Limits of In-Situ Reflectance for growth rate extraction in Aixtron Multi-Wafer Reactors

I really like this project, it was long awaited opportunity to work with excellent AMS-OSRAM team in Singapore, on newest Aixtron G4 planetary reactor, with state of the art in-situ LayTec Epi-Curve. At that moment LayTec VCSELs addons were not available. The project still has one feature to be added - curvature of 6" GaAs substrate influence to FP-dip - initial data analysis indicate, that this should allow improve AI model significantly.
To be updated when will find time and opportunity..

original PDF Poster

1 Introduction

Currently, Aixtron Planetary MOCVD Reactor Aix 2800 G4 is main flagship of compound photonics. The Planetary reactor concept was polished over the many years of development, and multiple generations of Aix 2000, Aix 2400 G1, Aix 2600 G2, Aix 2600 G3, and Aix 2800 G4. In-situ monitoring tools significantly contributed to the quick development of nitride materials, and offer indespensable support in development of some challenging devices like VCSELs or QCLs.
Inherent feature of the slow susceptor (planet) rotation in contemporary multi-wafer Aixtron reactors (both Planet and Showerhead), is that in-situ monitoring tools could only probe wafers for limited time, when they are passing under viewport. This have significant consequences in ability to resolve and monitor growth of thin layers and seeing effects of gas switching and layer interfaces.
This is why single wafer reactors like horizontal Aix-200 (15 or more years old, are still used for R&D) or VEECO Propel reactors are ideal platforms for process and product development.

Today‘s advanced requirements e.g. for VCSEL emission wavelength (940 ± 1)nm:
Growth rate = 0.9 ± 0.0009 nm/s = ~±0.1%

Is it possible to achieve such growth rate accuracy from in situ monitoring of thin layers in planetary reactor?

2 Simulations

Theoretical simulations of in situ reflectance of VCSEL’s DBR growth in 8x6” planetary reactor were used as test case for analyzing sensitivity of reflectance fitting algorithm to noise, wafer to wafer non-uniformity, and ability of algorithm to discover true growth rate changes during process.

The time of test layer growth in DBR was set to 60s, planet rotation speed 7.5s, which give 8 data points for single layer for each wafer, 50 repetitions of layer. Reflectance fitting algorithm is perfect, but for such small number of data points, noise became a big factor in growth rate fitting. 5 parameters of generated layer reflectance profiles are given below[1]:
n=4.5, k=1.85, GrR=0.9 nm/s, Ri=9e-3, Sigm=3.3

Fig. 1a) Idea of Merge Line analysis (Laytec). Fig 1b) Example of generated reflectance profile for 4 wafers.

Multiple fittings with varying number of Merge Line wafers, and range of noise SDEV amounts, allowed to obtain fig. 2, which shows relation between noise SDEV and Growth Rate SDEV.
Assume 4x wafer Merge Line, for example with SDEV of noise 4e-4, the growth rate SDEV in simulated case is 4e-3 nm/s. Assuming, that S/N=3 is needed to be able to reveal true growth rate from noise, then it is possible to discern variations of 1.2e-2 nm/s which with growth rate of 0.9nm/s give only 1.33%, quite far away from desired 0.1% (for 4x wafers Merge Line).
Situation get better if layer is repeated multiple times (fig. 3), then noise get averaged \(\sqrt n\), but reaching 0.1% takes 150 repeats.

Fig. 2 SDEV of noise vs SDEV of Growth Rate.

The uncertainty of gr rate extraction affect Feed Forward recipe correction, as illustrated on Fig. 3. If we would extract GrR from single wafer, without Merge Line, then 10 and 20 periods are not enough to reveal true growth rate drift (Fig. 3 left).