Copyright Plaspro GmbH 2012
Non-
Capability
- TSV & Deep Trench Measurements
- Repeatability typically < 0.1 um 3 sigma
- High-
Speed ~ 30 msec per data point
The Wafer Thickness Sensor (WTS) is an optical, non-destructive, sensor that directly measures the etch depth of vias, without regard to aspect ratio. The high throughput measurements of via depth after the etching of through silicon vias (TSV) will allow timely feedback for process control, process development, and the prevention of process excursions in 3D IC process technology. In this paper, we report the capabilities and limitations of the WTS for the measurement of TSV etched depth. Etch depth measurement results are presented for a variety of vias, including 1μm, 3μm, and 5μm diameter vias, vias with an aspect ratio of 28:1, and both isolated and densely packed vias. Results include accuracy and repeatability data, with a route towards providing a high volume manufacturing TSV etch metrology solution for 3D IC process technology.
The non-destructive and high throughput measurements of etch depth in the fabrication of etched through silicon vias (TSV) will allow timely feedback for process control, process development, and the prevention of process excursions in 3D IC process technology. Current TSV metrology techniques have limitations in terms of throughput, reproducibility, flexibility, wafer coverage, and sample preparation (mostly destructive). These limitations are especially severe for TSVs with feature sizes in the 1-5 μm range and aspect ratios greater than 20:1. The Wafer Thickness Sensor (WTS) is an optical, non-destructive, sensor that directly measures the etch depth of vias and wafer thickness, without regard to aspect ratio. With the capability to measure etched TSV depth over an entire wafer in minutes, the performance of the TSV
The non-
etcher can be monitored using advanced statistical process control (SPC) with information such as via depth variation across a wafer, etch depth variation with via diameter, and total thickness variation (TTV).
n this paper, we report the capabilities and limitations of the WTS for the measurement of TSV depth. The theory of operation of the sensor will not be presented. The fast (seconds) and non-destructive measurements allow immediate process feedback with information such as via depth variation across the wafer and etch depth variation with via diameter for both process development and process sustaining purposes. The sensor illuminates the wafer from the bottom, the side opposite the etched surface, with infrared light. The illumination spot size is 4μm or 14μm, depending on configuration. However, an isolated trench narrower than the illumination spot size can be measured, and the ultimate limitation to trench diameter comes from the signal-to-noise ratio. For practical purposes, the current limit to TSV diameter for the WTS is approximately 1μm. When the diameter of the TSV is larger than the spot size, the minimum etch depth that can be measured is approximately 0.1μm. Otherwise, the minimum is limited to the width of the resolution curve, which is approximately 5.5μm. The maximum depth is limited only to the full thickness of the wafer less the 5.5μm resolution curve. With the capability to accommodate such high aspect ratios, this metrology technique is suitable to support the high volume manufacturability of TSVs.
The primary method currently used to measure the etch depth of high aspect ratio TSVs is cross-section scanning
n this paper, we report the capabilities and limitations of the WTS for the measurement of TSV depth. The theory of operation of the sensor will not be presented. The fast (seconds) and non-
The primary method currently used to measure the etch depth of high aspect ratio TSVs is cross-
electron microscopy (x-SEM) [1]. However, the x-SEM method is slow, expensive, and destructive, and it cannot be used as in-line metrology to support high volume manufacturing needs. While other optical instruments, such as white light interferometers, laser triangulation sensors, and chromatic confocal sensors, are capable of measuring depth, they are sensitive to aspect ratio and are not typically able to make reliable measurements of TSV etch depth for vias in the realm of 1μm to 5μm wide by 20μm, or more, deep. These systems typically illuminate the etched feature from the top, i.e. the first surface to receive the illumination is the surface that was etched.
