Issue



Optical in situ monitoring in LED device production


11/01/2008







The most significant issue in LED MOCVD growth today is precise control of the wafer temperature throughout the whole growth process. As wafer temperature is the key parameter for controlling the final indium content of active layers for LED emission (InGaN multi-quantum wells), even a small deviation results in a shift in emission wavelength of the final LED. Process optimization methods that provide wafer temperature control are described.

As both the demand for high brightness LEDS and number of applications increases, higher throughput and yield are mandatory to satisfy current and future markets. Reactor temperature control and wafer bowing figure prominently in achieving yield.

Temperature control

Currently, reactor temperature is mostly controlled by a thermocouple or a back-side pyrometer. Therefore, the true wafer temperature on the wafer top side might change during the run and go unnoticed by the temperature control loop. Figure 1 gives a typical example of these effects [1]. It shows reflectance and temperature measurements during a typical GaN growth on sapphire. At around 10000s, the reactor pressure is reduced, leading to a drop in wafer temperature (red curve). This remains unnoticed by the back-side light pipe providing the control temperature for the recipe (black curve). The drop in temperature is accompanied by a change in curvature (blue curve, lower graph). While the process temperature is constant during the run, the measured wafer temperature is changing significantly due to changed process conditions (in this case, reduced pressure). Furthermore, run-to-run wafer temperature variations can occur due to different pre-coating conditions of quartz parts in the reactor, or different satellite rotation speeds, which would remain unnoticed without true temperature measurement, and could be fatal for the final LED devices.

Wafer bowing

Wafer bowing also strongly influences the strained growth temperature of GaN on sapphire. Due to lattice mismatch of the two materials, the material experiences tensile strain during growth. During growth of a necessary thick GaN buffer layer, the strain adds up to a significant amount and leads to considerable wafer bowing. The effect increases linearly with the growing buffer thickness (see Fig. 1, bottom part). This wafer bow effect influences the temperature uniformity by introducing a temperature gradient from the wafer’s center to edge. A concave bowed wafer is hotter in the center and cooler at the edge, while a convex bowed wafer is cooler in the center and hotter at the edge. These bowing-related temperature differences across the wafer, especially during thin quantum well layers, limit homogeneity of indium concentration and thereby, limits yield severely. Therefore, the aim of any growth process is to obtain flat wafers during this crucial step of quantum well growth.


Figure 1. Data from the measurement software, showing an InGaN laser test structure grown in an AIX G3 planetary system. The reflectance curves are shown in blue (405nm) and light blue (950nm) in the upper graph.
Click here to enlarge image

Yield improvements are possible by strain engineering, i.e., combining tensile and compressively strained layers and by taking advantage of the materials’ different thermal expansion coefficients.

For a complete process optimization and control of the wafer curvature, both the bow and the temperature must be measured simultaneously in situ throughout the whole LED growth process. Laser deflection measurements combined by emissivity corrected pyrometry (e.g., EpiCurve TT sensors) are an ideal solution to this problem. These laser and LED-based measurement tools measure the wafer curvature, reflectance, and pyrometry in real time during the complete growth and provide the user with all information necessary to obtain flat wafers and optimized yield.

While for 2" wafers the effect of wafer bow is rather moderate, it is becoming a real challenge for 4" and 6" wafers. A wafer curvature of 20km-1 during multi-quantum well (MQW) growth, for instance, results in a 2nm LED wavelength offset on a 2" wafer, and an 8nm wavelength offset on a 4" wafer. As a rough estimate for the impact of wafer bowing on InGaN LED wavelength yield, the following example will help: A bow of 10km-1 will produce a loss of yield of ~3% (assuming a 5nm yield tolerance), while the loss of yield will increase to >10% on a 4" wafer [2]. Therefore, without an in situ curvature monitor, it will not be possible to obtain sufficient yield from a 4" run in today’s LED production, let alone for companies that have already announced 6" projects.


Figure 2. Wafer curvature measurement for two MQW growth runs following an identical recipe. The blue wafer has a significantly higher concave bowing making the wafer center hotter compared to the red wafer.
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With regard to growth-related bowing effects, it was found that sapphire wafers exhibit a different pre-bow. This finding accounts for several wafer-to-wafer and run-to-run variations observed in the past. Figure 2 illustrates how the pre-bow of wafer can be measured in-line by in situ curvature measurements in production runs. Even if taken from the same batch, different sapphire wafers already showed a different bowing at room temperature. This difference in bow remained throughout the whole growth process. As a result, the temperature during InGaN quantum well growth also deviated and thus led to an inhomogeneous emission from wafer to wafer.


Figure 3. The corresponding reactor temperature shows a higher temperature for the red wafer, which was needed to compensate for the lower tempearture due to the different bowing. The objective was to achieve the same wafer temperature during MQW growth. The wafer temperature was controlled by feed-back on the EpiTT true temperature [3].
Click here to enlarge image

To compensate for this inevitable pre-growth bow difference, the bow of each wafer has to be controlled individually. In the future, the reactor temperature will be controlled individually for each wafer to obtain the same wafer temperature during growth. Figure 3 shows the first results of a closed-loop controlled wafer temperature in metal-organic chemical vapor deposition (MOCVD).

Conclusion

Practical application proves that the conventional method of temperature measurements by a thermocouple is not sufficient for growth control because it is not sensitive to changes of growth conditions within the chamber. So, emissivity corrected pyrometry measured in situ is indispensible for accurate growth control in today’s LED production and elsewhere. Furthermore, the wafer bow strongly influences wafer temperature. Temperature differences across the wafer have considerable impact on yield. The combination of emissivity corrected pyrometry and wafer bow measurements has tremendously enhanced yield in LED production.

To fully ensure faultless growth of high-brightness LEDs, it is necessary to measure temperature, reflectance, and curvature simultaneously during the run, as only the combination of all three methods provides the complete information.

Acknowledgments

EpiCurve is a registered trademark of LayTec GmbH. Planetary is a registered trademark of AIXTRON.

References

  1. F. Brunner, A. Knauer, T. Schenk, M. Weyers, J-T. Zettler, “Quantitative Analysis of in situ Wafer Bowing Measurements for III-Nitride Growth on Sapphire,” Jour. of Crystal Growth 310, pp. 2432 ??? 2438, 2008.
  2. Presentation of S. Srinivasan et al. during China SSL Forum 2007.
  3. A. Knauer,1, T. Kolbe, S. Einfeldt, M. Weyers, M. Kneissl, T. Zettler, “Optimization of InGaN/(In,Al,Ga)N-based Near UV-LEDs by MQW Strain Balancing with in situ Wafer Bow Sensor,” Physica status solidi, Proc. 3rd International Symposium on Semiconductor Light Emitting Devices (ISSLED) Phoenix, USA, April 27-May 2, 2008.

Kolja Haberland received his PhD in physics at the Technical U. Berlin, and is a co-founder and senior manager at LayTec GmbH, Helmholtzstr. 13-14, Berlin, Germany; ph.: +49 30 39 800 800; email [email protected].