Vaporizing material at a uniform rate

1. A method of vaporizing particulate organic material at a uniform rate for forming an organic layer on a substrate, comprising:

a) providing a chamber under low pressure conditions;

b) providing a heating element within the chamber that produces heat;

c) feeding at least one particulate organic material into the chamber to form an annular column of vaporizable material, wherein the cross-sectional area of the annular column is constant, but the volume of the annular column varies according to changes in the feed rate;

d) providing a cold sink in operative relationship with the annular column to maintain the sides and bottom portion of the annular column at a controlled temperature below the effective vaporization temperature; and

e) applying a constant current to the heating element so that a heat flux is delivered to the top surface of the annular column to vaporize the material to form the organic layer on the substrate,

wherein the heating element has a low thermal mass so that a uniform volume per unit time of the material is vaporized when the feed rate of the annular column varies.

2. The method of claim 1 wherein the length of the annular column between the temperature controlled region and the heating element varies during vaporization, while the cross-sectional area of the surface of the annular column is substantially constant.

3. The method of claim 1 wherein the heating element is a flash vaporization heating element.

4. The method of claim 1 further including providing a manifold in communication with the chamber for receiving vaporized material and the manifold defining an opening positioned in relationship to the substrate so that vaporized material passes through the opening to form the layer on the substrate.

5. The method of claim 4 further including delivering material into the chamber by a movable device.

6. The method of claim 5 wherein the movable device is an auger.

7. The method of claim 1 wherein the step of feeding at least one organic material comprises the step of feeding a first organic material and a second organic material wherein, over a portion of the vaporization temperature range, the vapor pressure of the first material exceeds the vapor pressure of the second material by an order of magnitude.

8. The method of claim 1 , wherein the heating element includes a pair of spaced apart mesh screen layers electrically connected in series, the vaporized material passing through the mesh screen layers.

9. A method of vaporizing particulate organic material at a uniform rate for forming an organic layer on a substrate, comprising:

providing a chamber under low pressure conditions;

providing a heating element within the chamber that produces heat;

feeding at least one particulate organic material into the chamber to form an annular column of vaporizable material, wherein the cross-sectional area of the annular column is constant, but the volume of the annular column varies according to changes in the feed rate;

providing a cold sink in operative relationship with the annular column to maintain the sides and bottom portion of the annular column at a controlled temperature below the effective vaporization temperature; and

applying a constant heat flux from the heating element to the surface of the column so that a uniform volume per unit time of the material is vaporized to form the organic material on the substrate,

wherein the column length is at least half of the distance between the cold sink and the heating element.

10. The method of claim 9 , wherein the heating element includes a pair of spaced apart mesh screen layers electrically connected in series, the vaporized material passing through the mesh screen layers.