How to specify an SOI wafer for MEMS

Start with your process, then choose the lot

An SOI order needs more than a diameter and a device-layer thickness. Write the required device, buried oxide and handle properties as separate groups, and distinguish fixed requirements from preferences. That makes it possible to compare stock fairly with a custom build.

First identify which face you will process, what thickness the finished silicon structure needs, whether the BOX stays or is removed, and which tools must handle the wafer. If the stack is unfamiliar, start with the SOI structure guide.

Specify each layer independently

Device silicon

Give the nominal thickness and acceptable variation, doping type and dopant, resistivity range, crystal orientation and surface-finish requirements. Keep units explicit: 500 nm is 0.5 µm. Resistivity is reported in Ω·cm; it is not the same quantity as resistance in Ω.

Buried oxide

State the BOX thickness and tolerance separately. Note whether it acts as an insulating layer, an etch stop or a sacrificial layer in your process. Ask about oxide type and any process constraints that matter to your application. A thicker BOX is not automatically a better match.

Handle silicon

Specify thickness, electrical properties, orientation and backside finish where your process requires them. Do not assume the handle has the same doping or resistivity as the device layer. If backside alignment, etching or bonding is planned, include those requirements in the inquiry.

Also confirm the actual diameter in millimeters, its tolerance, and the flat or notch requirements. Inch labels are nominal product categories and should not substitute for the listed dimensions. SSP means single-side polished; DSP means double-side polished. MicroChemicals’ specification reference (PDF) explains these conventions.

Make the meaning of a tolerance explicit

Ask whether a stated thickness tolerance applies across one wafer, between wafers in a lot, or to a particular measurement location. Agree on the measurement area, edge exclusion and reporting method when they are critical. An unqualified “±” value does not answer all of those questions.

Total thickness variation (TTV) is the difference between maximum and minimum measured thickness. Bow and warp describe wafer shape relative to a reference plane; surface roughness describes a different scale of surface variation. A specification for one should not be treated as a guarantee for the others.

Read an actual MEMS stock lot

The following fields come from the same catalog record as SOI-3-202. This is an example of reading a listing, not a recommendation for a particular device design.

The device, BOX and handle values describe different parts of the stack. Read each tolerance with its own layer; do not add the listed “±” values together and assume that gives an agreed total-stack tolerance. Confirm any missing acceptance criteria with the quote.

In other MEMS listings, “BPR,” “P.R” or “per request” means the device thickness is specified with the order. It is not a numeric thickness. The inventory notation reference explains the other abbreviations.

What to include in an SOI quote request

• Lot or custom build: give the stock ID if a listed lot fits, or identify which requirements differ from the standard specification.
• Diameter and handling: millimeter diameter, flat or notch, and any tool constraints.
• Device layer: thickness and tolerance, doping, resistivity, orientation and finish.
• BOX: thickness and tolerance, plus any required oxide type or process constraint.
• Handle: thickness, doping, resistivity, orientation and backside finish requirements.
• Acceptance criteria: required measurements, sampling, edge exclusion and documentation. Ask whether these can be supplied.
• Quantity and schedule: required quantity, delivery destination and target date, with essential requirements clearly marked.

Compare this checklist with our standard SOI specification. A stock count is not a delivery commitment; MEMS Engineering confirms price, availability and delivery with your quote.

Request an SOI quote