deposition  industry

Electron-beam liftoff: Collection efficiency &#38; paths to improvement

Liftoff metallization has been classically accomplished in the compound semiconductor industry with masks to achieve a uniform batch process. New process methodologies promise to significantly reduce those mask losses and offer significant improvements in both uniformity and collection efficiency in the future. By Gregg Wallace, Ferrotec USA Corporation, Temescal Division.

L

iftoff carriers orient wafers to maintain the perpendicularity rule from a virtual point source near the electron beam impact point to a point on the center of each wafer in a carrier. Considering perfect 90° incidence to an essentially “flat wafer center” it is then obvious that incidence from the same point source at the e-gun to the outer edge of the wafer causes an imperfection to perfect normal incidence. The lack of perpendicularity at the wafer edge represents a process limit for “ideal liftoff conditions”.

These constraints lead to larger diameter wafers requiring greater distance between the E-beam source and the wafers surface to maintain near normal incidence at the wafers edge. In general, +/- 5° from 90° incidence is an accepted rule for good liftoff processing geometry.

Figure 1 maps the radiating zones of gold vapor flux by effective deposition rate at locations above an E-gun. Most simply, this is the flux topography above an E-gun. It is important to note, the map is for a specific set of evaporation conditions as described at the top of the chart.

The flux lines would be different if a material other than gold were evaporated even under the exact same E-Gun conditions.

This image can be mathematically described as cosine radiation from a point source. The product of the equation will be deposition rate which is highest close and directly above the source.

This material flux, under similar conditions, is highly reproducible and this is what offers engineers the benefit of predictable production. However the flux pattern’s shape is ruled by cosine curve mathematics and is highly dependent on control of the conditions listed below:

 Bulk Characteristics of the material being evaporated: density, melting point, thermal coefficients of expansion &#38; conductivity, etc

 Maximum power capacity of the material being evaporated

 Size of crucible used (thermal mass)  Deposition Power  Beam Shape  Use of sweep

Figure 1

Cosine curves are expressed in exponential form as cosine to the “n” power of an angle theta describing a position above the (point) source. As the exponent “n” changes (becoming &#62;1) the cosine

March 2013 www.compoundsemiconductor.net 29

Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39  |  Page 40  |  Page 41  |  Page 42  |  Page 43  |  Page 44  |  Page 45  |  Page 46  |  Page 47  |  Page 48  |  Page 49  |  Page 50  |  Page 51  |  Page 52  |  Page 53  |  Page 54  |  Page 55  |  Page 56  |  Page 57  |  Page 58  |  Page 59  |  Page 60  |  Page 61  |  Page 62  |  Page 63  |  Page 64  |  Page 65  |  Page 66  |  Page 67  |  Page 68  |  Page 69  |  Page 70  |  Page 71  |  Page 72  |  Page 73  |  Page 74  |  Page 75  |  Page 76  |  Page 77  |  Page 78  |  Page 79  |  Page 80  |  Page 81  |  Page 82  |  Page 83  |  Page 84  |  Page 85  |  Page 86  |  Page 87  |  Page 88  |  Page 89  |  Page 90  |  Page 91  |  Page 92  |  Page 93  |  Page 94  |  Page 95  |  Page 96  |  Page 97  |  Page 98  |  Page 99  |  Page 100  |  Page 101  |  Page 102  |  Page 103  |  Page 104  |  Page 105  |  Page 106  |  Page 107  |  Page 108  |  Page 109  |  Page 110  |  Page 111  |  Page 112  |  Page 113  |  Page 114  |  Page 115  |  Page 116  |  Page 117  |  Page 118  |  Page 119  |  Page 120  |  Page 121  |  Page 122  |  Page 123  |  Page 124  |  Page 125  |  Page 126  |  Page 127  |  Page 128  |  Page 129  |  Page 130  |  Page 131  |  Page 132  |  Page 133  |  Page 134  |  Page 135  |  Page 136  |  Page 137  |  Page 138  |  Page 139  |  Page 140  |  Page 141  |  Page 142  |  Page 143