemistry and composition. The data were charge corrected to carbon 1s at 284.8eV. The data were reduced using a non-linear least squares method to determine the chemical states to determine the organic and inorganic oxygen species. Quantification calculations were made using established sensitivity factors (bracketed values) and the reported values should be regarded as semi-quantitative. It should be noted that the analysis is of only the first 7 nm of the surface.

[0021]The sputter depth profiles were acquired by alternately sputtering and acquiring spectral data. The samples were sputtered using an Ar+ ion beam with a sputter rate of about 5 nm/min relative to Si0₂ with a 2kV beam. The Y- axis is the atomic percentage of an element, and the X-axis is the sputter time (how long the sample was bombarded by Ar, how much was removed in prior analyses). The sputter time is proportional to the depth of the films, such that "0" represents the sample surface and as the times get longer deeper portions of the sample were analyzed.

[0022] The 800°C sample (Figure 3E) begins to show diffusion of the zinc oxide into the substrate. At 900°C (Figure 3F) there is more pronounced diffusion, and by 1000°C (Figure 3G) the zinc oxide is fully mixed in the silicon dioxide matrix.

[0023] Figure 4 illustrates results of an X-ray diffraction (XRD) characterization of 50 nm of zinc oxide deposited on silicon dioxide and annealed as disclosed herein. [0024]GIXRD measurements, as employed in this study, typically probe much less than a micron into a sample. The actual penetration depth of the incident radiation is highly dependent on the density and stoichiometry of the sample as well as the incident angle of the copper radiation. A precise estimate of the penetration depth of the incident radiation was computed from known mass absorption coefficients so as to optimize the analysis for the study of the films of interest.

[0025] The XRD analysis illustrated in Figure 4 suggests the presence of crystalline phases of both ZnO and Zn_xSiO_y materials. The zinc oxide is predominately of hexagonal symmetry at all temperatures. A slight shift in crystallographic texture of this phase of zinc oxide is observed at about 800°C. The orientation of the Zn_xSiO_y phase changes with increasing temperature, but the overall phase symmetry likely remains constant. A stoichiometry of Zn₂SiO₄ for the mixed phase is suspected.

[0026] Figure 5 is a schematic view of a portion of an inkjet printhead 100, with an expanded view of a thin film stack 101 provided to illustrate an example thin film stack employed in the illustrated inkjet printhead 100. Though an inkjet printhead is shown in Figure 5 with various specific layers, it is understood that this is not intended to limit the scope of the present disclosure. This is provided to show examples of various thin film stacks that can be used in various devices, such as piezoelectric actuators or sensors.

[0027] In Figure 5, a silicon support is fabricated to include multiple ink chambers 112 for receiving and jetting ink therefrom. It is noted that often, ink chambers or other areas where ink may contact the printhead can be coated with any of a number of protective coatings. Those coatings are not shown, but it is understood that such a coating may be used for protective purposes without departing from the scope of the present disclosure. For example, tantalum or tantalum oxide coatings, such as Ta₂O₅, are often used for this purpose. Other support material(s) can be used alternatively or in addition to the mentioned silicon support and optional protective coatings. Thus, the term "support" typically includes structures comprising semi-conductive materials such as silicon wafer, either alone or in assemblies comprising other materials applied thereto. Metallic supports can also be used, including metallic materials with an insulating material applied thereto. Certain specific materials that can be used for the support material include silicon, glass, gallium arsenide, silicon on sapphire (SOS), germanium, germanium silicon, diamond, silicon on insulator (SOI) material, selective implantation of oxygen (SIMOX) substrates, or other similar materials. Furthermore, the substrate described herein can actually be the support material, particularly when the support material inherently includes an oxidized surface. However, in many typical examples, a separate membrane of oxidized material is applied to the support and acts as the substrate.

[0028] In Figure 5, the printhead 100 includes a substrate 10, an adhesive layer 1 16, a first metal electrode 118, a piezoelectric layer 120, a second metal electrode 122, and a passivation layer 124. Some typical printheads could additionally include further layers, including other insulating, semi-conducing, conducting, or protective layers that are not shown. However, one skilled in the art would recognize other layers that could optionally be used, or optionally omitted from the illustrated structure.

[0029] In the system shown, the first metal electrode 118 and the second metal electrode 122 are used to generate an electric field with respect to the piezoelectric layer 120, and as the piezoelectric layer is actuated, the thin film stack bends into an appropriate ink chamber 12, causing inkjetting to occur. The substrate layer 10 can be the support material with an oxide layer inherently present on its surface, but is typically prepared as an oxide membrane applied to the support material, e.g., SiO₂, ZrO₂, HfO₂, Ta₂O₅, AI₂O₃, SrTiO₃, etc. These membranes can be applied as multiple layers, and/or be prepared using multiple materials in a common layer. Thus, the materials are typically applied as one or more layer to the silicon or other support material as described above. When the substrate is in the form of a thin film or membrane, the substrate can be formed at a thickness from 10 A to 10 pm, for example. In an example piezoelectric actuator device, the thickness of this substrate, e.g., oxidized membrane, can be approximately the same thickness as piezoelectric layer, e.g., at a 1 :2 to 2:1 thickness ratio of substrate layer to piezoelectric layer, and both layers can be about 50 nm or greater.

[0030] In the printhead 100 illustrated in Figure 5, a passivation layer 124 is shown, which can be formed of any suitable material, including, but not limited to wet or dry process silicon dioxide, aluminum oxide (e.g., Al₂0₃), silicon carbide, silicon nitride, tetraethylorthosilicate-based oxides, borophosphosilicate glass, phosphosilicate glass, or borosilicate glass, Hf0₂, Zr0₂, or the like.

Suitable thicknesses for this layer can be from 10nm to 1 pm, though

thicknesses outside of this range can also be used.

[0031]The metal electrodes 118, 122 can be applied at a thickness from about 5 nm to 5 microns, though thicknesses outside this range can also be used. Materials that can be used, particularly for electrodes, typically include noble metals or other metals or alloys, including but not limited to, platinum, copper, gold, ruthenium, iridium, silver, nickel molybdenum, rhodium, and palladium. In other examples, oxides of these or other metals can also be used, such as Ir0₂ or SrRuOs, if the adhesive properties of the adhesion layers of the present disclosure would be beneficial for use. Platinum is of particular interest as a metal that benefits from the adhesive layers of the present disclosure because its surface does not become readily oxidized. Metal electrodes (or metals applied for another purpose, such as for conductive layers or traces) can be deposited using any technique known in the art, such as sputtering,

evaporation, growing the metal on a substrate, plasma deposition,

electroplating, etc.

[0032] Figure 6 shows an example of the thin film stack 101 , with additional aspects of the adhesion layer 1 16 illustrated. In Figure 6, the adhesion layer 1 16 is similar to that shown in Figure 1 B resulting from the process illustrated in Figure 2, including the annealing process 24. Thus, the first layer 10 is a substrate including silicon dioxide, the second layer 14 includes a blend of zinc oxide and silicon dioxide, and third layer 16 includes zinc oxide. After the annealing process 24 of Figure 2, the first metal layer 1 18 (platinum, for example) is deposited on the zinc oxide layer 16, and the piezoelectric layer 120 (PZT in Figure 6) is deposited on the first metal layer 1 18.

[0033] A suitable material for the piezoelectric layer 120 includes, as mentioned, lead zirconium titanate (PZT). In general, with respect to PZT, the general formula can be Pb(Zri_-xTi_x)0₃, where x is from 0.1 to 0.9. However, it is notable that different dopants can be used, such as La, Nb, etc. Thus, other materials for the piezoelectric layer can also be used, including lead lanthanum zirconium titanate (PLZT, or La doped PZT), lead niobium zirconium titanate (PNZT, or Nb doped PZT), and PMN-PT (Pb(Mg,Nb)0₃-PbTi0₃). Lead-free piezoelectric layers may also be used, examples of which include LiNb0₃, BCTZ

[Ba(Tio.₈Zr₀.2)0₃-(Bao.₇Ca₀.₃)Ti0₃], tungsten bronze structured ferroelectrics (TBSF), BNT-BT [(Bi_0.5Nao_.5)TiO₃-BaTiO₃], BT [BaTi0₃], AIN, AIN doped with Sc, and ternary compositions in the BKT-BNT-BZT [(Bi_0.5Ko.5)Ti0₃-(Bio.5Nao.5)Ti0₃~ Bi(Zn_0.5Ti₀.5)O₃] system, a specific example of which includes 0.4(Bi₀.₅Ko.5)TiO₃- 0.5(Bio_.5Na_0.5)Ti0₃-0.1 Bi(Zno.5Tio.5₎0₃). Other suitable piezoelectric materials can be used for the piezoelectric layer, or combinations of materials or multiple layers can likewise be used in accordance with examples of the present disclosure. Referring back to Figure 2, the zinc oxide 12 is deposited on the substrate 10 in block 22, and then annealed in block 24. The annealing process in block 24 results in creating the three-layer (silicon dioxide 10/zinc oxide and silicon dioxide blend 14/zinc oxide 12) composite structure 26 that is more robust to later processing. After creating the composite structure 26 by the annealing process 24, the metal layer 1 18 and piezoelectric layer 120 are deposited in blocks 28 and 30, respectively.

[0034]The example adhesion layer 1 16 illustrated in Figures 5 and 6 promotes uniform mechanical performance and provides acceptable barrier properties to lead and other impurities that may migrate through the adjacent electrode.

Thus, the adhesive layer 116 of the present disclosure provides reliable adhesion between many noble and other metallic materials, including platinum, copper, gold, ruthenium, and iridium. Furthermore, acceptable adhesion to non- metallic materials can also be achieved, making it a good adhesive to use between metallic and non-metallic layers or surfaces. [0035] ln piezoelectric systems in particular, the metal electrodes 1 18, 122 selected for use should be those which can effectively cause appropriate movement of the piezoelectric materials, such as those used in the piezoelectric layer 120. This is particularly true with respect to the metal electrode 1 18, which is in direct contact with the adhesive layer 1 6. As PZT contains lead, and lead cations are migratory though other metals under the proper conditions, there can be problems associated with lead migrating into and through the metal electrode, e.g., lead migrates fairly readily through platinum when a titanium oxide adhesive layer is applied to the opposite side of the metal electrode. This is believed to occur because after annealing platinum and titanium oxide during the manufacturing process, especially at high temperatures, lead cations will diffuse into or through the platinum (preferably along the Pt grain boundaries) and into the titanium oxide, forming lead titanate (PbTiO₃). The annealed adhesion layer 16 including the second layer 16 zinc oxide and silicon dioxide blend and the zinc oxide third layer 15 decreases migration of lead cations through the metal layer.

[0036] Figures 7A and 7B are charts illustrating depth profile data from an XPS analysis of a piezoelectric thin film stack. More specifically, the analyzed thin film stack was essentially as illustrated in Figure 6. A 50nm layer of zinc oxide was deposited on an 800nm silicon dioxide substrate and annealed to form a structure such as illustrated in Figure 1 B. A 100nm layer of platinum was deposited on the zinc oxide layer 12, with a 1.1 pm PZT layer on the metal layer. No significant lead diffusivity into the adhesion layer was detected.

[0037] With respect to the various layers described herein, any of a number of deposition methods or techniques can be used. For example, as mentioned, a PZT layer can be grown on or otherwise applied to the surface of a metal in some examples. Deposition techniques that can be used for depositing piezoelectric material or other layers on top of one another include sol-gel deposition, physical vapor deposition (PVD), pulsed laser deposition (PLD), atomic layer deposition (ALD), Metal Organic DVD (MOCVD), etc. Metal can be deposited, for example, by sputtering or other known deposition methods.

Semi-conductive, non-conductive, or passivation layers can be deposited by plasma enhanced chemical vapor deposition (PECVD), a low pressure chemical vapor deposition (LPCVD), an atmosphere pressure chemical vapor deposition (APCVD), atomic layer deposition (ALD), sputter deposition, evaporation, thermal oxidation, or other known methods. Any combination of these or other methods can be used.

[0038]Although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that a variety of alternate and/or equivalent implementations may be substituted for the specific embodiments shown and described without departing from the scope of the present invention. This application is intended to cover any adaptations or variations of the specific embodiments discussed herein. Therefore, it is intended that this invention be limited only by the claims and the equivalents thereof

Claims

CLAIMS What is Claimed is:

1. A thin film stack, comprising:

a first layer including silicon dioxide;

a second layer directly on the first layer, the second layer including a blend of zinc oxide and silicon dioxide; and

a third layer directly on the second layer, the third layer including zinc oxide.

2. The thin film stack of claim 1 , further comprising a metal layer directly on the third layer.

3. The thin film stack of claim 2, further comprising a piezoelectric layer directly on the metal layer.

4. The thin film stack of claim 2, wherein the meal layer includes platinum.

5. The thin film stack of claim 3, wherein the piezoelectric layer includes PZT.

6. The thin film stack of claim 5, wherein the thin film stack is an actuator for fluid ejection device.

7. A method, comprising:

providing a first layer including silicon dioxide;

depositing zinc oxide on the first layer; and

heating the first layer and the zinc oxide to form a second layer including a

blend of zinc oxide and silicon dioxide and a third layer including zinc oxide, wherein the second layer is between the first and third layers.

8. The method of claim 7, wherein heating the first layer and zinc oxide includes heating to at least 500°C.

9. The method of claim 7, further comprising depositing a metal layer on the third layer.

10. The method of claim 9, wherein the metal layer is deposited on the third layer after heating the first layer and the zinc oxide to form the second layer and the third layer.

11. The method of claim 9, further comprising depositing a piezoelectric layer on the meal layer.