Thermal treatment boat comprising a cylinder having a central axis and a plurality of band slots having opposed upper and lower surfaces in planes perpendicular to said central axis and spaced at predetermined locations along said central axis. At least one slot in each set extends around at least 180.degree. and less than of the full circumference of said cylinder. Pairs of adjacent band slots define an annular band therebetween. The height of each slot being from about 3.8 to 12.7 mm. Each of the bands having a height, Height.sub.Band, in mm, according to the equation: ##EQU1## wherein Height.sub.Band is always .ltoreq. wafer thickness; ColumnHeight is the total height of the cylinder, mm; BandSlotHeight is the height of the slot, mm; and NumberBands is the total number of bands in the treatment boat. The cylinder can include a wafer loading effector slot therethrough in a plane of the central axis extending along the length of the cylinder. Each band preferably includes wafer support means for supporting a wafer therein. The wafer support means preferably includes at least three inwardly extending projections. The spacing between the wafer edge and the cylinder wall is within the range of from about 1.5 to 6.3 mm. In the optimum process, the heat provided by the heater is sufficient to raise the temperature of the wafers from 21.degree. C. up to 1100.degree. C. at a rate of from 50.degree. C./min to 100.degree. C./min without causing mechanical damage from thermal stresses to the wafers.