Patent Application: US-51283804-A

Abstract:
the invention relates to an effusing source for film deposition made of a reservoir comprising one hole characterized by the fact that the hole diameter is less than one order of magnitude than the mean free path of the molecules determined by the pressure and its thickness is at least one order of magnitude smaller than the diameter . preferably the source has several holes .

Description:
a typical effusion source is a hole of 0 . 5 mm drilled in a foil of 0 . 05 mm of thickness between a reservoir ( pre - chamber ) with a pressure of 10 − 3 - 10 − 2 mbar and the deposition chamber with a pressure below 10 − 3 mbar . the hole dimensions however , depend on the pressure in the pre - chamber and on the substrate size , and could vary from 0 . 001 and 50 mm . furthermore , the thickness of the hole is about one order of magnitude ( or more ) smaller than the diameter , while the distance of the source to the deposition area is one order of magnitude ( or more ) larger than the diameter of the hole . the combination of several sources may allow substrate rotation avoidance . several holes are uniformly distributed on an annular geometry ( see fig1 ). the formula that describes the distribution of impinging molecules on a planar surface for several cost distributions of the effusion sources distributed on a ring is the following : i ⁡ ( ϑ ) = i 0 ⁢ cos n ⁡ ( ϑ ) ⇒ i tot = i 0 ⁢ ∑ i = 1 p ⁢ [ h h 2 + r 2 + r 2 - 2 ⁢ rr ⁢ ⁢ cos ⁡ ( β - i ⁢ 2 ⁢ π p ) ] n to illustrate the concept of reactor size reduction with opportune thickness uniformity , several examples of angular distribution shaping ( as reported in the summary of the invention point 5 ) are provided in table - 1 . analytical modelling of precursor transport is applied to find the adequate parameters providing molecular impinging uniformity better than 1 % on a 150 mm substrate for various arbitrary cos n distributions , distance h of the sources to the substrate , and radius r of the ring on which are distributed the sources ( see table - 1 ). with the parameters reported in table - 1 , the distributions are identical with an error less than 0 . 1 %. these cos n sources ( with n & lt ; 4 under - cosine sources ) are not existing sources nor are they to be considered optimal sources , but are used only to show that the trend of the reactor size is reduced with decreasing the focusing of the source . i tot = i 0 ⁢ ∑ i = 1 p ⁢ [ - x s ⁡ ( x - x s ) - y s ⁡ ( y - y s ) + h ⁢ x s 2 + y s 2 ⁢ tan ⁢ ⁢ φ ( ( x - x s ) 2 + ( y - y s ) 2 + h 2 ) ⁢ ( x s 2 + y s 2 ) ⁢ ( 1 + tan ⁢ ⁢ φ ) ] similar distributions ( within still an error of less than 0 . 1 %) are achieved with parameters reported in table - 1 and table - 2 . we can see that the dimensions can be further reduced with a small tilting angle of the sources . furthermore , several tilted concentric rings could be considered equivalent to planetary motion . it can be shown that this angle is smaller and less critical on uniformity distribution for under - cosine sources . the second point discussed is how to achieve the desired distribution shaping of the sources required for the already discussed reasons in point 5 of the summary of the invention . in particular , under - cosine distributions for small angles and over - cosine distributions for greater angles corresponding to regions outside the substrate ( see fig3 ) are aimed at . a tilt angle could be considered the first step in molecular beam modification ( relative to the substrate ), but this method is very limited . two other solutions are proposed , as examples , but should not be considered exhaustive . the first design is based on selecting and promoting molecules escaping the source with a given angle . two different types of molecules will escape the source : the molecules that had the last collision inside the pre - chamber with another molecule and those that had the last collision on a surface inside the source . on one hand , a volume below the effusing aperture is a forbidden region for gas phase collision . on the other hand , a pumping aperture will act as a trap for surface scattered molecules . counterbalancing both effects could lead to shaped distributions . furthermore , variable pressure configuration could lead to variable angular distributions without any moving part or modifications of the set - up . an example to reduce molecules effusing at small angles is reported in fig4 with a cone - like shaped forbidden volume . any kind of structure could serve to reach this purpose . in the cone - case , the apex may be cut and a hole provides a pumping aperture . if the cone is positioned under the hole at a distance b ≠ 0 , we will have a progressive increase of the volume , as angle will increase that will depend on the ratio between the distance b and the mean free path λ . in particular , variable pressure configuration will lead to variable effect of the cone as the mean free path is changed . a particular case of this configuration could be a negative parameter b ; i . e . the cone exits the pre - chamber through the hole . as multiple sources are to be used , mechanical complexity of multiple cones can be avoided by producing a continuous structure . as a general rule , any kind of molecular angular distribution can be achieved by opportune disposition of several holes a 3 - d surface . however , only in the case in which these sources are dispatched close together compared to their distance to the deposition area we can consider them as a single point source with the advantage of easy mathematical modelling . furthermore , the total area of the holes should be small compared to the area separating the 3d surface from the pre - chamber to avoid gas depletion and pressure gradients . furthermore , each single hole must satisfy the rules introduced previously in the summary of the invention . a particular shape of interest is a hemisphere ( see fig5 ). in particular asymmetric sources are easily produced leading to gas waste reduction . disposition of effusion holes with annular geometry ( 1 ). r is the radius of the ring on which are distributed the holes , r is the distance from the centre on the substrate ( 2 ), and h is the distance of the substrate from the holes containing plane ( 3 ). φ is the tilt angle of the surface on which is the source s and is oriented towards the z - axis . fig3 : shape of the ideal angular distribution ( 1 ), to achieve high impinging rate uniformity and small reactor size , compared to knudsen effusion ( 2 ). both curves are normalised to achieve same number of effusing molecules . in the ideal curve ( 1 ), 60 ° is assumed to be the cut - off plane that discriminates between deposition on and outside the deposition area . a rapid decrease of the molecules occurs after this critical angle and the under - cosine distribution ( 4 ) is modified resulting in an over - cosine distribution ( 3 ). asymmetric sources could also prove useful . fig4 : the source is composed of a hole ( 1 ), a volume ( 2 ) that reduce the region where gas phase collisions are allowed , and a pumping aperture ( 3 ) that reduce the surface from which scattered molecules can exit the source . gas phase collisions are restricted to larger angles in fig4 a . surface scattered molecules are reduced for small angles fig4 b . when the parameter b is not null , a variation in pressure induces a variation of the mean free path λ . a different contribution of the cone to angular distribution is hence achievable as a function of pressure in fig4 c . the structure can also exit the source as reported in fig4 d . fig5 a : fractal source composed of a distribution of effusing holes on a hemispherical surface . fig5 b : asymmetric fractal source with preferential orientation of the molecular beam . 1 . suzuki , k ., state of the art in large area vacuum coatings on glass . thin solid films , 1999 . 351 : p . 8 - 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