Method of electrophoretic deposition of ferroelectric films...

Details Method of electrophoretic deposition of ferroelectric films... Method of electrophoretic deposition of ferroelectric films...

2000-12-11

2002-11-12

Sherry, Michael J. (Department: 2829)

Semiconductor device manufacturing: process

Coating of substrate containing semiconductor region or of...

Insulative material deposited upon semiconductive substrate

Reexamination Certificate

active

06479406

ABSTRACT:

FIELD AND BACKGROUND OF THE INVENTION
The present invention relates to methods of manufacturing piezoelectric products used in the fields of electronics and electro-optics, by deposition of ferroelectric films and, more particularly, to an improved method for electrophoretic deposition (EPD) of ferroelectric films by using a tri-functional additive in EPD suspensions, compositions of suspensions for effecting same, and ferroelectric films formed therefrom.
Ferroelectric materials are used in many applications in electronics and electro-optics. Due to growing demand for miniaturization of electronic products, particularly for micro-electro-mechanical (MEM) devices or systems, there is a complementary growing need for miniaturized piezoelectric elements or piezoelectric films, of thickness on the order of less than 100 microns. Production of thin and thick piezoelectric films, where, in general, thin and thick films are of thickness of less than about 2 microns and greater than about 2 microns, respectively, started in the 1980's. Piezoelectric films are applied in the production of high dot per inch (DPI) ink jet printing nozzle arrays for printing heads, and for functional devices possessing specific electrical, mechanical, and piezoelectric properties or characteristics, such as solid state devices including actuators, micro-machinery, ultrasound transducers, pyroelectric sensors, electro-optic displays, ferroelectric field-effect transistors, high value capacitors, non-volatile memory field-effect transistors, infrared sensors, and optical switches.
It will be appreciated that application of piezoelectric films is an important basis for next century's electronic and electro-optic devices.
Several methods are currently used for manufacturing piezoelectric films and elements by deposition, varying greatly both with respect to methodology and with respect to obtained results. These methods include physical vapor deposition (PVD) such as sputtering, laser ablation and evaporation or molecular beam epitaxy (MBE), chemical vapor deposition (CVD) methods such as metal-organic chemical vapor deposition (MOCVD) and plasma enhanced chemical vapor deposition (PECVD), and wet chemistry methods such as sol-gel processing, electrophoretic deposition, liquid phase epitaxy, tape casting, and slip casting.
Each of these methods has advantages and disadvantages. PVD and CVD deposition methods ordinarily require sophisticated and expensive equipment, making them either undesirable or impracticable for implementation in manufacturing environments. PVD methods involving vacuum techniques have the advantage of being based on dry processes featured by high levels of system purity and cleanliness, translating to relatively high compatibility with fabrication of semiconductor integrated circuits. However, sputtering and MOCVD deposition methods are limited by difficulties in controlling component stoichiometry, especially in multi-component systems, and deposition rates are low. In these methods, there is also a need for post deposition annealing due to occurrence of internal stresses. The technique of laser ablation may be quick for producing films of usable quality, but yields poor thickness uniformity for deposition areas larger than about 1 cm
2
, and is limited by particulate ejection from the target, thereby causing defects in the deposited ferroelectric film.
Simpler and less expensive ferroelectric deposition methods include sol-gel processing, tape casting, slip casting, and electrophoretic deposition (EPD). Sol-gel processing techniques combine the advantages of high compositional control, thin coating capability, and low equipment costs, but sol-gel processing techniques are limited by the occurrence of large volume changes during the deposition process. Sol-gel techniques are useful for thin film production, but are relatively slow for producing thick films having thickness larger than about 1 micron because multiple deposition is required. Tape casting and slip casting are part of wet methods of film deposition, and are used for conventional production of piezoelectric elements of thicknesses of tens to several hundreds of microns, and are not thin film technologies.
Electrophoretic deposition (EPD), a relatively new ferroelectric materials processing technique, in which charged ferroelectric particles, typically submicron in size, dispersed or suspended in a liquid medium are attracted and deposited onto a substrate acting as an electrode of opposite charge, during application of an electric field through the liquid, has been successfully applied to the production of thin, less than 2 microns, and thick, greater than 2 microns, ferroelectric films and elements for the manufacture of piezoelectric devices. Commonly used ferroelectric materials are of the perovskite type, such as lead-zirconate-titanate, PbZrTiO
3
(PZT), lead titanate, PbTiO
3
(PT), and barium titanate, BaTiO
3
(BT).
For better understanding and appreciation of the present invention, an EPD process may be considered to involve two principle stages. The first stage involves electrophoretic movement or migration of the ferroelectric particles through an EPD suspension, and the second stage involves deposition of the ferroelectric particles onto an electrode or treated substrate. Hereinafter, the term ‘EPD suspension’ or ‘electrophoretic suspension’ refers to the combination or suspension of an ‘EPD liquid medium’, and the ferroelectric particles, with or without addition of one or more liquid or solid phase additives and/or binders. Furthermore, hereinafter, an ‘EPD liquid medium’ refers to a pure solvent, or combination of two or more pure solvents, without addition of any liquid or solid phase additive and/or binder. Ferroelectric particles may be added to an EPD liquid medium either before or after addition of one or more liquid or solid phase additives and/or binders.
Effective electrophoretic deposition of ferroelectric materials is strongly dependent upon using a suitable EPD liquid medium which exhibits three important characteristics of rapidly and uniformly (i) dispersing the ferroelectric particles while preventing particle sedimentation and agglomeration, prior to and during the migration stage of the EPD process (ii) charging the ferroelectric particles, prior to and during the migration stage of the EPD process, and (iii) binding of the ferroelectric particles to the selected substrate and to each other, during the deposition stage of the EPD process. Exhibiting all three characteristics is critical for enabling the manufacture of piezoelectric elements or devices having suitable electrical and piezoelectric properties such as high dielectric constant, and other physico-chemical properties such as uniform film thickness, and high film strength.
Commonly used EPD liquid media include pure, or mixtures of two or more miscible, polar organic solvents such as alcohols, ketones, aldehydes, providing strong ionization effects to the dispersed particles. Typically, ferroelectric particles are initially dispersed in a selected pure solvent, and if it is determined that there is insufficient dispersion, or particle charging, mixtures of varying concentration ratios of two or more solvents are then used for improved particle dispersion and/or charging in the EPD liquid medium. Concentration ratios of two solvents as EPD liquid media, for example, are usually in the range of about 90/10 to 10/90, volume/volume.
An example of electrophoretic deposition of PZT films is described in U.S. Pat. No. 5,462,647, issued to Bhattacharya et al., wherein PZT is dispersed in an aqueous dimethylsulfoxide (DMSO) solution. Electrophoretic deposition of PT and PZT films is described in Japanese Patent Application No. 127439, of Apr. 28, 1995, wherein either a PZT or a PT powder is dispersed in pure acetone.
In both teachings, a main objective is recovery of deposited PZT or PT in powdered form. Extensive elemental analysis was performed for characterizing composition of the recovered PZT or PT powders, but minimal attention

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