Chemistry: electrical and wave energy – Processes and products – Coating – forming or etching by sputtering
Reexamination Certificate
2002-07-22
2004-04-20
VerSteeg, Steven (Department: 1753)
Chemistry: electrical and wave energy
Processes and products
Coating, forming or etching by sputtering
C204S298060, C204S298080, C204S298230, C204S298280
Reexamination Certificate
active
06723209
ABSTRACT:
FIELD OF INVENTION
The present invention is directed generally to novel systems and methods for performing thin film deposition or chemical treatment of substrates, and to optical devices manufactured using such systems and methods.
BACKGROUND OF THE INVENTION
U.S. Pat. No. 6,402,904 (the '904 patent) and U.S. patent application Ser. No. 09/810,688 filed Mar. 16, 2001, entitled “System and Method for Performing Sputter Deposition Using Ion Sources, Targets and a Substrate Arranged About the Faces of a Cube” (also incorporated herein by reference) (the '688 application) disclose systems and methods for ion sputter deposition of thin films on a substrate which involve directing ions from at least one plasma ion source generally towards at least one sputter target, each target having its own associated ion source. In operation, negative voltages applied to the target(s) attract ions from the plasma and accelerate the ions toward the target to sufficient kinetic energies (50 to 5000 eV) to cause sputtering of the target. Electron sources may also be provided. For reasons of avoiding charge build-up on insulating targets, the negative voltages may be pulsed or even alternately pulsed with positive pulses which serve to attract electrons to the target to neutralize it.
The '904 patent discloses yet another ion source directed generally at the substrate, and its main purpose is to bombard the growing film on the substrate with ion species chemically reactive with the sputter-deposited atoms from the target(s) to form compound thin films. For example O
2
+
, N
2
+
, H
2
+
and other ions collide with the growing film surface, dissociate and chemically react with the depositing atom flux sputtered from targets composed of pure Si, Al, Ti, Ta and others. In their respective combinations, compound thin films such as SiO
2
, Si
3
N
4
, Si
x
H
1−x
, Al
2
O
3
, AlN, TiO
2
, TiN, Ta
2
O
5
, TaN and many others may be formed on the substrate. In the prior art, such an added ion source directed at the substrate enables so-called ion-assisted deposition (IAD).
IAD has many benefits. IAD supplies highly reactive O, N, H and other atomic species via surface-collisional dissociation of molecular ions, while the parent molecular species would not necessarily react (e.g., N
2
). This makes it easy to attain complete stoichiometry of various oxides, nitrides, hydrides and other compounds. IAD may also affect film growth by momentum-transfer physics, even with inert gas ions. IAD adds several controllable parameters to the process of thin film deposition. These are IAD ion species (and mixtures thereof), ion mass, ion kinetic energy, ion current density and ion incidence angle. Singly and in combination, variations in these parameters generally have a strong effect on film density, degree of crystalline versus amorphous character of the film, intrinsic film stress, refractive index, crystalline texture (grain orientation), crystalline grain size, grain boundary morphology, surface flatness/roughness and others. In addition, IAD may have the effect of pre-cleaning the substrate surface, removing “native oxides”, removing loosely-bound atoms, intermixing the substrate and film atoms and other effects, which collectively may improve film adhesion, environmental degradation rate, nucleation/seeding of desired thin film morphologies and other properties.
It would be desirable to retain as many benefits of IAD as possible while eliminating certain drawbacks. Many practitioners of thin film deposition art consider IAD too violent and damaging when applied to highly sensitive, damage-prone substrates. Damage-prone substrates include semiconductor laser diode emission facets, metal layers adjacent to electron tunnel-barrier in magnetic tunnel junctions, electron tunnel-barrier layers themselves, advanced transistor conduction channels when a gate dielectric is to be deposited on them, advanced transistor gate dielectrics themselves and advanced transistor gate metal contacts. Generally, the critical zones of these devices are on the order of 0.5 to 5 nm (5 to 50 Å) in depth and at the deposition surface.
With respect to these devices, practitioners typically fear two aspects of IAD. These are momentum transfer effects and electric charge effects. Regarding momentum transfer damage to sensitive substrates, practitioners would prefer surface collision energies to be below ~5 eV, to avoid disruption of the substrate lattice, intermixing and/or sputtering. Yet the dissociation energies of the reactive IAD ions, O
2
+
, N
2
+
, H
2
+
, etc. lies between 5 and 10 eV, and the kinetic energy of the incident ions must be at least approximately equal to these values or there is not enough energy to dissociate the chemical bond. At these low energies, the dissociation fraction of these species is <10%, and, in practice, to achieve reasonably high dissociation efficiencies, the collision energy must be 5 to 10 times higher. In this higher (25 eV to 100 eV per ion) kinetic energy range, it is justifiably expected that significant momentum-transfer damage may occur, either by lattice disruption, intermixing or sputtering.
Regarding electric charge damage to sensitive substrates, the arriving ions may build up a macroscopic positive charge on the surface of an insulating substrate, which can lead to dielectric breakdown with attendant local heating or lattice disruption. In addition, these typical IAD ions, including non-reactive Ar
+
assist ions, induce a phenomenon called Auger neutralization microscopically at the surface at the location each ion collides with the surface. In Auger processes, the ion abstracts an electron from the atoms at the surface with the result that an amount of energy equal to the ionization potential of the ion (or the molecule which was ionized to form the ion) minus the binding energy of the electron abstracted from the surface atom must be dissipated. Most of this energy is dissipated to the surface atoms, potentially causing lattice disruption and/or local heating. Both types of electric charge damage are avoided if ions are simply not used.
A number of authors report the fact that reactive neutral atoms, radicals and molecular fragments may be produced in the gas phase (within a plasma) by various processes (electron impact excitation being the main one), and that these atoms, radicals and molecular fragments may find their way to the surface of the growing film and participate in film growth. Reactive neutrals in the plasma are either formed at low translational kinetic energy or become thermalized to low translational kinetic energy due to collisions with the background gas. Because of these same processes, all directionality of the reactive neutrals towards the substrate and the growing film is likewise absent or lost. Therefore the concept of using these reactive neutrals produced in the gas phase in place of IAD ions fails. H. F. Winters [“Elementary processes at solid surfaces immersed in low pressure plasmas,” In: Topics in Current Chemistry, Vol. 94, p. 69, M. J. S. Dewar, et al (eds.), Berlin, Springer-Verlag, 1980] states in Sect. 2.2.6.2 (p. 106 ff) that, when a molecular ion collides with a solid surface, it is usually dissociated into its various constituent atoms and that some of these atoms (or radicals) are reflected away from the target. He further speculates that these reflected atoms may be incorporated into growing films. P. Martin et al [“Optical properties and stress of ion-assisted aluminum nitride thin films”, Applied Optics 31(31) p.6734 (1992)], plus others they cite, speculate that energetic back-reflected neutrals, including N atoms, from a sputtering target may contribute to some anomalous film stress results they obtained. The research group of K. W. Hipps [e.g., L. Huang, X.-D. Wang, K. W. Hipps, U. Mazur, R. Heffron and J. T. Dickinson, “Chemical etching of ion beam deposited AlN and AlN:H,” Thin Solid Films 279 p.43 (1996)] deposited aluminum nitride and silicon ni
Baldwin David Alan
Hylton Todd Lanier
4-Wave, Inc.
VerSteeg Steven
LandOfFree
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