Uncooled Infrared Detectors: VOx Vs. α-Si

As the mainstream materials for thermal components of uncooled infrared detectors, what are the similarities and differences between vanadium oxide (VOx) and amorphous silicon (α-Si)?

1. Similarities

1.1 Same Production Process

Microbolometer technology is compatible with CMOS technology. It can be monolithically integrated with CMOS readout circuits, and achieves mass production based on semiconductor manufacturing processes. It is the mainstream technology of uncooled infrared focal plane detectors.

1.2 Same Type of Film

The thin film of both vanadium oxide and amorphous silicon are semiconductor heat-sensitive thin films. And the TCR of the thin film is directly proportional to the resistivity.

2. Differences

2.1 Technology Development Time

In 1978, a breakthrough was made in the use of microbolometer VOx materials to make detectors. Amorphous silicon is 10 years late. The performance and technology of detectors made of vanadium oxide are more mature than amorphous silicon (The vanadium oxide detector has a 10μm pixel pitch product, and the amorphous silicon detector currently only has a 12μm pixel pitch product with 320×240 pixels).

2.2 Different Thin Film Deposition Method

The vanadium oxide thin film is prepared by reactive sputtering deposition method, which needs to modify the standard CMOS process PVD equipment and introduce O2 as the reaction gas to realize the thin film oxidation.

Amorphous silicon thin films are prepared by chemical vapor deposition (CVD), which requires the modification of standard CMOS process CVD equipment, and the introduction of H2 as a reaction gas to realize the hydrogen doping process of thin films.

2.3 Different Film Performance Indicators

The indicators are mainly including TCR resistance temperature coefficient, 1/f noise coefficient, resistivity and resistance uniformity.

The thin film of both vanadium oxide and amorphous silicon thin film are semiconductor heat-sensitive thin films, and the TCR of the thin film is directly proportional to the resistivity.

Under the same resistivity condition, the TCR of vanadium oxide film is better than that of amorphous silicon film.

Under the same TCR conditions, the 1/f noise factor of amorphous silicon thin films is 2 orders of magnitude higher than that of vanadium oxide thin films (high noise and poor imaging quality), which seriously restricts the inherent sensitivity and fixed pattern noise of detectors based on amorphous silicon thin films, and this limitation will become more and more obvious as the pixel size shrinks.

2.4 Different Technical Specifications

The sensitivity of vanadium oxide detector can reach 20~30mK, while the sensitivity of amorphous silicon detector is usually around 50mK.

The residual fixed pattern noise of amorphous silicon is large, which is more than an order of magnitude larger than that of vanadium oxide. The specific performance is that the image has a sense of veil, and the infrared image shows not sharp and transparent enough.

2.5 Different Imaging Principle

From the principle of imaging, a single pixel of vanadium oxide is an accurate temperature, while polycrystalline silicon thin films are relatively insensitive to temperature changes due to the characteristics of material growth. With the development of software algorithms, this drawback can be addressed to a certain extent through image algorithm programs. Using image algorithms, the average temperature of N * N (N ≥ 2) pixels is taken as a temperature measurement value. Then provide a simulated artificial value for the adjacent area. If close observation is still possible, sometimes when an object has only a few pixels at a long observation distance, it is often difficult to recognize or analyze errors.

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