Atomic Layer Epitaxy (ALE) method was invented by Dr. Tuomo Suntola in the early 1970's.  Another generic name for the method is Atomic Layer Deposition (ALD) and it is nowadays used instead of ALE.  ALD is a special chemical deposition method based on the sequential introduction of at least two reactive precursor species to a substrate that is located within a heated reaction space.  The growth mechanism of ALD relies on the bond strength differences between chemical adsorption (chemisorption) and physical adsorption (physisorption).  ALD utilizes chemisorption and eliminates physisorption during the deposition process.  During chemisorption a strong chemical bond is formed between atom(s) of a solid phase surface and a molecule that is arriving from the gas phase.  Bonding by physisorption is much weaker because only van der Waals forces are involved.  Physisorption bonds are easily broken by thermal energy when the local temperature is above the condensation temperature of the molecules. 
By definition the reaction space of an ALD reactor comprises all the heated surfaces that can be exposed alternately and sequentially to each of the ALD precursor used for the deposition of thin films.  A basic ALD deposition cycle consists of four sequential steps: pulse A, purge A, pulse B and purge B. Pulse A typically consists of metal precursor vapor and pulse B of non-metal precursor vapor, especially nitrogen or oxygen precursor vapor.  Inactive gas, such as nitrogen or argon, and a vacuum pump are used for purging gaseous reaction by-products and the residual reactant molecules from the reaction space.  A deposition sequence comprises at least one deposition cycle.  Deposition cycles are repeated until the deposition sequence has produced a thin film of desired thickness. 
Precursor species form through chemisorption a chemical bond to reactive sites of the heated surfaces.  Conditions are typically arranged in such a way that no more than a molecular monolayer of a solid material forms on the surfaces during one precursor pulse.  The growth process is thus self-terminating or saturative.  For example, the first precursor can include ligands that remain attached to the adsorbed species and saturate the surface, which prevents further chemisorption.  Reaction space temperature is maintained above condensation temperatures and below thermal decomposition temperatures of the utilized precursors such that the precursor molecule species chemisorb on the substrate(s) essentially intact.  Essentially intact means that volatile ligands may come off the precursor molecule when the precursor molecules species chemisorb on the surface.  The surface becomes essentially saturated with the first type of reactive sites, i.e. adsorbed species of the first precursor molecules.  This chemisorption step is typically followed by a first purge step (purge A) wherein the excess first precursor and possible reaction by-products are removed from the reaction space.  Second precursor vapor is then introduced into the reaction space.  Second precursor molecules typically react with the adsorbed species of the first precursor molecules, thereby forming the desired thin film material.  This growth terminates once the entire amount of the adsorbed first precursor has been consumed and the surface has essentially been saturated with the second type of reactive sites.  The excess of second precursor vapor and possible reaction by-product vapors are then removed by a second purge step (purge B).  The cycle is then repeated until the film has grown to a desired thickness.  Deposition cycles can also be more complex.  For example, the cycles can include three or more reactant vapor pulses separated by purging steps.  All these deposition cycles form a timed deposition sequence that is controlled by a logic unit or a microprocessor. 
Thin films grown by ALD are dense, pinhole free and have uniform thickness.  For example, aluminum oxide grown from trimethylaluminum (CH3)3Al, also referred to as TMA, and water at 250-300° C. has usually about 1% non-uniformity over the 100-200 mm wafer.  Metal oxide thin films grown by ALD are suitable for gate dielectrics, electroluminescent display insulators, capacitor dielectrics and passivation layers.  Metal nitride thin films grown by ALD are suitable for diffusion barriers, e.g., in dual damascene structures.  Precursors for the ALD growth of thin films and thin film materials deposited by the ALD method are disclosed, for example, in a review article M. Ritala et al., “ Atomic Layer Deposition” Handbook of Thin Film Materials, Volume 1: Deposition and Processing of Thin Films, Chapter 2, Academic Press, 2002, p. 103, and R. Puurunen, “Surface chemistry of atomic layer deposition: A case study for the trimethylaluminium/water process”, Journal of Applied Physics, vol.  97 (2005) pp.  121301-121352 which are incorporated herein by reference. 
Apparatuses suited for the implementation of ALE and ALD methods are disclosed, for example, in review articles T. Suntola, “Atomic Layer Epitaxy”, Materials Science Reports, 4(7) 1989, Elsevier Science Publishers B. V., p. 261, and T Suntola, “Atomic Layer Epitaxy”, Handbook of Crystal Growth 3, Thin Films and Epitaxy, Part B: Growth Mechanisms and Dynamics, Chapter 14, Elsevier Science Publishers B. V., 1994, p. 601 which are incorporated herein by reference. 
A precursor source is disclosed in U.S. patent application publication no.  US 2007/0117383 A1, which is incorporated herein by reference. 
Another precursor source is disclosed in WO patent application publication no.  WO 2006/111618 A1, also incorporated herein by reference. 
Various existing precursor sources have a number of problems.  One general problem is that preventing the condensation of precursor vapor in source chemical lines has required complicated and expensive heating systems.  Another general problem is that preventing crust formation on the solid precursor surface has required complicated source structures.  Still another general problem is that existing precursor sources are very bulky and service of the precursor source has been time-consuming.