Printability and detection of backside defects on photomasks

Abstract

Reticles are contaminated during its lifetime and can catch particles as large as several tens of microns. Such defects on the backside of photomasks are usually considered as uncritical and thus do not receive much attention. Backside defects are out of focus by the mask thickness during wafer exposure and cannot be directly imaged on wafer. However, the shadow of the defects changes the local illumination of the mask patterns and may result in spatial variation of critical dimension (CD) on wafer depending on numerical aperture (NA) and pupil shape. There have been only a few investigations on printability of backside defects in the past, and no data are available for the most advanced technology nodes. Reticles are regularly inspected for particles on the glass side in the wafer fab but limits for acceptable defect size are based on estimations. Detection of non-acceptable particles causes exposed wafers being either delayed or reworked with impact on throughput and cost performance. It is therefore important to gain better understanding of critical sizes of backside defects and of appropriate detection capabilities. We have designed and manufactured a test mask with repeating patterns of 20nm, 28nm and 40nm technology node ranging from contact and line/space critical layers to non-critical implant layers. Programmed chrome defects of varying size are placed on the backside of the mask opposite to the repeating front side patterns in order to measure the spatial variation of transmission and wafer CD caused by the backside defects. The test mask was printed on a bare Silicon wafer and the size of printed features was measured by spatial sampling. Wafer CD variation for different backside defect sizes are demonstrated and compared for 28nm node first metal layer. Although the opaque chrome defects on the backside do not behave like real particles they aim on deriving a print threshold for backside particles based on actual wafer data. After such critical size of backside defects is obtained the reticle was also utilized to investigate the detection ability of backside defects by defect inspection of the reticle.