METHOD OF CLAMPING A SUBSTRATE AND CLAMP PREPARATION UNIT
FIG2 is a sectional view schematically illustrating processes with a negative impact on the clamp stability of the capillary layer of FIG1  FIG1 is a sectional view schematically illustrating a capillary layer between two structures
FIG2 is a sectional view schematically illustrating processes with a negative impact on the stability of the clamping action performed by means of the capillary liquid layer 1 of FIG1. Hereafter, the expression "clamp" will be used for the arrangement where the substrate 2 is clamped to the substrate support structure 3 by means of the capillary layer 1. If pre-existing bubbles are present in the liquid, introduction of the clamp into a vacuum environment will lead to expansion of such bubbles within the capillary layer. The size of initially small bubbles can grow by several orders of magnitude if ambient pressure decreases, e.g. from 1 bar to 10-6 mbar. As can be readily seen in FIG2, a bubble of the size of bubble 11 may seriously influence the clamping strength, at least locally, and may have a negative influence on the stability of the clamp. Another mechanism that may lead to clamp instability is spontaneous void formation, for example caused by cavitation of or dissolved gas precipitation in the capillary liquid layer. An example of such a void has been denoted by reference numeral 13 in FIG2. Voids formed by cavitation may grow in a similar way as discussed before with respect to pre-existing bubbles if the clamp is brought into a vacuum environment. The resulting voids may have a negative influence on the clamp stability. Besides decrease of the clamp stability due to the presence of bubbles and/or voids, the clamp stability will also be negatively affected by evaporation of liquid at the capillary layer interface, i.e. evaporation at the concave liquid surface. FIG2 schematically shows the effect of such evaporation. Due to evaporation, the position of the outer liquid surface 8 has shifted towards a new position to form outer liquid surface 8'. As a result of that shift, the surface area covered by the capillary layer, and thus the stability of the clamp, has decreased. As mentioned earlier with reference to FIG2, the layer of capillary liquid evaporates in a vacuum environment. Experiments have shown that the remaining volume of the capillary liquid layer tends to accumulate at one side of the clamp. Due to this asymmetric distribution of the capillary layer, one side of the substrate "peels off' the table. Hereafter, the effect will be referred to as substrate peeling.  FIG1 is a sectional view schematically illustrating a capillary liquid layer 1 of a capillary liquid, e.g. water, between a first substrate 2, e.g. a wafer, and a second substrate 3, e.g. a substrate support structure like a wafer table. The first and second substrates 2, 3 have a substantially flat surface 5, 6 respectively. The nominal distance between the opposing surfaces 5, 6 of the first and second substrates 2, 3 is given by height h. The capillary liquid layer 1 has an outer liquid surface 8, which is generally concavely shaped due to adhesive connection of the liquid to the first substrate 2 and the second substrate 3.
sectional view
28
surface
23
substrate support structure
11
bubble
1
layer
22
substrate
29
sealing structure
2
substrate substrates
41
reservoir
31
moat
27
contact elements burls burl
43
channels
26
surface
21
capillary layer
44
flow control unit
3
second substrate
13
reference numeral
['7. Arrangement according to any one of the preceding claims, wherein the clamp preparation unit comprises a liquid dispensing unit (124) for applying liquid onto the surface of the substrate support structure, and wherein the substrate support structure comprises a substrate transfer unit for lowering the substrate onto a liquid layer formed on the surface of the substrate support structure, and for lifting the substrate from the liquid layer on the substrate support surface, wherein the substrate transfer unit comprises a plurality of separately controlled movable support pins (127).']