This article discusses the role of rapid heat treatment (RTP) in the semiconductor manufacturing process.
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What is RTP?
RTP is a semiconductor fabrication technique in which silicon wafers are heated to temperatures above 1000 ohC using lasers or high intensity lamps for a few seconds. During wafer cooling, wafer temperatures are gradually reduced to prevent wafer breakage and dislocations due to thermal shock.
RTP is more efficient than conventional furnace annealing because it produces higher silicides and oxides and reduces thermal budget/integral of temperature and time. Other advantages of RTP include lower manufacturing cost, ease of process development, high throughput, and process consistency.
An RTP system is typically used to process silicon wafers to manufacture semiconductors for high-speed computing applications and computing devices. Single wafer processing produces the best uniformity, especially for large wafer sizes.
RTP systems perform heat-related manufacturing steps such as chemical vapor deposition, thin dielectric film formation, and annealing. Annealing is used to add impurities to the semiconductor by thermally activated diffusion.
Importance of Efficient RTP Systems in Semiconductor Manufacturing
A nearly uniform temperature distribution must be maintained across the wafer during the transient and steady state situations of the annealing process to achieve uniform resistivity and conductivity throughout the wafer. In addition, the uniform temperature distribution must be obtained for different operating conditions, including different gases, pressures and process temperatures.
An RTP system must be able to alter the spatial distribution of radiant energy flux to the wafer to maintain temperature uniformity under different process conditions, since wafer temperature uniformity requirements vary with different Working conditions.
An RTP system with several independently controlled concentric circular rings of lamps, such as Stanford’s Rapid Thermal Multiprocessor (RTM), can be used to meet this requirement.
In the Stanford RTM, an automatic control strategy is used to control the power of each of the three zones of the lamp to achieve a uniform temperature across the wafer under transient and steady state conditions. The control strategy uses a multi-point sensor reading to provide real-time temperature distribution measurement.
In an article published in SPIE Acts for Rapid Thermal and Integrated Treatmentthe researchers derived a first-principle lower-order model of semiconductor wafer RTPs and experimentally verified the model at torr pressure and an operating temperature range of 400 ohC to 900 ohC in an inert nitrogen medium.
Researchers also developed an automatic multivariable controller for multipoint sensor and multizone lamp RTP systems and applied it to RTM. In the real-time multivariable controller, a look-ahead mechanism was used to predict temperature transients and a feedback mechanism was used to correct prediction errors.
Ease of implementation and identification of predictive applications in temperature control and real-time signal processing was the main advantage of the lower-order model over the detailed model. Nonlinear effects of temperature were demonstrated to validate the model.
The controller was successfully applied to achieve a ramp of 20 ohC to 900 ohC at age 45 ohC/second with less than 15 ohC non-uniformity during the ramp and less than 1 ohC average non-uniformity during hold at 900 ohC for five minutes.
Controller performance was satisfactory in the presence of several challenges, including system nonlinearities, sensor noise, actuator saturation, slow disturbances, and large delays. Thus, the results demonstrated that the automatic multivariable controller can help achieve wafer temperature uniformity under different process conditions in an RTP system.
In another study published in the Mathematics in Industry Case Study Review, researchers derived a model of radiative heat transfer that occurs in an axially symmetric RTP chamber using form factor theory and used the model to predict the influence of chamber materials and geometry on the temperature uniformity of the silicon wafer.
A series of numerical experiments were performed to predict the effects of shower head reflectivity and size, chamber height, and guard ring on temperature uniformity across the wafer.
The results showed improved wafer temperature uniformity when the showerhead radius was the same or greater than the outer radius of the guard ring. However, the size of the chamber should be less than 300mm in diameter to ensure that the radius of the shower head is smaller than the radius of the chamber.
Similarly, temperature uniformity across the wafer increased with increasing guard ring radius, indicating the effectiveness of larger rings in achieving uniform temperature. However, guard rings should be less than 1 inch wide for practicality, as larger rings require more power to stay warm, making the process expensive.
Lower reflectivity led to a more uniform temperature across the wafer. However, some reflectivity was helpful in minimizing the amount of power required from the lamp to reach the maximum temperatures. The best temperature uniformity was achieved in the smallest chamber height.
The optimal control of rapid thermal annealing (RTA) is used to fabricate the ultra-shallow junctions required in microelectronic devices. In a study published in the Process Control Logthe researchers designed a state-of-the-art annealing program that can optimize junction depth by limiting sheet resistance.
The study demonstrated that the optimal RTA program can minimize transient enhanced diffusion (TED) while achieving the desired sheet strength consisting of fast linear cooling and heating profiles.
Model-based optimal control directly calculated the maximum annealing temperature and avoided the commonly used heuristic and trial-and-error approach, reducing the cost of identifying the optimal annealing schedule and the number of experiments.
Observations from worst-case analysis of optimal junction depth indicated the need to improve existing RTA controllers and metrology to minimize control implementation inaccuracies.
Non-thermal equilibrium conditions in RTP make modeling and prediction difficult, while absolute temperatures remain unknown. Additionally, uniform heating is extremely critical in RTP compared to traditional baking due to the high ramp rates that lead to stress.
Recent studies involving RTP
In a study published in the journal Advanced functional materialsresearchers have demonstrated a new method for making kusachiite (CuBi2O4) photoelectrodes with improved photo-electrochemical stability and charge separation.
Dibismuth trioxide (Bi2O3) and copper oxide (CuO) layers were sequentially deposited on fluorine-doped tin oxide (FTO) substrates using pulsed laser deposition (PLD), followed by RTP for 10 min at 650 ohC to obtain highly crystalline and phase-pure CuBi2O4 movies.
The combined PLD and RTP approach allowed excellent control of the Bi:Cu stoichiometry, resulting in the synthesis of CuBi2O4 photoelectrodes with superior electronic properties compared to photoelectrodes obtained by spray pyrolysis.
Uncoated CuBi synthesized2O4 the photoelectrodes showed only a 26% reduction in photocurrent after five hours, which represented the highest stability of this material reported so far. The results demonstrated that the RTP/PLD fabrication approach offered new possibilities to fabricate highly crystalline complex metal oxide photoelectrodes with good electronic properties at higher temperatures.
In summary, RTP has become indispensable in semiconductor manufacturing as it satisfies both production and device requirements, and the technique will contribute to future advances in microelectronics technologies.
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References and further reading
Saraswat, K., Kailath, T., Franklin, G., Boyd, S., Balemi, S., Hoffmann, G., Gyugyi, P., Norman, S., Park, P., Cho, Y., Schaper, C. (1991). Modeling and control of rapid heat treatments. SPIE Acts for Rapid Thermal and Integrated Treatment. DOI: 10.1117/12.56658. https://www.researchgate.net/publication/234065612_Modeling_and_Control_of_Rapid_Thermal_Processing
Wacher, A., Seymour, BR (2011). A radiation model of a rapid heat treatment system. Mathematics in Industry Case Study Review3, 1-18. http://www.fields.utoronto.ca/journalarchive/mics/35-32.pdf
Gunawan, R., Jung, MYL, Seebauer, EG, Braatz, RD (2004) Optimal control of rapid thermal annealing in a semiconductor process. Process Control Log14, 4, 423-430. https://doi.org/10.1016/j.jprocont.2003.07.005
Rapid heat treatment. [Online]. https://alan.ece.gatech.edu/ECE6450/Lectures/ECE6450L6-Rapid%20Thermal%20Processing.pdf (Accessed June 26, 2022)
Gottesman, R., Song, A., Levine, I., Krause, M., Islam, ATMN, Abou-Ras, D., Dittrich, T., van de Krol, R., Chemseddine, A. (2020). pure CuBi2O4 Photoelectrodes with increased stability by rapid heat treatment of Bi2O3/CuO grown by pulsed laser deposition. Advanced functional materials. https://doi.org/10.1002/adfm.201910832