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Powering Production For The Future

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The manufacturing world of semiconductors has seen great change with emerging opportunities and technology arriving every day. Many issues drive the manufacturing challenges and companies look at every part of the process to improve performance, cost and yields. Maintaining an accurate power supply during manufacturing has become a key issue and Dr. Dirk Ochs and Dr. Thomas Rettich of Huettinger Elektronik in Germany discuss application needs for power stability.
There is a strong demand in semiconductor and flat panel display industry for decreasing production costs. This is realised on the one side by increasing the substrate size. In flat panel display production glass sizes of more than two by two meters are becoming a standard, while in semiconductor processing wafer size has been increased to 300mm diameter and a further increase to 450mm diameter is under discussion. The scaling in substrate size has a direct consequence on the related process power used for deposition and etching processes. On the other side for semiconductor applications as well as for thin film transistor displays an ongoing shrinking of the structural elements leads to the requirement of better film uniformity, a minimum amount of defects and a more and more precise control of the sputter power. The defect size is directly proportional to the remaining arc energy delivered by the power supply. For this reason a main demand for the next generation power supply is an arc energy as low as possible. Especially for RF power supplies another very important feature is a stable power delivery even at very low output power down to 50W or less to fit today's demands for etching processes.

Flat Panel Displays can be divided into passive and active matrix displays. In a passive matrix display the visible information represented by a row of pixels fades during the period of time needed to address all the other rows in the display. A non-flickering image requires a balance of the display fade rate and the persistence of vision in the human eye. In an active matrix display each pixel is connected to its corresponding row and column electrodes by additional transistors keeping the pixels on or off, even when the row in which the pixel resides is not being addressed. Flickering is not a problem since the row of pixels remains static. An example of a layer stack of a thin film transistor needed for active matrix displays is shown in fig. 1. Plasma deposited layers for display applications are PECVD deposited amorphous silicon (a-Si), micro-crystalline silicon (µc-Si), SiO2 (buffer layer), SiNx (insulator), and sputter deposited ITO and metallic layers as Al for gate and contact layers. The PECVD deposition is done using standard RF power while for sputtering often MF or DC power supplies are used.

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In the case of semiconductor processing plasma processes are used for etching and deposition. Main etching applications are reactive ion etching of Si3N4 and SiO2 as well as plasma etching of Al, W, Si, GaAs, InP and polyamide layers.

Another trend in semiconductor etching is the use of dual frequency capacitive discharges i.e. a high frequency RF source and a low frequency bias source. This enables the possibility to separately control the ion density and the ion energy. RF powered PECVD deposition is used for a-Si, µc-Si and dielectric layers as SiO2, Si3N3, SiC. Metallic layers as Cu, Al, W, Mo, NiV, Ti, TiW and several silicides are sputter deposited using DC power supplies.

RF systems with an output power of up to 50kW, MF systems with up to 150kW and DC systems with up to 120kW are needed to meet the requirements of Semiconductor and Flat Panel Display production.

All generators need a high precision process control and a supreme arc management with adaptable parameters to provide minimal disturbances in the plasma process and to obtain optimised results in terms of film quality, homogeneity and optical properties of the deposited film or during an etching process.

DC and MF coating processes
Magnetron sputtering is used for the deposition of several layers of the display layer stack . An important application is the coating withtransparent conductive oxides as ITO. This is done using DC or MF power supplies. Also metallic contact or gate layers as for example Al are standard magnetron sputtered using DC processes. In the case of semiconductor application metallic layers as Cu, Al, W, Mo, NiV, Ti, TiW and several silicides as WSi2, TiSi2, MoSi2 and TaSi2 are sputter deposited using DC power supplies. For all these layers a good thickness homogeneity of the deposited layers at high deposition rates is very important.

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To improve the power supply performance and the cost situation a new concept for a modular DC system has been realised. The system is divided in modules that are capable of 20 or 30 kW output power each (fig. 2). The modules have a compact size, are air or water cooled and can be combined and stacked (master/slave concept). In that way a max generator power of 120kW can be realised. The output voltage is orientated at the various target materials and has a range of 400-800 V at full rated power of 30 kW. With a high power factor of 0.9 and an efficiency of >90% this concept meets requirements of modern power regulations. To ensure the ignition of the plasma an ignition voltage source is integrated to provide a voltage of >1400 V.

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Fig.3 shows the concept of the power supply with the power conversion stage and the output module with the ignition and arc handling unit. The arc management and control of the DC module is a very important part for plasma applications and has an intelligent control. Recipes and parameter sets can be set up by the external control via DeviceNet or Profibus. The arc detection has a fast response (<1µs) and the arc delay time is adjustable in a broad range (<1µs up to 100 ms). To have a high flexibility the modules can be combined in a master/slave concept to increase the output power.

In the case of a MF power supply the fast and effective arc management is an even more important tool for keeping reactive processes stable. With an arc cycle time of below one millisecond, the disturbance in the gas concentration is negligibly small. MF generators in the BIG 200 P range, which are equipped with ARCtelligent, can minimise the arc cycle times to an average of 0.2 milliseconds at nominal power. Because of the more sophisticated MF process, the arc management offers more parameters than with DC sputtering. Sets of these parameters can be used as presets in recipes.

ARCtelligent makes a significant contribution to the economy of the systems, with both DC and MF processes. The fast and flexible arc handling leads to an improvement in the film quality by minimising flaws and defects. Beside this very high sputter rates can be achieved because of the high process stability, which leads to an increase in productivity.

The minimisation of the residual energy to only around 12 mJ per kilowatt with the BIG 200 P model yields additional advantages. Even when the cathode material is almost used up and consequently the arc probability increases, the low residual energy values contribute to an extended service life of the cathode and to better utilisation of the target material.

Essentially, the outstanding feature of effective arc management is an undisturbed plasma operation. The fast arc suppression minimises the interruption in the sputtering process to negligibly small values. However, because arcs also serve to remove unwanted materials on the cathode, power supplies must be able to control and regulate the arc energy precisely. Fig. 4 shows a picture of a 150kW MF power supply BIG 150P.

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RF coating processes
RF powered PECVD deposition is used for a-Si, µc-Si and dielectric layers as SiO2, Si3N3, SiC. Etching applications using RF power are reactive ion etching of Si3N4 and SiO2 as well as plasma etching of Al, W, Si, GaAs, InP and polyamide layers. The typical RF operating frequency is 13.56 MHz but also higher frequencies are under investigation. Used process power ranges from 50W for some semiconductor etching application to about 50kW for display application.

For the low energy range a fully transistorised and µC-controlled RF generator was developed. This generator has been designed for a max. power of 3kW.

Important features are the compact size, robustness, reliability and easy maintenance. Protection against mismatches and overload is provided by over dimensioning in conjunction with electronic monitoring and protection circuits. The generator can be used in synchronous operation mode using an external master oscillator as well as the generator can serve as a master oscillator. Fig. 5 shows the panel and the compact size version of the 2kW generator (Qinto 2kW).

The 3kW generator has the same size. To have a cost effective design for a RF generator in the high power range an oscillator - amplifier concept has been chosen with a solid state driver in combination with a tube type endstage amplifier (fig.6). Starting from 1 µW the signal is modulated for power control and pulsing functionality and then gets to the chain of amplifiers where the power of the modulated signal is increased in the pre-amplifier to a level of 100 mW and in the driver stage to a level of 4 kW. Finally, the robust tube type end stage creates the output power level of 50 kW. The output power, which is delivered into the 50 Ohm coaxial cable is controlled and the incident and reflected part is measured by the directional coupler.

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This system has a compact size to be easily integrated in a system. The height of the cabinet is only 1600 mm so the RF generator can be installed below the chamber (fig.7) The impedance of the cathode varies with the process in the range of 1-5 Ohms with a strong capacitive part. In contrast, the output impedance of the generator has 50 Ohms. In order to deliver the maximum output power of the generator to the chamber an impedance transformation network is needed: the matchbox.

The impedance of the cathode is matched to the 50 Ohms by a network of three elements as shown in fig.8. To compensate the varying impedance of the process two of the three elements are variable by motor controlled vacuum capacitors. The resulting impedance of the matchbox and process parameters like the RF peak voltage and the DC bias voltage are measured and delivered to the controller inside the matchbox. These signals are also transferred to the generator that acts as a system controller.

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By these means even critical processes can be ignited and stabilised for long cycle times. Depending on the need of the process the system can operate in cw mode as well as in pulsed mode. For the various semiconductor and flat panel display applications high precision DC, MF and RF generators are available which cover the broad process power range and meet the specific process related technical and economical requirements. These are a high precision process control and a supreme arc management with adaptable parameters to provide minimal disturbances in the plasma process and to obtain optimised results in terms of film quality, homogeneity and uniformity over the whole substrate.

Also the stable operation at very low process power is an important feature of the generators to meet the requirements for several applications.


REFERENCES
[1] R. Waser, Nanoelectronics and Information Technology, Wiley-VCH 2003, ISBN 3-527-40363-9
[1] C. May, J. Strümpfel, D. Schulze, 43rd Annual Technical Conference Proceedings (2000) p 137-142
[1] M. Armacost et al., IBM Journal of Research and Development 43 No. 1/2 (1999) 39-72
[1] A. Elshabini-Riad, F.D. Barlow, Thin Film Technology Handbook, McGraw-Hill 1997, ISBN 0-07-019025-9
[1] P. van Zant, Microchip Fabrication, Mc Graw-Hill, 2000, ISBN 0-07-135636-3, p 263-274
[1] M.A. Lieberman, XXVIIth ICPIG, Eindhoven, the Netherlands, 18-22 July 2005, p 6-9
[1] I. Sakai, H. Hayashi, T. Ohiwa, SEMI Technology Symp. Japan 2005, p 3/19 - 3/20
[1] Y. Kuo, K. Okajima, M. Takeichi, IBM Journal of Research and Development 43 No. 1/2(1999) 73-88
[1] P. van Zant, Microchip Fabrication, Mc Graw-Hill, 2000, ISBN 0-07-135636-3, p 407-423
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