EUV

Mauricio M. Ortega

When it comes to Physics, there is definitely a wide array of topics that come to mind. We’ve learned so much this semester and there’s still so much more to learn. I, personally, am interested a lot in radiation; so I set myself to find out how it’s being used in today’s world to better our technology. The answer was lithography. Optical exposure technology has already reached its limit. Extreme Ultraviolet lithography will hopefully take over, becoming the leading contender, establishing the new generation of lithography. One critical problem is to develop a source-emitting radiation with high power and life time at a short wavelength such as 13 to 14 nanometers. The production of EUV radiation includes two possible processes, the laser-produced plasma and the discharge-produced plasma. Oxygen, Lithium, Tin and Xenon were considered for the development of the source, but due to Xenon’s low conversion efficiency we might be able to produce a higher quantity of photons. Laser-Produced Plasma is focused on a Xenon beam which also presents some problems, for example, it is required an exact pulse duration with an exact power output. Additionally the difficulty to control this process is very high. Due to these LPP issues, interest in DPP has increased. But the biggest advantage of DPP it has to be its simplicity to control and its cost-effectiveness. With Z-pinch discharge the magnetic field compresses the plasma to a capillary of 3mm and 5mm diameter, for later production of EUV radiation. Xenon will emit EUV, which will flow through the discharge tubes of 3mm and 5mm. 

The Z-pinch plasma is characterized by measurement of its temporal intensity. The discharge current is detected by a pick-up coil. The energy for this process will be provided by a magnetic pulse compressor exceeding a 90% of energy efficiency. The EUV energy is measured with a monitor that consists of a filters and diode tools. A pinhole camera, filter, a multichannel plate-image intensifier and a digital camera will be used to record time-resolved images of the discharge plasma around the EUV wavelength. The Z-pinch design and circuit pulsed-power excitation parameters will be optimized to achieve the required characteristics for the source-emitting radiation needed in the first place.

In today’s market, between laser-produced plasmas (LPP) and gas discharge-produced plasma (DPP), DPP is currently the most powerful EUV source. Despite its superiority to LPP, DPP still faces problems such as the thermal load on the components. This particular problem causes the degradation of the lifetime of the components, such as the electrodes, the insulator wall, and the light-handling mirrors. This problem was solved by shortening the discharge current pulses, reducing thermal problems and improving conversion efficiency from the discharge energy to the in-band EUV emission power (CE).

As for the monitoring of the DPP, the temporal development of the relative EUV emission intensity is monitored by a fast PIN photodiode (Hamamatsu, 3883, 300MHz) covered with a 150-nm-thick Zr filter Luxel). The intensity of the EUV radiation depends on the Xenon flow rate. These two are directly proportional. However, at a too large flow rate, EUV intensity tends to decrease and maximum emission is delayed. Moreover, the vacuum level in the discharge chamber changes with the xenon gas flow rate which absorbs, partially, the EUV radiation. Now, EUV emissions not only strongly depends upon discharge current (amplitude, duration), but also on the filling-gas pressure or xenon gas flow rate delivered to the discharge chamber. The emission spectrum of the radiation was measured with a reflection grating spectrograph including a flat-field grating and a charge-coupled device detector. For the amplitude of the current and the Xenon flow rate is influenced by the temperature. We find proof for this in several studies on the EUV spectra of DPPs reported that the 4d-4f transition of Xe¬11+ becomes stronger for higher electron temperature.

Lastly, and in general, the EUV emission energy from these pinched plasmas tends to be proportional to the square of the peak current amplitude. On the other hand, the increase in electrical energy input to the discharge causes the ablation of electrodes and insulator wall, which results in the generation of large amount of debris and the impurity contamination in the target plasmas. In order to increase EUV power without increasing energy input, a key issue is to improve the conversion efficiency for the discharge energy to the in-band CE. One is to use tin or lithium plasmas. The other is to optimize the plasma temperature and density for the 13.5 nm emission. The formation process of the high-energy-density plasmas in a DPP scheme is flexible compared with an LPP scheme and is dependent on the current waveform, electrode geometry, and the target gas, gas pressure, and gas initial ionization state.

So what do we make out of all of this? EUV is evidently the way of the future as far as lithography, for the time being. The ability to transfer a geometric pattern from a photomask to a light-sensitive chemical "photoresist", or simply "resist," on the substrate is a priceless for circuits. However, the smaller we can make these patterns, the better, and the more efficient. That’s what EUV is bringing to the table, the ability to do this at an even smaller scale than previous technologies such as photolithography, and allow us to create smaller electronics. Paving the way for nanotechnology.

 

Works Cited 

Zhang, C. H., P. Lv, Y. P. Zhao, Q. Wang, S. Katsuki, T. Namihira, H. Horta, H. Imamura, Y. Kondo, and H. Akiyama. "Xenon Discharge-Produced Plasma Radiation Source for EUV Lithography." IEEE Transactions on Industry Applications 46.4 (2010): 1661-666. Print.