SEMI 3D12 - Guide for Measuring Flatness and Shape of Low Stiffness Wafers
The control of parameters, such as thickness, total thickness variation (TTV), bow, warp, and flatness, is well known to be essential to many, if not all, semiconductor processes as reflected in a number of existing standards.
The general trend in wafer geometry is to a larger diameter but even so the thickness increases correspondingly, it may not be enough to sustain adequate stiffness of the wafer. With growing aspect ratio of a wafer (e.g., diameter/thickness), gravity compensation errors increase until they become larger than the actual wafer surface variations. This is already the case for many special purpose wafers with lower modulus of elasticity and is becoming more common in mainstream applications. For example many wafer substrates are very thin such as germanium wafers used in specialized solar applications, and many other substrates are being made thinner such as lithium tantalate, and lithium niobate used in communication filters.
Manufacturing processes incorporating bonded wafers typically require thinned wafers; these wafers are then extremely low stiffness. In addition, since non-planarity of the wafers can lead to problems in subsequent processing steps, including lithography and electrical contact between metal layers on the bonded wafers, these technologies demand higher precision wafers and more accurate depiction of the true topography of the wafer.
This Guide is designed to provide definitions for describing a more suitable measurement strategy for low stiffness wafers and geometries.
The more suitable measurement process consists of an alternative mounting for wafers with high aspect ratios and the use of high resolution measurements.
This Guide also provides a measurement procedure for local bow, which denotes small areas of imperfection of the otherwise flat wafer or substrate.
This Guide’s alternative measurement process is suitable for use in materials acceptance and process control, but may also be useful in other applications, such as wafer design and production.
This Guide is intended in general for metrology on low stiffness substrates like wafers or other geometrical shapes such as panels. As low stiffness can occur with any material, this guide can be applied to all materials that show low stiffness in certain deformation modes.
This Guide applies in particular to glass and silicon wafers with a diameter equal to or exceeding 300 mm and their thickness equal to or less than 775 µm ± 20 µm.
This Guide also applies to wafers or other geometrical shapes of any material type that show deflections more than 200% of the allowable bow tolerance when tested for bow using the conventional three-point mounting method.
Although the accuracy of TTV measurement is not as dependent on deflection as bow and warp are, it makes sense to use the same wafer support configuration when measuring TTV.
This Document is a guide for a nondestructive procedure that uses a semicontinuous flat mounting surface and high resolution measurement methods.
Referenced SEMI Standards
SEMI M1 — Specifications for Polished Single Crystal Silicon Wafers
SEMI M59 — Terminology for Silicon Technology
SEMI MF533 — Test Method for Thickness and Thickness Variation of Silicon Wafers
SEMI MF534—— Test Method for Bow of Silicon Wafers
SEMI MF657 — Test Method for Measuring Warp and Total Thickness Variation on Silicon Slices and Wafers by a Noncontact Scanning Method
SEMI MF1390 — Test Method for Measuring Bow and Warp on Silicon Wafers by Automated Noncontact Scanning
SEMI MF1451 — Test Method for Measuring Sori on Silicon Wafers by Automated Noncontact Scanning
SEMI MF1530 — Test Method for Measuring Flatness, Thickness, and Thickness Variation on Silicon Wafers by Automated Noncontact Scanning
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