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Figure 1 – Anisotropic and Isotropic Etch
(Unetched Wafer, Isotropic, Anisotropic)
Each layer of a circuit will require a unique mask. For example, a simple resistor circuit would require two masks. The first mask (Composite Layer Mask) will define the composite with a photoresist pattern. The Ion Beam Etch (IBE) process will etch away everything except for the defined photoresist pattern. The second mask will position and locate photoresist openings across the composite circuit that will be ion etched down to the resistor layer. Figure 2 illustrates the process.
Figure 2 – Simple Multi-layer circuit
Note: Dimension C is recommended to be at least 2 mil, if the circuit real estate permits. The oversize exposure area for the resistor layer is recommended to insure all the gold is removed. The dashed lines are not part of the Resistor Layer mask and are only shown to illustrate how the two masks work together.
A via hole is used to electrically connect the top-side of the substrate to the bottom-side of the substrate. Via holes should be designed with an aspect ratio of 1. That is the depth of the hole should equal the diameter of the hole. This is required to obtain proper coverage inside the hole for sputtered layers. In some special applications an aspect ratio of 0.75 is possible.
The pad (Ring or square), for a via hole, must be at least double the via diameter. This is to insure that there is sufficient material on the pad for adhesion. An example is illustrated in Figure 3. In some special applications, the pad can be as small as 1.5 times the diameter of the via hole. However, this reduction can reduce the yield. A filled via (additional cost) can improve the yield for a 1.5 pad.
Figure 3 – Via Dimensions
The Wrap-around geometry is popular for Laser Diode mount applications. This type of geometry requires a 1:1 aspect ratio of the depth to the width of the laser cut. As with the via hole, this requirement insure proper sputtering of metal layers after the laser cut. A typical pull-back after dicing is 0.002″. In some applications, a complete edge wrap is available where there is no pull-back. An example is illustrated in Figure 4.
Figure 4 – Wrap-around
(Prior to Dicing – Final Product After Dicing)