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Laytec introduces in-situ monitor and software just for PSS

Equipped for wafer bow control, Laytec's latest EpiTT system incorporates a blue laser and triple wavelength reflectance for the precise monitoring of AlN interlayers, AlGaN buffer layers and multiple quantum wells
A major challenge of in-situ metrology on single-port reactors with small viewport geometries is the combination of curvature measurements by a blue laser with reflectance measurement at 405nm.

LayTec believes a blue laser is a must for patterned sapphire substrates (PSS) and double-side polished substrates.

Sapphire is a commonly used substrate for LED growth. But it is now coming up against cheaper alternatives such as silicon. However, when it comes to growing compound semiconductors on silicon, the knowledge is still rather lacking.

Which is where the latest LayTec in-situ tool could come in useful.

The firm's new version of the EpiCurve TT system, shown in Figure 1 below, is suitable for monitoring the growth of III-nitrides on both silicon and sapphire substrates.

What's more, LayTec says the 405nm reflectance is indispensable for monitoring of InGaN MQW growth. And until now, it was impossible to have both features for reactors with only one small optical access because of the cross-talk effect.

Cross-talk is the capacitive and inductive coupling of signals from one signal line to another. As system performance and board densities increase, so does the problem of cross-talk.

LayTec claims the new optical and electronic design of EpiCurve TT eliminates this problem. The latest version of the tool has been installed on an Aixtron 200-4 RF/S reactor with just a 5mm hole in the ceiling at Otto-von-Guericke University of Magdeburg in Germany.

Figure 1: EpiCurve TT used in Magdeburg, Germany
The team of Alois Krost and Armin Dadgar uses the tool in development projects for in-situ monitoring of various GaN based optoelectronic and power electronic device structures on silicon and sapphire substrates.

This Epi-Curve TT is equipped with a blue laser (405nm) for wafer bow control and a triple wavelength reflectance (405, 633 and 950nm) for a precise monitoring of MQW layers, AlN interlayers, AlGaN buffer and further features.

After several years of experience with LayTec systems, Krost is convinced that “EpiCurve TT is the best in-situ tool available on the market to control strain, temperature uniformity, MQW formation and surface morphology during III-N device growth."

In the future, LayTec believes the use of PSS will further increase due to its high light extraction efficiency.

To address this issue, LayTec‘s in-situ metrology software EpiNet 2 can be individually customised for various kinds of PSS. And users can expand the substrate database themselves.

Once the initial reflectance values of the PSS substrates are uploaded, the operator can choose the needed substrate in the RunType‘s Material Spec window, as depicted in Figure 2. As a result, all PSS wafers can be monitored with the same accuracy as standard sapphire substrates.

Figure 2: Extract of a Run-Type’s Material Spec window of LayTec‘s EpiNet 2 software with a customised database extended by additional PSS types.
Figure 3 below shows a GaN growth on PSS. The initial 405nm reflectance on the bare PSS substrate is noisy because it senses the local non-uniformity of the PSS structure. As soon as the GaN buffer is thick enough, this noise reduces because the 405nm light does not reach the PSS pattern anymore through the UV absorbing GaN.


Figure 3: GaN growth on PSS measured by EpiCurve TT at FBH, Berlin, Germany: reflectance at 405 nm (blue), 633 nm (green), 950 nm (red); True Temperature (black).
The increasing reflectance of all 3 wavelengths after ~4000s shows the coalescence process of GaN.
After ~8050s, the 405 nm reflectance stays stable at 15.5 percent, which is an indication of the GaN surface quality improvement. However, the interference patterns of the 633/950nm reflectance look noisy during 2D buffer growth. The initial non-uniformity of the PSS structure causes a certain non-uniformity in the GaN thickness.

Along with the customised database, EpiNet 2 also provides fitting of thin layers, wafer bow calculation and many further features indispensable for growth on PSS.
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