Patent Application
20200224312
Embodiments of the present invention generally relate to thin film deposition and, more specifically, to deposition techniques for depositing thin films of lead titanate (PTO), lead zirconate (PZO) or lead zirconate-titanate (PZT).
Lead zirconate-titanate (PZT) films crystallized in the perovskite phase near the morphotropic phase boundary exhibit a very high dielectric constant and extremely large piezoelectric coefficients. For those reasons among others, PZT is a ubiquitous and technologically useful material. Decades have been spent developing and refining thin film deposition techniques for PZT; however, those techniques have typically yielded planar films (sputtering, pulsed-laser deposition, sol-gel) with few exceptions, such as off-angle sputtering, chemical vapor deposition. Atomic layer deposition (ALD) is a technique that can enable film deposition into micro-machined or self-assembled trenches and pores with very high aspect-ratios exceeding 50:1. A reliable ALD process to deposit high-quality PZT into micro-machined or self-assembled trenches and pores is desired to overcome the boundary posed by planar film deposition techniques.
Several attempts have been made to develop a reliable process to deposit high-quality PTO and PZT by ALD over the past 12 to 15 years. However, none of these techniques have shown to produce repeatable commercially viable results having preferred electrical characteristics.
Therefore, there is a need in the art for improved techniques to deposit thin films of PTO, PZO, and PZT on non-planar surfaces.
A method of depositing a thin film of lead titinate (PTO), lead zirconate (PZO) or lead zirconate titanate (PZT) comprising depositing a PTO, PZO or PZT layer upon a substrate whereby growth occurs primarily due to self-limited surface chemisorption of pulsed chemical vapor, and annealing the PTO, PZO, or PZT layer and substrate.
So that the manner in which the above recited features of the present invention can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments.
Table 1 lists ferroelectric properties of a PTO layer produced using an embodiment of the present invention.
Table 2 lists ferroelectric properties of a PZT layer produced using an embodiment of the present invention.
Embodiments of the invention utilize a commercially viable process comprising a specific combination of precursors (in particular, Pb(DMAMP)2 and amide 4+ cation precursors), a particular process temperature window, a particular precursor pulse-sequence, and a particular post-processing which yields a technologically useful phase of lead titanate (PTO), lead zirconate (PZO), and lead zirconate titanate (PZT). Embodiments of the invention further include methods to control the texture of the deposited films.
Embodiments of the invention include the specific administration of an atomic layer deposition (ALD) process that yields high-quality lead titanate (PTO), lead zirconate (PZO), and lead zirconate titanate (PZT) films following a post-deposition anneal. This process produces PTO, PZO or PZT films, whereby growth of the film occurs primarily due to self-limited surface chemisorption of pulsed chemical vapor. Four chemical precursors are used in the preparation of the film including Pb(DMAMP)2, TDMAT, TDMAZ, and H2O. The precursors Pb(DMAMP)2, TDMAT, and TDMAZ supply the lead, titanium and zirconium cations, respectively, though other precursors could be substituted to reveal other compatible combinations. H2O is the oxidizing species for each metal cation precursor in the deposition sequence, though other oxidizers would be compatible including but not limited to H2O2 and O3. Co-oxidizers may also be used, e.g., a sequence of H2O and O3, or O3 followed by H2O, or applying both oxidizers simultaneously.
The precursors are heated in order to supply sufficient vapor pressure for deposition with the exception of H2O which has sufficient vapor pressure at room temperature. The Pb(DMAMP)2 precursor is nominally heated to 80° C. and would work in the range of 50° C. to 100° C. TDMAT is nominally heated to 85° C. and would work in the range of 0° C. to 90° C., TDMAZ is nominally heated to 75° C. but would work in the range of 40° C. to 85° C. The film is deposited by sequentially pulsing the precursors into an appropriate reactor, and each pulse is separated by a purge step to ensure the reactor has been fully evacuated before the introduction of the subsequent precursor. The purge step typically involves flowing an inert gas, typically nitrogen or argon, through the reactor while simultaneously pumping downstream with an ALD process pump. Depending on the reactor design it may be desirable to only pump on the reactor without flowing inert gas during the purge step.
The PTO, PZO, and PZT films are deposited by repetitive sequencing of ALD cycles corresponding to the constituent oxides of TiOx, ZrOx, and PbOx. With each cycle, material growth occurs primarily due to self-limited surface chemisorption of pulsed chemical vapor. In one embodiment, one ALD cycle is defined as follows: one 0.5 second cation precursor pulse step, followed by a 10 second reactor purge step, followed by one 0.5 second oxidizing precursor pulse step and finishing with one 10 second reactor purge step. The exact duration of each dose/purge step will vary depending on individual reactor dynamics and the precursor used. The precursor dose may be increased either by increasing the dose time above 0.5 seconds, increasing the precursor temperature, or by adding additional pulses of the same precursor in series before the purge step. The use of a 0.5 second precursor pulse and a 10 second purge step is considered an exemplary embodiment of the invention. Other length pulses may be used, for example, the precursor pulse may range from 0.05 seconds to 30 seconds and the purge step could range from 1 seconds to 60 seconds. The specific selection of the pulse and step lengths to use are well within the skill in the art to derive in view of the materials used, the film type to be deposited and the desired thickness of the film.
Although ALD is discussed as one specific way of depositing the PZO, PTO, or PZT film, other forms of conformal deposition such as pulsed chemical vapor deposition may be utilized. The commonality of these types of deposition techniques all utilize pulses of chemical vapor to achieve a desired film thickness.
The chemical vapor pulsing cycles described above are combined to form a super-cycle according to
Sub-cycle 108 is a ZrOx precursor cycle that is repeated Z times (Z being a positive integer) to obtain a required thickness of ZrOx on the substrate. The sub-cycle 108 comprises, as mentioned above, a sequence of steps performed within a chamber comprising: a Zr precursor step, a purge step, an oxidizer step, and a final purge step. Sub-cycle 110 is a PbOx precursor cycle that is repeated 8 times (8 being a positive integer) to obtain a required thickness of PbOx. The sub-cycle 110 comprises, as mentioned above, a sequence of steps performed within a chamber comprising: a Pb precursor step, a purge step, an oxidizer step, and a final purge step. The sub-cycles 108 and 110 together form a super-cycle 114 that is used to yield a nominally 1:1 ratio of Pb and Zr content in a PZO film. The ratio is achieved by adjusting the values of Z and θ.
The PZT super-cycle 102 comprises the PTO and PZO super-cycles 112 and 114. As each sub-cycle is executed to deposit defined amounts of Pb, Ti, and Zr and adjusting the number of cycles performed as PTO and PZO super-cycles, the result is a desired ratio of Ti:Zr (nominally 1:1) in the overall PZT film, though elemental gradients could be engineered by modification of the layering sequence.
In one embodiment of the invention, the substrate temperature during ALD sequence is held at 200° C. However, the process will work within the range to 150° C. to 350° C.
Specifically, at step 1, the optional 5 nm buffer layer 204 of PbOx is deposited using the PbOx sub-cycle 106 of
At step 4 of process 200, the film stack of 202, 204, 206, 208 is annealed to produce a 25 nm thick layer 210 of PTO, PZO, or PZT having a perovskite structure. The anneal step may be performed in a rapid thermal anneal (RTA) oven, a heated substrate chuck (hot chuck) under vacuum or O2 ambient, or other common anneal methods. The anneal process conditions are as follows for both the hot chuck and RTA methods: the maximum temperature range would be 500° C. to 800° C., the anneal time can vary between 0 seconds and 4 hours, and the O2 flow can vary between 0 liters per minute to 100 liters per minute.
Texture control may be obtained by fabricating thin nucleation layers designed to template further growth. The nucleation layers are fabricated by first depositing 0-5 nm of ALD TiOx followed by 0-10 nm ALD PbOx followed by an annealing step. Two examples of annealing techniques would be rapid thermal anneal with an oxygen environment, or a hot chuck contained within a vacuum system with gas and pressure control. Identical to the above, the anneal conditions for the nucleation layers are as follows for both the hot chuck and RTA methods: the maximum temperature range would be 500° C. to 800° C., the anneal time can vary between 0 seconds and 4 hours, and the O2 flow can vary between 0 liters per minute to 100 liters per minute.
In one embodiment of the invention used to produce a PTO thin film, the PTO thin films were deposited by atomic layer deposition (ALD) using a Kurt J. Lesker Company ALD-150LX reactor. Laminar purge flow was constantly supplied using mass flow controller (MFC)-regulated ultrahigh purity (UHP) argon supplied by a cryogenic liquid argon dewar. The purge flow was used to provide a diffusion barrier to prevent deleterious chamber wall deposition and to serve as a carrier gas for the precursors. The process pressure was held at approximately 1.6 Torr. Pb(DMAMP)2 heated to 90° C. and TDMAT heated to 85° C. in stainless steel ampoules were used as the lead and titanium cation precursors, respectively. TDMAT was selected due to its high vapor pressure when heated above 40° C. and high reactivity at substrate temperatures of 250° C. Pb(DMAMP)2 was selected as the lead precursor due to its reasonable vapor pressure and the quality of the electrical properties demonstrated previously. The vapor pressure of the Pb(DMAMP)2 was increased by briefly pulsing argon into the ampoule prior to dosing into the reactor. Demineralized H2O at ambient temperature and ozone were both used as oxidizers. Ozone was supplied via an Absolute Ozone® Nano Ozone Generator. Ozone flow was controlled via MFC to 200 sccm, and the ozone concentration was measured to be approximately 10% by volume. The ozone generator was continuously running during the deposition process, and the ozone was collected in a 1 liter stainless steel reservoir which was evacuated into the reactor using an ALD valve during the ozone dose step. As depicted in
PTO growth was achieved using a combination of binary oxide processes for PbOx and TiOx with the relative number of PbOx:TiOx sub-cycles varied over the range from 1:1 to 4:1. The cycle ratio refers to the relative number of PbOx to TiOx cycles in one super-cycle, which is repeated to achieve the desired thickness. For example, PTO films grown with a 3:1 PbOx:TiOx cycle ratio indicates that the super-cycle consists of three PbOx ALD cycles performed in sequence followed by a single TiOx cycle. The films grown with fractional cycle ratios such as 3:2 and 5:2 are grown with the constituent binary ALD cycles occurring in back-to-back sequence as follows: (PbOx)x3-(TiOx)x2 for 3:2 and (PbOx)x5-(TiOx)x2 for 5:2.
In an exemplary embodiment of the invention, the PTO films were annealed by rapid thermal anneal (RTA) at 700° C. for 1 minute with a 90° C./sec standard ramp rate in O2 atmosphere using an AG Associates 610 system for crystallization prior to electrical characterization. The sample temperature was measured by a thermocouple in contact with the backside of the substrate near the center of the RTA. Each sample selected for electrical characterization received at least one additional deposition layer and anneal to help to avoid electrical shorting due to pinholes. A 50-nm Pt thin film, sputtered at 500° C. to promote adhesion, was used as the top electrode. Capacitors with 4.92×10−4 cm2 area were patterned using photolithography, and the electrode area was defined using UV-stabilized resist and etched via ion milling. The capacitor array was evenly spaced over a 100-mm diameter working area. In other embodiments, additional piece-part samples were annealed for varying times, temperatures, and ramp rates in an Allwin 21 810 RTA using a carrier wafer to evaluate a variety of thermal treatment recipes.
Table 1 summarizes the properties of the PTO film of
Generally atomic layer deposition is a technique designed to improve the areal utilization of substrates. In the context of electronics manufacturing, substrates are typically thin wafers of a structural material, often silicon, sized from 150 to 350 mm in diameter and typically around 1 mm thick. Taking the simple case of square capacitor, nominally 50 microns×50 microns in area and 1 micron thick, there is an obvious limit to the number of capacitors than can fit on a single substrate if they are in the plane of the wafer. However, trenches could be patterned and etched such that the capacitors could be fabricated vertically, 40-50 capacitors could fit on the same area as a single in-plane capacitor, increasing the number of active devices on a single wafer by more than an order of magnitude. A further example that may use the conformal deposition technique described herein includes the three-dimensional MEMS device described in commonly assigned U.S. Pat. No. 8,966,993, granted Mar. 3, 2015, and hereby incorporated herein in its entirety.
In the extreme case, nanotubular structures could be coated using the ALD technique described herein.
The capacitor is not an arbitrary example, as PTO and PZT are both ferroelectric materials that could be incorporated into a 3D capacitor design. In fact, Texas Instruments (TI) utilizes PZT for non-volatile ferroelectric random access memory units with 50 ms read/write speeds, which are essentially capacitors arrayed into a memory architecture. Currently TI uses Metal Organic Chemical Vapor Deposition (MOCVD) to deposit PZT, though MOCVD is primarily a planar technique incapable of coating high aspect-ratio topologies. The areal density could be greatly improved with a robust technique to deposit PZT by ALD. Commercially available FRAM products are typically low in capacity (2-4 MB), which limits the overall integration into consumer electronics.
In other embodiments, 3D deposition of PZT by ALD is needed to improve actuation force, efficiency, and density of MEMS structures as we attempt to modernize technologies such as electromechanical actuators, gyroscopes, and resonators. Embodiments of the invention greatly enhance the signal or actuation force density per unit area of many microelectromechanical systems that employ PZT as the piezoelectric material such as transducers, actuators, benders, or resonators. The high aspect ratio characteristic of this approach could significantly enhance the dynamic range of MEMS fabricated gyroscopes used in inertial measurement units thereby providing a more stable navigation solution for assured position, navigation, and timing (PNT). Additionally, the revolutionary new actuator performance would enable high mobility mm-scale robotics for emergency search and recovery.
Another benefit is that the technique permits the tailoring of film growth by manipulation of surface chemistries. Chemical precursors are used in some cases to promote film nucleation on certain surfaces while preventing nucleation on others. ALD could potentially replace sol-gel, sputtering, pulsed-laser deposition (PLD), and MOCVD as a more efficient method even for planar device architectures by reducing the number of required lithographic masks and etch steps by making use of surface selective deposition.
Embodiments of the invention display superior results in the following exemplary ways: the XRD evidence provided for PTO deposited by ALD incontrovertibly shows the perovskite phase with no deleterious phases, the nominal growth rate is high with an ideal sub-cycle ratio of one lead dose cycle to one titanium dose cycle required for perovskite films, and finally texture control has not been observed in any of the PTO, PZO, or PZT as-deposited obtained by ALD but has been attained with the proposed strategy.
Other technologies that may benefit from conformal layers of PZT include:
While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.
This application claims benefit to U.S. Provisional Patent Application Ser. No. 62/790,489, filed Jan. 10, 2019, entitled “Atomic Layer Deposition and Texture Control of PbTiO3, PbZrO3, and PbZrxTi1-xO3,” which is hereby incorporated herein in its entirety.
The invention described herein may be manufactured, used, and licensed by or for the U.S. Government.
| Number | Date | Country | |
|---|---|---|---|
| 62790489 | Jan 2019 | US |
