Gen3: new features of the next in-situ generation
Starting in 2016, LayTec offers a new generation of its proven and market leading in-situ metrology systems EpiTT, Pyro 400 and EpiCurve®TT. This 3rd generation offers a lot of significant technology advances and is more flexible in customization to the needs of specific epitaxial processes. What is new in Gen3?
The backbone of the metrology’s hardware and software is modularity. The new concept offers a much wider range of process specific customizations without compromising the robust and accurate performance which our products are known for. Furthermore, the 24/7 operation is improved by separating data acquisition (based on ARM processors) from metrology control and analysis (based on MS Windows PC).
Gen3 offers a significantly extended choice of process interfaces, e.g., SECS/GEM for communication with MES systems and Modbus for RIBER’s latest Crystal XE software for MBE.
The real-time and post-growth data analysis functionality has been further improved. Several completely new hardware components can be now combined with well proven working-horse modules that, of course, have been integrated into the new Gen3 platform, too.
Here are some application examples:
High resolution wafer bow measurements for CCS reactors with EpiCurve®TT Gen3
Detecting thin-film strain in-situ during epi growth through the tiny openings of the showerhead view-ports is a challenge. However, with our advanced software algorithms we have further improved the signal-to-noise ratio of EpiCurve®TT by a full order of magnitude.
The wafer bow data in Fig. 1 (green line) shows that the wafer curvature noise in this Close Coupled Showerhead® (CCS) reactor is now down to 0.3 km-1. With this improvement, in-situ strain balancing or AlGaN lattice constant tuning (Fig. 1, strain changes from compressive towards tensile during AlGaN growth at ~1000 s) is now possible with accuracy levels formerly known only for ex-situ XRD methods.
Fig. 1: Growth of AlN/AlGaN(60%Al) on a Sapphire/AlN template in Aixtron CCS 6x2 reactor (UV-C LED): black – 280 nm reflectance; blue – 405 nm reflectance; green – high-resolution wafer bow; red – true temperature.
Emissivity corrected GaN temperature with Pyro 400 Gen3
Ultra-violet (UV) pyrometers like Pyro 400 have to collect very low intensity thermal emission in the UV region from the GaN buffer layers. Until recently, emissivity correction was not possible for low noise UV temperature measurement and Fabry-Perot oscillations (FPOs) in the temperature signal were unavoidable. However, LayTec’s new Pyro 400 Gen3 is now the first UV pyrometer with full emissivity correction. It uses a separate UV broad-band reflection for emissivity correction and the three narrow-band reflectance channels of EpiTT (405/633/950 nm) remain in full operation. The system is now available for Aixtron G4/G5 planetary reactors and Veeco Epik 700 reactors. Fig. 1 shows a "stress test" on a Veeco Epik 700 reactor of a leading LED manufacturer
Fig. 1: Pyro 400 Gen3 on Veeco Epik 700: (1) – UV reflectance (broad band) for Emissivity Corrected Pyrometry (ECP); (2) – Pyro 400 raw data with FPOs; (3) – Pyro 400 Gen3 temperature signal with basically no remaining FPO artifacts in the temperature signal.
The measurement in Fig. 1 shows that maximum FPOs were generated by growing a 40 nm AlN marker layer on the GaN buffer layer with subsequent overgrowth by a GaN cap layer. This caused large FPOs both in UV reflectance and thermal UV emission during GaN cap layer growth. These FPO artifacts are fully removed by Pyro 400 Gen3. .
UV LED challenges under control
EpiTT Gen3 and EpiCurve®TT Gen3 families of our products offer a lot of advantages for UV LED related MOCVD processes:
- Emissivity corrected pyrometry for high temperature AlN/AlGaN processes up to 1500°C
- 3 wavelength reflectance (405 nm, 633 nm, 950 nm) with improved short wavelength reflection measurement for precise AlN/AlGaN analysis and for monitoring of highly doped AlGaN superlattices
- Parallel beam heads (PBH) to overcome the off-focus situation, e.g., when the gap between the susceptor and the showerhead is changed (see below)
- TT 950 pocket temperature configuration for ssp (single side polished) and dsp (double side polished) saphire as well aspss (patterned sapphire substrates)
- EpiNet software with an improved material database for the complete UV LED layer stack and different substrates, e.g. AlGaN and AlInGaN material classes, double-side polished (dsp) sapphire and many more.
You will find UV LED related application examples here
Accurate temperature for pss and double-side polished sapphire
Patterned sapphire substrates (pss) and double-side polished (dsp) sapphire often cause unrecognized artifacts in temperature sensing. EpiTT Gen3 facilitates emissivity correcting effects as well as straylight scattering/defracting effects of the wafer’s backside/frontside and of pss structures. Since these substrate specifics are often used in UV LED epitaxy, we show some temperature data on the UV LED page of our website
Overcoming the wafer-showerhead gap variation in UV LED epitaxy
In some specific application (e.g. UV LED epitaxy), the gap between the susceptor and the showerhead is varied during the process. Gen3 tools do not suffer from such off-focus situation due to their parallel beam heads and, therefore, provide an extremely stable reflectance signal without a complex multi-gap calibration. Find more on our UV LED page