Patent Application
20060207503
The present disclosure relates generally to a vaporizer and a method for vaporizing precursor gases for the formation of thin films.
The manufacture or fabrication of semiconductor devices is very complicated and often requires, for example, the careful synchronization and precisely measured delivery of as many as a dozen gases to a process chamber. Various recipes are used in the manufacturing process, and many discrete processing steps can be required for a semiconductor device, such as cleaning, polishing, oxidizing, masking, etching, doping, and metalization. The steps used, their sequence, and the materials involved all contribute to the making of particular devices.
As device sizes continue to shrink below 90 nm, the semiconductor roadmap suggests that atomic layer deposition, or ALD processes will be required for a variety of applications, such as the deposition of barriers for copper interconnects, and the creation of tungsten nucleation layers. In the ALD process, two or more precursor gases flow over a wafer surface in a process chamber maintained under vacuum. The two or more precursor gases flow in an alternating manner, or pulses, so that the gases can react with the sites or functional groups on the wafer surface. When all of the available sites are saturated from one of the precursor gases (e.g., gas A), the reaction stops and a purge gas is used to purge the excess precursor molecules from the process chamber. The process is repeated, as the next precursor gas (i.e., gas B) flows over the wafer surface. A cycle is defined as one pulse of precursor A, purge, one pulse of precursor B, and purge. This sequence is repeated until the final thickness is reached. These sequential, self-limiting surface reactions result in one monolayer of deposited film per cycle.
The pulses of precursor gases into the processing chamber are normally controlled using on/off-type gas valves which are simply opened for a predetermined period of time to deliver a desired amount of precursor gas from a heated holding container into the processing chamber. Alternatively, a gas mass flow controller, which is a self-contained device consisting of a transducer, gas control valve, and control and signal-processing electronics, is used to deliver repeatable gas flow rate in short time intervals.
In many cases the precursor gases are formed by vaporizing liquids and solids. The current standard technique for vaporizing a liquid is to use a liquid mass flow controller with a separate vaporizer device, or a combined liquid mass flow controller and vaporizer, to deliver vaporized precursor gases to the heated holding container. The liquid is first metered from a source container by the liquid mass flow controller and then vaporized by the vaporizer device before being delivered to the heated holding container. Then the on/off-type gas valve or gas mass flow controller is used to deliver a desired amount of precursor gas from the heated holding container into the processing chamber. A disadvantage of this technique, however, is cost. A liquid mass flow controller and vaporizer device can cost thousands of dollars. In addition, many of the ALD precursers have rather demanding heating requirements, are difficult to accurately flow control due to condensation, and are liable to decompose in an undesirable manner prior to use.
ALD Precursors can vary greatly depending on application. New precursors are still being developed and tested for different substrates and deposition film requirements. Three very common precursors are Al(CH3)3 (Al2O3 deposition film), HfCl4 (HfO2deposition film), and ZrCl4 (ZrO2 deposition film). The oxygen precursor for each of these gases is typically H2O, or O2 or O3. Other film types that may be deposited via ALD or CVD techniques include Ni, W, SiO2, Ta2O5, TaN, TiO2, WN, ZnO, ZrO2, WCN, Ru, Ir, Pt, RuTiN, Ti, Mo. ZnS, WNxCy, HfSiO, LaxCayMnO3, CuInS2, In2S3, HfN, TiN, Cu, V2O5, and SiN. It should be noted, however, that the present disclosure is not limited to use with any particular precursor or process.
What is still desired is a new and improved vaporizer and a method for vaporizing precursor materials for the formation of thin films, such as in atomic layer deposition (ALD) processes. Preferably, the new and improved vaporizer and method for vaporizing precursor gases will be relatively simple in design and relatively inexpensive in comparison to existing methods and devices for vaporizing precursor materials. In addition, the new and improved vaporizer and method for vaporizing will preferably provide vapor on demand at the point where the vapor is actually metered, as opposed to creating the vapor and storing the vapor prior to use.
The present disclosure provides a vaporizer including an inlet for receiving liquid and an outlet for delivering gas, a gas valve controlling gas flow to the outlet of the vaporizer, and means for heating the liquid flowing between the liquid inlet and the gas valve. The vaporizer also includes means for increasing the heat transfer rate to liquid flowing between the liquid inlet and the gas valve, and for causing a pressure drop to liquid flowing between the liquid inlet and the gas valve, so that a pressure of the liquid drops below a vapor transition pressure of the liquid upon reaching the gas valve.
According to one aspect of the present disclosure, the means for causing a pressure drop between the fluid inlet and the gas valve and for increasing the heat transfer rate to the liquid flowing between the fluid inlet and the gas valve comprises a porous plug connecting the fluid inlet to the gas valve.
According to another aspect of the present disclosure, the vaporizer is incorporated into a system for delivering pulsed mass flow of precursor gases into semiconductor processing chambers, wherein the system actually measures the amount of material (mass) flowing into the process chamber and delivers highly repeatable and precise quantities of gaseous mass.
Among other aspects and advantages, the present disclosure provides a new and improved vaporizer, and a method for vaporizing precursor gases for the formation of thin films, such as in atomic layer deposition (ALD) processes as well as other chemical vapor deposition (CVD) processes. The new and improved vaporizer and method for vaporizing precursor gases is relatively simple in design and is relatively inexpensive. In addition, the new and improved vaporizer allows liquid to be vaporized on demand, when metered (i.e., only when the gas valve is opened).
Additional aspects and advantages of the present disclosure will become readily apparent to those skilled in this art from the following detailed description, wherein an exemplary embodiment of the present disclosure is shown and described, simply by way of illustration. As will be realized, the present disclosure is capable of other and different embodiments and its several details are capable of modifications in various obvious respects, all without departing from the disclosure. Accordingly, the drawings and description are to be regarded as illustrative in nature, and not as restrictive.
Reference is made to the attached drawings, wherein elements having the same reference characters represent like elements throughout, and wherein:
Referring first to
As shown in
The vapor pressure is the pressure (if the vapor is mixed with other gases, the partial pressure) of a vapor. At any given temperature, for a particular substance, there is a pressure at which the vapor of that substance is in equilibrium with its liquid or solid forms. This is the saturated vapor pressure of that substance at that temperature. The term vapor pressure is often understood to mean saturated vapor pressure. When the pressure of any liquid equals its saturated vapor pressure, the liquid is partially vaporized: liquid and vapor are in equilibrium. Given a constant temperature, if the pressure is reduced, the equilibrium is changed the substance's gas phase: The liquid eventually gets totally vaporized.
According to one exemplary embodiment of the present disclosure, the means for causing a pressure drop and for increasing the heat transfer rate comprises a porous media plug 18. It is important to note that the pressure at the exit of the valve must be below the vapor pressure of the liquid. Otherwise, liquid will exit the porous media and the gas valve may not be able to close. The high heat transfer rate is necessary to ensure constant temperature because the vaporizing liquid creates a heat sink. If the local temperature of the porous media plug 18 drops, the local saturated vapor pressure will drop with it, sometimes below the pressure at the exit. Then there won't be any vaporization in the porous media plug 18.
Suitable porous media is available, for example, from Mott Corporation of Farmington, Conn. (http://www.mottcorp.com). According to one exemplary embodiment, the porous media is made from sintered metal. The sintered metal can be formed from metal powder having a pre-sintered mean particle size of less than 20 microns. According to another embodiment, the mean particle size of the sintered elements is less than 10 microns and the sintered metal has a density of at least 5 g/cc. The metal used to make the plug 18 is selected from, but not limited to, a group consisting of stainless steel, nickel and nickel alloys, and titanium, to meet special requirements, such as greater temperature and corrosion resistance. In particular, the metals and alloys include, but are not limited to, Stainless Steel 316L, 304L, 310, 347 and 430, Hastelloy C-276, C-22, X, N, B and B2, Inconel 600, 625 and 690, Nickel 200 and Monelo® 400 (70 Ni-30 Cu), Titanium, and Alloy 20.
According to a further exemplary embodiment of the present disclosure, the porous media plug 18 is made of Stainless Steel 316L and is adapted to provide a flow rate of 10 sccm (Standard Cubic Centimeters per Minute) of nitrogen at a pressure of 30 psig (gage pressure using atmospheric pressure as a zero reference). According to other exemplary embodiments, the porous media plug 18 is made of: Stainless Steel 316L and is adapted to provide a flow rate of 50 sccm of N2 at 30 psig; Stainless Steel 316L and is adapted to provide a flow rate of 250 sccm of N2 at 30 psig; or Nickel 200 and is adapted to provide a flow rate of 50 sccm of N2 at 30 psig. According to one exemplary embodiment of the present disclosure, the porous media plug 18 is cylindrical and elongated and may comprise a single, elongated piece of porous media, or may comprise individual inserts stacked to form an elongated assembly of porous media.
The porous media plug 18 may also be assembled of an inner cylinder coaxially receive in an outer sleeve, and the inner cylinder and the outer sleeve may be comprised of different porous media. Such an assembly will provide parallel flows of vapor.
The porosity or pore size of the porous media plug 18 controls the pressure drop. According to one exemplary embodiment, the porous media plug 18 comprises a one piece insert made by sintering one piece from two different frit sizes. Each frit size will correspond to a different pore size, or porosity.
The porous media plug 18 may comprise materials other than metals, such as quartz, ceramic (sapphire, alumina, aluminum nitride, etc.), and polymeric materials, including Teflon, PFA, PTFA, etc.
The means 18 for causing a pressure drop and for increasing the heat transfer rate may alternatively take other forms, such as, but not limited to, a long, thin, straight capillary tube, a long, thin, coiled capillary tube, a bundle of thin capillary tubes, or a laminar flow element.
As shown in
Referring to
Referring to
An input/output device 108 of the mass flow delivery system 100 receives a desired mass flow (either directly from a human operator or indirectly through a wafer processing computer controller), and a computer controller (i.e., computer processing unit or “CPU”) 102 is connected to the pressure transducer 104, the temperature sensor 106, the outlet valve 150 and the input/output device 108. The input/output device 108 can also be used to input other processing instructions, and can be used to provide an indication (either directly to a human operator or indirectly through a wafer processing computer controller) of the mass delivered by the system 100. The input/output device 108 may comprise a personal computer with a keyboard, mouse, and monitor, for example.
According to one exemplary embodiment of the disclosure, the controller 102 of the mass flow delivery systems 100 of
After a predetermined waiting period, wherein the gas inside the holding volume 140 can approach a state of equilibrium, the outlet valve 150 is opened to discharge a mass of gas from the holding volume 140, as shown at 212 of
Alternative modes of operation are possible. For example, in some instances it may be desirable to fill the holding volume 140 to a given pressure, such that multiple doses of the gas within the holding volume may be delivered through the outlet valve, before the holding volume is refilled. In other instances it may be desirable to deliver a dose of gas over a much longer period of time, wherein the outlet valve will be opened for longer periods (e.g. 0.5 to 30 seconds). In addition, the operational lives of the valves may be prolonged by using multiple valves at the inlet or outlet. Only one of the two valves would be pulsed at a time, and upon failure of that first valve, the second valve would take over. To accommodate failure of a value in either open or closed mode, four values would be needed: two in series, and these in parallel with another pair. U.S. patent application Ser. No. 11/015,465, filed on Dec. 17, 2004, which is entitled “Pulsed Mass Flow Deliver System and Method” and is assigned to the assignee of the present disclosure, discloses such an arrangement and is incorporated herein by reference.
For high pressure applications, the temperature of the gas within the holding volume 140 of the system 100 can be measured using the temperature probe 106. For low pressure applications and fast temperature transients, however, using a probe to measure the temperature may not be fast enough for accurate readings. In the case of low pressure applications and fast temperature transients a real-time physical model that estimates gas temperature is used, as described below.
For some precursor materials, it may be difficult to achieve an adequate vapor pressure of the material simply by heating it. For example, some materials may decompose excessively on heating to an adequate vapor pressure for delivery, reducing the effectiveness of the deposition process. In other cases the temperature required to vaporize sufficiently may be undesirably high. In such cases, the precursor material may be dissolved in an appropriate solvent, and then vaporized. Examples of solvent materials include various alcohols, ethers, acetone, and oils. Either organic or inorganic solvents may be suitable, depending on the particular application. Supercritical CO2 can act as an effective solvent for various materials.
The total mass m in the holding volume 140 based on the ideal gas law is:
m=ρV=(P/RT)V (1)
where ρ equals density, V equals volume, P equals absolute pressure, T equals absolute temperature, and R is equal to the gas constant.
The density dynamics within the holding volume 140 is:
dρ/dt=−(QoutρSTP/V) (2)
Where Qout is the flow out of the holding volume 140, and ρSTP is the gas density under standard temperature and pressure (STP) conditions.
The Temperature dynamics within the holding volume 140 is:
dT/dt=(ρSTP/ρV)Qout(γ−1)T+(Nuκ/l)(AW/VCνρ)(Tw−T) (3)
Where γ is the ratio of specific heats, Nu is Nusslets number, κ is the thermal conductivity of the gas, Cν is the specific heat under constant volume, l is the characteristic length of the delivery chamber, and Tw is the temperature of the wall of the holding volume 140 as provided by the temperature probe 106.
The outlet flow Qout can be estimated as follows:
Qout=−(V/ρSTP)[(1/RT)(dρ/dt)−(P/RT2)(dT/dt)] (4)
To compute the total mass delivered Δm from the holding volume 140, equation (4) is substituted for Qout in equation (3) to calculate the gas temperature T(t), at time=t, within the holding volume 140, as opposed to using the temperature probe 106 in
The total mass delivered Δm from the holding volume 140 between time t0 and time t* is:
Δm=m(t0)−m(t*)=V/R[(P(t0)/T(t0))−(P(t*)/T(t*))] (5)
Among other aspects and advantages, the mass flow delivery system and method, such as the exemplary embodiments shown in
The vaporizer 300 of
In the exemplary embodiment shown in
The porous plug assembly 318 is positioned in the inlet passageway 354 of the valve body 350. In the exemplary embodiment shown, the porous plug assembly 318 is cylindrical and elongated and comprises a single, elongated piece of porous media received in a solid metal sleeve. Alternatively, the porous plug assembly may comprise individual inserts stacked to form the elongated assembly, and the inserts may be comprised of different porous media (i.e., provide different flow rates at the same pressures).
The exemplary embodiments described in this specification have been presented by way of illustration rather than limitation, and various modifications, combinations and substitutions may be effected by those skilled in the art without departure either in spirit or scope from this disclosure in its broader aspects and as set forth in the appended claims.
