Patent Application
20110023775
1. Field of the Invention
This invention relates to an apparatus for atomic layer deposition of material(s) on a substrate.
2. Description of the Art
Atomic layer deposition (“ALD”) is a thin film deposition technique that offers extremely precise control over the thickness of a layer of a compound material deposited on a substrate. As the name implies, the film growth in ALD is layer by layer, which allows the deposition of extremely thin, conformal coatings that are also free of grain boundaries and pinholes. Deposition of this coating is typically done through the application of two molecular precursors. The surface of the substrate is exposed to a first precursor (“precursor I”) molecule, which reacts chemically with the surface. This reaction is self-limiting and proceeds until there is a uniform monolayer coating of reacted precursor I covering the surface. The surface is then exposed to a second precursor (“precursor II”), which reacts chemically with the surface coated with precursor I to form the desired compound. As before, the reaction is self limiting, and the result is a completed monolayer coating of reacted precursor II covering the surface, and therefore a completed monolayer of the desired compound material.
The process can then be repeated, exposing the surface first to precursor I and then to precursor II, until a coating of the desired thickness has been formed. Since each completed I-II layer has a thickness on the order of 0.1 nm, very thin layers, with a very precisely controlled thickness are possible.
Historically, ALD has been carried out by placing the substrate to be coated in a vacuum chamber and introducing a low pressure carrier gas containing some small percentage of precursor, also in the gas phase. However, because the time to completely purge the precursors from the deposition chamber can be long, ALD has typically been regarded as a slow process.
An alternative form of ALD coating head is known that allows deposition at much higher rates. In this head arrangement the precursor gases (again, precursor molecules in an inert carrier gas) are delivered by long narrow channels, and these channels alternate with vacuum uptake channels and purge gas channels. The head is then traversed across the substrate to be coated in a direction perpendicular to the long axis of the output channels (or alternatively held in one position while the substrate is translated underneath it). U.S. Published Patent Application 2008/166,880 (Levy) is representative of the structure of such a head.
The head disclosed in this referenced published application requires that the separation between the head and the substrate be very small (˜ thirty microns) and very closely controlled. In fact, jets of gas emanating from the face of the device are used as a means to float the coating head, in a manner analogous to a hovercraft, over the substrate to be coated.
It view of the foregoing it is believed to be advantageous to provide an apparatus for ALD coating of a substrate that is not sensitive to the precise distance between the coating head and the substrate, but is, instead, independent of the separation between the head and the substrate and tolerant of dimensional variations in that separation. In that way, no extraordinary measures would be needed to keep this separation distance fixed. In particular, it is believed to be advantageous not to require the gases exiting the head to do double duty: i.e., the gases are not be required to serve the function of maintaining a fixed separation at the cost of compromising the main function of the device, the deposition of an ALD coating.
The present invention is directed to an apparatus for atomic layer deposition of a material on a moving substrate comprising a conveying arrangement for moving a substrate along a predetermined path of travel through the apparatus and a coating bar having at least one precursor delivery channel. The precursor delivery channel is able to conduct a fluid containing a material to be deposited on a substrate toward the path of travel. When in use, a substrate movable along the path of travel defines a gap between the outlet end of the precursor delivery channel and the substrate. The gap defines an impedance Zg to a flow of fluid from the precursor delivery channel.
The apparatus further comprises a flow restrictor disposed within the precursor delivery channel. The flow restrictor presents a predetermined impedance Zfc to the flow in the precursor delivery channel. The restrictor is sized such that the impedance Zfc is at least five (5) times, and more preferably at least fifteen (15) times, the impedance Zg.
The impedance Zfc has a friction factor f. The restrictor in the precursor delivery channel is sized such that the impedance Zfc has a friction factor f that is less than 100, and preferably less than 10.
The coating bar also has first and second inert gas delivery channels respectively disposed on the upstream and downstream sides of the precursor delivery channel.
The outlet end of each inert gas delivery channel also defines a gap between the end of each inert gas flow delivery channel and the substrate. Each gap defines an impedance Z′g to a flow of fluid from each respective inert gas delivery channel. A flow restrictor is disposed within each inert gas delivery channel. Each flow restrictor presents a predetermined impedance Z′fc to the flow in the respective inert gas delivery channel. Each restrictor within each inert gas delivery channel is sized such that the impedance Z′fc is at least five (5) times, and more preferably at least fifteen (15) times, the impedance Z′g. The impedance Z′fc has a friction factor f′. The restrictor in the inert gas delivery channel is sized such that the impedance Z′fc has a friction factor f′ that is less than 100, and preferably less than 10.
The path of travel of the substrate through the apparatus could be a planar or a curved path of travel.
The invention will be more fully understood from the following detailed description taken in connection with the accompanying Figures, which form a part of this application and in which:
Throughout the following detailed description similar reference numerals refer to similar elements in all figures of the drawings.
A conveying arrangement 14 is provided within the enclosure 12 for moving the substrate S along a predetermined path of travel 16 through the apparatus 10. In the arrangement illustrated in
The apparatus 10 incorporates at least one coating bar 20 in accordance with the present invention.
As illustrated in
There is an advantageous efficiency in combining multiple deposition modules (e.g., modules 21, 21″,
Structurally, the precursor deposition module 21 within the bar 20 can be constructed in any convenient manner. For example, in the embodiments depicted in this application the precursor deposition module 21 is formed as a layered stack of structural plates 22 bolted between end members 24A, 24B. As will be discussed in more detail each of the plates 22 is configured such that when the sandwich is assembled the space between adjacent plates 22 defines the various internal channels to be explained herein. In addition, the plates have appropriately positioned openings that cooperate to define the necessary supply headers and fluid transport passages within the bar 20.
In its most basic form a precursor deposition module 21 able to deposit a single precursor on a substrate is configured to include a precursor delivery channel 28, a pair of exhaust channels 32, and a pair of inert gas channels 36. Flow arrows depict the direction of the fluid flow in each of the channels to be described. The precursor delivery channel 28, each of the exhaust channels 32, and the inert gas channels all have a predetermined width dimension (measured in the X-direction) on the order of 0.5 to two (2) millimeters, and typically approximately one (1) millimeter.
The precursor delivery channel 28 has an inlet end 28I and an outlet end 28E. As shown by the flow arrows the precursor delivery channel 28 conducts a flow of a fluid containing a precursor material (“I”) supplied at the inlet end 28I of the channel 28 toward the outlet end 28E thereof. The inlet end 28I of the precursor delivery channel 28 is connected to a supply fitting indicated by the reference character 28F. Precursor material carried in the flow exiting from the outlet end 28E of the channel 28 is deposited on the substrate S as the substrate S moves beneath the bar.
An upstream exhaust channel 32U and a downstream exhaust (or “uptake”) channel 32D respectively flank the precursor delivery channel 28 on both its upstream and downstream sides. As generally used herein the terms “upstream” and “downstream” are defined relative to the direction 16 of the substrate S along its path of travel through the apparatus 10 and respectively correspond to negative and positive directions along the reference X-axis. Each exhaust channel 32U, 32D has a collection end 32C and an exhaust end 32E. The collection end 32C of each exhaust channel is adjacent to the path of travel of the substrate S. The exhaust end of each of the exhaust channels 32U, 32D is connected to a common exhaust fitting diagrammatically indicated by the reference character 32F.
The coating bar 20 further includes upstream and downstream inert gas delivery (or “purge”) channels 36U, 36D, respectively. As illustrated, the purge channel 36U is deployed immediately upstream of the upstream exhaust channel 32U, while the purge channel 36D is deployed immediately downstream of the downstream exhaust channel 28D. Each purge channel 36U, 36D serves to deliver an inert fluid, such as nitrogen gas, from a supply end 36S to a discharge end 36H located adjacent to the path of travel of the substrate S. The supply end 36S of each purge channel 36 is connected to a common supply fitting diagrammatically indicated by the reference character 36F.
The outlet end 28E of the precursor delivery channel 28, the collection end 32C of each respective exhaust channel 32U, 32D, as well as the discharge end 36H of each respective purge channel 36U, 36D, all have a transverse dimension (extending the positive Z-direction) that encompasses the entire transverse dimension of the substrate S.
In operation, a gas containing a precursor material (material “I”) is supplied via the fitting 28F to the precursor delivery channel 28. The precursor material is conducted through the precursor delivery channel 28 toward the outlet end 28E thereof. At the outlet end 28E the flow of precursor gas exits the precursor delivery channel 28 and is drawn into a gap 42 defined between the edges of the plates 22 forming the delivery channel 28 and the substrate S. The gap 42 defines an impedance Zg to a flow of fluid from the precursor delivery channel. The magnitude of the impedance Zg is directly controlled by the size of the gap 42.
Simultaneously, a flow of inert gas is introduced via the supply fitting 36F into each of the purge channels 36U, 36D. Each of these flows is conducted toward the respective discharge end 36D of these channels. The inert gas flows are similarly drawn into gaps 43 defined between the edge of the plates 22 forming these channels and the substrate S. These gaps 43 similarly define an impedance Z′g to a flow of fluid from the inert gas delivery channels. The size of the gap 43 directly controls the magnitude of the impedance Z′g.
The precursor gas flow as well as the inert gas flows are drawn toward and collected by the collection ends 32C of the exhaust channels 32U, 32D. As the precursor flow squeezes through the gap 42 a layer of precursor “I” material is deposited on the substrate S.
As noted in connection with the discussion of a known ALD coating head as set forth in the Background portion of the application, the dimension of the gaps between the coating head and the substrate S must be rigorously controlled to insure that these dimensions remain relatively constant, since small changes in the dimension of a gap results in large changes in the flow. However, a coating bar in accordance with the present invention is able to maintain a substantially steady (i.e., variable but within tolerable process limits) flow of precursor material toward the substrate even if the dimension of the gap(s) 42 and/or 43 change(s).
The elimination of dependence on the gap dimension in accordance with the present invention is obtained by disposing a flow restrictor 22R in the precursor delivery channel 28 as well as in each of the inert gas delivery channels 36U, 36D. The presence of the flow restrictor 22R narrows each of these channels and creates a restriction to the flow of gas therethrough.
The restriction in the precursor delivery channel 28 caused by the restrictor 22R presents a predetermined impedance Zfc to the flow therethrough. In accordance with the present invention the restrictor is sized such that the impedance Zfc is at least five (5) times the impedance Zg. More preferably, the impedance Zfc is at least fifteen (15) times the impedance Zg.
Similarly, the presence of a restriction 48 in each of the inert gas delivery channels 36 presents a predetermined impedance Z′fc to the flow through these channels. The restrictor in each of these channels 36 should also be sized such that the impedance Z′fc is at least five (5) times, and more preferably at least fifteen (15) times, the impedance Z′g.
By appropriately sizing the restriction to exhibit the defined relationship between the flow impedance in the channel with respect to the impedance in the gap at the outlet of the channel the delivery of the precursor and the purge gas, as the case may be, is made independent of the gap impedance made tolerant of variations in the dimension of the gap(s) 42 and/or 43 and therefore substantially of the gap impedances Zg and/or Z′g.
The various impedances Zg, Z′g, Zfc and Z′fc relate the volumetric flow Q through the gap or channel (as the case may be) to the pressure drop ΔP along the path of the fluid according to
Flow impedance is discussed in S. Dushman, The Scientific Foundations of Vacuum Technique, 2 Ed., John Wiley & Sons, New York, 1962.
The impedances Zfc and/or Z′fc relate can also have friction factors f and f′, respectively. Such friction factors f, f′ relate the shear stress at the wall of a restriction τw to the kinetic energy K of the moving fluid according to
τw=fK (2).
The friction factor is discussed in F. A. Holland and R Bragg, Fluid Flow for Chemical Engineers, Elsevier, Amsterdam, 1995.
In accordance with the present invention the impedance Zfc in the precursor delivery channel has a friction factor less than 100, and more preferably less than 10. In addition, in accordance with the invention the impedance Z′fc in each inert gas delivery channel has a friction factor less than 100, and more preferably less than 10.
The flow restrictor 22R may take any convenient form. In the arrangement illustrated the flow restrictor takes the form of a rectanguloid projection that extends transversely across either one (or both) of the plates defining the particular delivery channel. In the preferred case the restrictor defines a flow restriction that extends the full transverse (Z direction of the bar). Preferably the flow restrictor should include a transition surface 22C at the end of the restriction to minimize the formation of eddies in gas flow through the channel. The transition surface 22C may be planar, as illustrated. However, the shape of the surface may be otherwise configured.
As noted earlier, a coating bar may contain multiple precursor deposition modules 21.
As seen from
As seen in
As shown from the exploded view of
As is apparent from
Appropriate ones of the through openings 22G, 23G in the plates 22 and gaskets 23 respectively register with each other to define supply headers that extend appropriate predetermined distances into the bar 20′. The supply headers communicate with fittings provided on one of the ends bars 24A, 24B.
The relieved supply slot region 22S and enlarged flow region 22F in one surface of each plate confronts the opposed surface of the adjacent plate to define the various delivery and exhaust channels present in the bar 20′. The furrow 22U in each plate connects the supply slot in that plate to the appropriate passage formed in the bar. The presence of a gasket 23 intermediate between adjacent plates 22 serves to space the restrictor bar 22R on the surface of one plate away from the opposite surface of the adjacent plate, thereby defining the restriction in each channel. The impedances and friction factor of a restrictor 22R may be determined from a measurement of both the pressure drop across the restriction and the mass flow through it, the equipment and methods necessary for such a measurement being well known. The value of the impedance of a restrictor 22R may be adjusted by changing the thickness of the associated gasket 23.
Since the delivery of precursor material is independent of the gap dimension, the path of travel of the substrate the can be curved.
If the radius of curvature of the curved path is sufficiently large, and/or if the individual coating bars are sufficiently narrow, the output face of the coating bar whereby the precursor and purge gases emerge need not be shaped to match the curve. If, however, such is not the case, the individual plates 22 may be shaped such that the gaps 42 and 43 remain constant across the output face of the bar without adversely impacting the performance of the apparatus.
The operation of an atomic layer deposition apparatus having a coating bar in accordance with the present invention may be understood more clearly from the following Examples.
A coating bar capable of depositing a single precursor layer according to the embodiment of
This model included a single precursor delivery channel (28) flanked by a pair of exhaust channels (32U, 32D). The exhaust channels were flanked by a pair of inert gas channels (36U, 36D). The gap (42) was defined between a flat substrate S and the end of the precursor delivery channel. Finally, the module was flanked with a pair of wider regions (50,
The vertically disposed fluid delivery and uptake channels had a width w=1 mm, except in the region of the flow restrictions. The channels were separated by solid plates of thickness t=1 mm. The substrate surface, designated E, was disposed a distance g=0.1 mm below the output face of the bar.
The open volume in the model was considered to be filled with a fluid having the properties of nitrogen gas at a temperature of 373 K. This gas was considered to be an Incompressible Navier-Stokes fluid and, in the bulk, to obey the equations:
ρ({right arrow over (u)}·{right arrow over (∇)}){right arrow over (u)}={right arrow over (Δ)}·[pI+μ({right arrow over (∇)}{right arrow over (u)}+({right arrow over (∇)}{right arrow over (u)})T)], (E1.1a)
{right arrow over (∇)}·{right arrow over (u)}=0, (E1.1b)
where ρ is the fluid density, {right arrow over (u)} is the fluid velocity, and μ is the fluid viscosity. I is the identity tensor. In order to solve any system of equations within a defined region, the behavior on the boundaries that define the region must be specified. In
Along these boundaries, the conditions on the fluid entering or leaving the modeled region were given by
μ({right arrow over (∇)}{right arrow over (u)}+({right arrow over (∇)}{right arrow over (u)})T){circumflex over (n)}=0, (E1.2a)
p=constant, (E1.2b)
where {circumflex over (n)} is an inward pointing unit vector normal to the boundary. Along each indicated boundary, the pressure was held constant at the value given in Table E1.1.
All of the remaining boundaries with the exception of E (i.e. all the unlabeled boundaries), represented physical walls where the well known “no slip” boundary condition, {right arrow over (u)}=0, was applied. The last boundary, indicated as E, represented the substrate. Here also the no slip condition was applied: the velocity of the fluid with respect to the substrate was taken as zero at the line of contact, but the substrate itself was in motion with speed v0 directed in the positive x direction, so that the calculated fluid flow would be correct for a coating bar in close proximity to a moving substrate.
Transport of the precursor within the fluid was calculated according to the convection and diffusion equation,
where c is the molar concentration of precursor dispersed in the inert carrier gas, and the fluid velocity is given by the solution to Eq. E1.1, with the boundary conditions discussed. D12 is the diffusion coefficient for the precursor in the carrier gas. This quantity was taken as
with a value for D*, calculated according to J. C. Slattery and R. B. Bird (A. I. Ch. E. Journal vol. 4, p. 137, 1958) for a trimethylaluminum precursor in a nitrogen carrier gas, of 1.2 Pa-m2/s.
The boundary condition for Equation E1.3 along all unlabeled boundaries in
{circumflex over (n)}·(−D12{right arrow over (∇)}c+c{right arrow over (u)})=0, (E1.5)
which specifies that no precursor may be carried through these boundaries. The condition along boundary A was taken as
{circumflex over (n)}·(−D12{right arrow over (∇)}c+c{right arrow over (u)})=c0uy(x), (E1.6)
representing an inward flux of precursor at a concentration c0 of 1 molar %. Along B, C, and D, the boundary condition was taken as
{circumflex over (n)}·(−D12{right arrow over (∇)}c)=0. (E1.7)
This so called convective flux condition allowed precursor to be carried in or out across the boundary as the local values for the concentration and the fluid velocity indicated. Finally, along E, the boundary condition was taken as
{circumflex over (n)}·(−D12{right arrow over (∇)}c+c{right arrow over (u)})=−ksσc(θ0−cs), (E1.8)
where cs is the surface concentration (mol/m2) of precursor already chemically bound to the substrate, θ0 is the surface concentration of a completed monolayer of precursor, σ is the probability that a precursor molecule striking the surface will react and bind rather than deadsorbing, and ks is the surface rate constant.
The rate constant was calculated from elementary kinetic theory (F. Reif, Fundamentals of Statistical and Thermal Physics, McGraw-Hill, New York, 1965) to be ks=2.27×106 m3 mol−1 s−1. The sticking probability was taken as (C. Soto and W. T. Tysoe, J. Vac. Sci. Technol. A, vol. 9, p. 2686, 1991) σ=0.01, and θ0 was calculated from the known density of ALD deposited Al2O3 films (Groner et al., Chem. Mater. vol. 16, p. 639, 2004) to be 2.66×10−5 mol/m2. Eq. E1.7 therefore gives a flux of precursor leaving the gas phase to deposit on to the substrate.
On the substrate surface, the concentration of deposited precursor was given by the solution to the equation
with the point boundary conditions
at x=0 (the left-hand end of boundary E in
at x=15 mm (the right-and end of boundary E in
For the purposes of computational efficiency, the system of equations was solved in a two step process. First the Navier-Stokes component only was solved as a stationary problem, then the full coupled system of equations was solved as a transient problem. The initial conditions for the fluid flow in the transient problem were taken from the solution to the stationary problem. The initial condition for the convection-diffusion component was c=0 everywhere. The initial condition for the deposited precursor was cs=0 all along boundary E.
Example 2 The coating bar of Example 1 was analyzed in the case that the substrate was disposed a distance g=0.2 mm below the output face of the bar. All other particulars of the analysis remain the same as in Example 1.
Taken together these examples show that the deposition as performed by an apparatus having the impedances having the relations as defined herein and friction factors in the ranges as defined herein is insensitive to moderate variations in the separation between the coating bar and the substrate. The variation in g from Example 1 to Example 2 is of a size that might reasonably be expected in a mechanical apparatus containing moving or translating parts. For example, if the substrate was held to a rotating drum, as in
Those skilled in the art, having the benefit of the teachings of the present invention as hereinabove set forth may effect numerous modifications thereto. Such modifications are to be construed as lying within the contemplation of the present invention as defined by the appended claims.
This application claims priority of the following United States Provisional Application, hereby incorporated by reference: Apparatus For Atomic Layer Deposition, Ser. No. 61/230,336, filed Jul. 31, 2009.
| Number | Date | Country | |
|---|---|---|---|
| 61230336 | Jul 2009 | US |
