There are four primary masks that are each associated with one of the wafer levels outlined below. The SOIHOLE mask is an extra, special-use masking level that helps designers define holes in the silicon layer. The buried oxide layer has no mask that directly patterns it, but it can be etched using the TRENCH mask and DRIE techniques.
| Material layer | Lithography mask name | Field type | Lithography purpose |
|---|---|---|---|
| Pad metal | PADMETAL | light | Defines the geometry of metal for electrical connections. Metal is 20 nm Cr and 500 nm Au. |
| Silicon | SOI | light | Defines structures in the silicon layer. |
| Silicon | SOIHOLE | dark | Defines holes in the silicon layer. |
| Buried oxide | - | - | - |
| Substrate | TRENCH | dark | Defines through-hole structures in the substrate layer. |
| Blanket metal | BLANKETMETAL | dark | Defines the geometry of metal to be deposited through a shadow mask. Metal is 50 nm Cr and 600 nm Au. |
The below table summarizes Science’s naming conventions and design limitations for each masking level.
| Mnemonic mask name | GDS mask number | Min. feature length (µm) | Min. space (µm) | Max. feature length (µm) | Max. etched area mm² |
|---|---|---|---|---|---|
| PADMETAL | 5 | 3 | 3 | 5000 | N/A |
| SOI | 10 | See note below ↓ | See note below ↓ | See note below ↓ | 33 |
| SOIHOLE | 11 | 3 | 3 | - | - |
| TRENCH | 20 | 200 | 200 | 5000 | 25 |
| BLANKETMETAL | 30 | 100 | 100 | 5000 | 20 |
SOI mask notes
Features in the SOI layer with a width greater than 6 µm have no maximum length. Silicon features in the SOI layer that are less than 6 µm wide may curl out of plane from the substrate due to the intrinsic stresses in the silicon layer. To minimize the possibility of curling, Science recommends a maximum length of 500 µm if the structure is anchored at both ends and 100 µm if it is anchored at only one end.
Features and spaces on non-orthogonal axes may not print on wafers at their nominal sizes due to the pixelation of the 0.25 µm photomask resolution. This may cause bridging between closely spaced features in the SOI layer. Science recommends a slightly higher minimum feature and minimum space requirement for non-orthogonal features at 2.5 µm instead of the 2 µm used with orthogonal features.
Mask to mask design rules
The SOI MEMS process has some rules specific to how each of the masking levels interact with one another.
| # | Rule | Min. Value (µm) | Function | Required? |
|---|---|---|---|---|
| A | SOI encloses PADMETAL | 3 | This rule ensures the pad metal layer is not accidentally exposed to the SOI mask etch. | Yes |
| B | SOI spaces TRENCH | 50 | This guideline ensures the edge of the substrate does not undermine the edge of the silicon during the TRENCH etching process. | No |
| C | SOI feature size is at least | 10 | This guideline, together with rule B, ensures the silicon structure is anchored to the substrate layer. | No |
| D | TRENCH encloses silicon structure | 5 | This guideline ensures the silicon structure is released from the substrate layer. | No |
| E | BLANKETMETAL alignment to SOI | ± 35 | The center of the blanket metal layer is deposited with a ± 35 µm tolerance. | No |
| F | Shadow mask alignment to SOI | ± 40 | The center of the shadow mask is bonded with a ± 40 µm tolerance. | No |
| This example wafer features:
|
SOI encloses PADMETAL - A
This rule ensures the pad metal layer is not accidentally exposed to the SOI mask etch.
SOI spaces TRENCH - B
This guideline ensures the edge of the substrate does not undermine the edge of the silicon during the TRENCH etching process.
SOI feature size is at least - C
This guideline, together with rule B, ensures the silicon structure is anchored to the substrate layer.
TRENCH encloses silicon structure - D
This guideline ensures the silicon structure is released from the substrate layer.
BLANKETMETAL alignment to SOI - E
The center of the blanket metal layer is deposited with a ± 35 µm tolerance.
Shadowmask alignment to SOI - F
The center of the shadow mask is bonded with a ± 40 µm tolerance.