Keh-Chyang Leou

Real-time control of ion density and ion energy in chlorine inductively coupled plasma etch processing

In this study, we have experimentally demonstrated the real-time closed-loop control of both ion density and ion energy in a chlorine inductively coupled plasma etcher. To measure positive ion density, the trace rare gases-optical emission spectroscopy is used to measure the chlorine positive ion density. An rf voltage probe is adopted to measure the root-mean-square rf voltage on the electrostatic chuck which is linearly dependent on sheath voltage. One actuator is a 13.56 MHz rf generator to drive the inductive coil seated on a ceramic window. The second actuator is also a 13.56 MHz rf generator to power the electrostatic chuck. The closed-loop controller is designed to compensate for process drift, process disturbance, and pilot wafer effect and to minimize steady-state error of plasma parameters. This controller has been used to control the etch process of unpatterned polysilicon. The experimental results showed that the closed-loop control had a better repeatability of plasma parameters compared with open-loop control. The closed-loop control can eliminate the process disturbance resulting from reflected power. In addition, experimental results also demonstrated that closed-loop control has a better reproducibility in etch rate as compared with open-loop control.

Real-time feedback control of electron density in inductively coupled plasmas

The real-time feedback control of electron density was performed in an inductively coupled plasma (ICP). A 36 GHz heterodyne interferometer was adopted as the sensor for electron density measurement. The actuator was rf power which drove the ICP antenna. The results show that the electron density in an ICP system is a type 0 system. Therefore, a proportional-integral controller is necessary to eliminate steady-state error. A prefilter was designed to smooth the desired step change of electron density so that the variation of rf power became mild. A feedforward compensator was added to reduce the disturbance effect resulting from pressure change. The experimental results showed that the control system could quickly track the desired electron density and compensate the electron density variation resulting from pressure disturbance.

Feedback control of chlorine inductively coupled plasma etch processing

Feedback control has been applied to poly-Si etch processing using a chlorine inductively coupled plasma. Since the positive ion flux and ion energy incident upon the wafer surface are the key factors that influence the etch rate, the ion current and the root mean square (rms) rf voltage on the wafer stage, which are measured using an impedance meter connected to the wafer stage, are adopted as the controlled variables to enhance etch rate. The actuators are two 13.56 MHz rf power generators, which adjust ion density and ion energy, respectively. The results of closed-loop control show that the advantages of feedback control can be achieved. For example, with feedback control, etch rate variation under the transient chamber wall condition is reduced roughly by a factor of 2 as compared to the open-loop case. In addition, the capability of the disturbance rejection was also investigated. For a gas pressure variation of 20%, the largest etch rate variation is about 2.4% with closed-loop control as compared ...

Flexible x-ray imaging detector based on direct conversion in amorphous selenium

In this paper, the authors propose a mechanically flexible direct conversion x-ray detector as a potential solution for portable and conformal digital x-ray imaging. It consists of a micropillar structured layer of 100-μm-thick amorphous selenium (a-Se) on a flexible thin film transistor (TFT) backplane with a pixel size of 70u2009μm as a substrate. The flexible substrate is made of an optically transparent polyimide with a heat resistance of more than 200u2009°C. It is bonded on a glass carrier for rigid substrate handling during the amorphous silicon (a-Si) TFT process. Separating the flexible substrate from the glass carrier is partly facilitated by a debonding layer sandwiched between them. A two-dimensional electrical simulation analysis revealed a possible charge generation and collection mechanism within the micropillared a-Se layer. An x-ray image captured by the curved flexible detector indicated that a pillarlike a-Se conversion layer can be used to perform x-ray imaging. This is, to the best of our kno...

Feedback control of plasma electron density and ion energy in an inductively coupled plasma etcher

Here the authors report the development of a fuzzy logic based feedback control of the plasma electron density and ion energy for high density plasma etch process. The plasma electron density was measured using their recently developed transmission line microstrip microwave interferometer mounted on the chamber wall, and the rf voltage was measured by a commercial impedance meter connected to the wafer stage. The actuators were two 13.56 MHz rf power generators which provided the inductively coupled plasma power and bias power, respectively. The control system adopted the fuzzy logic control algorithm to reduce frequent actuator action resulting from measurement noise. The experimental results show that the first wafer effect can be eliminated using closed-loop control for both poly-Si and HfO2 etching. In particular, for the HfO2 etch, the controlled variables in this work were much more effective than the previous one where ion current was controlled, instead of the electron density. However, the pressu...

Feedback control of HfO2 etch processing in inductively coupled Cl2/N2/Ar plasmas

The etch rate of HfO2 etch processing has been feedback controlled in inductively coupled Cl2/N2/Ar plasmas. The ion current and the root mean square rf voltage on the wafer stage, which are measured using a commercial impedance meter connected to the wafer stage, are chosen as controlled variables because the positive-ion flux and ion energy incident upon the wafer surface are the key factors that determine the etch rate. Two 13.56 MHz rf generators are used to adjust the inductively coupled plasma power and bias power which control ion density and ion energy, respectively. The adopted HfO2 etch processing used rather low rf voltage. The ion-current value obtained by the power/voltage method is underestimated, so the neural-network model was developed to assist estimating the correct ion-current value. The experimental results show that the etch-rate variation of the closed-loop control is smaller than that of the open-loop control. However, the first wafer effect cannot be eliminated using closed-loop c...

Measurements of time resolved rf impedance of a pulsed inductively coupled Ar plasma

An rf impedance meter was developed to measure the time resolved rf power and impedance characteristics of a square-wave time modulated inductively coupled Ar plasma. The impedance meter consisted of a voltage/current sensor head and a signal processing circuit unit. The voltage and current sensors were a capacitive voltage divider and a magnetic flux pick-up coil, respectively. Bridge rectifiers were used to determine the voltage and current amplitudes, while the relative phase signal was extracted using a double balanced mixer. The meter was calibrated against a commercial impedance meter under continuous wave operation. A multilayer feedforward neural network was developed for the phase angle estimation, and an accuracy of around ±0.1° was obtained. Measurements of the rf impedance and absorbed power of an inductively coupled Ar plasma under square-wave time modulation reveal that the time dependence of the ion saturation current follows the rf power closely. A spike in the real part of the rf impedance was observed at the beginning of the modulation pulse due to the transition from capacitive to inductive coupling. At a lower modulation frequency about 0.7 ms is needed to attain a steady state compared with cw operation in this experiment.

Development of a ridged microstrip microwave interferometer for plasma electron density measurements

Here we report the development of a microwave interferometer based on a ridged microstrip transmission line structure for the monitoring of plasma density in plasma processing tools. A special ridged shaped microstrip structure with a quartz dielectric is adopted for yielding a large phase shift of the microwave, and thus a higher sensitivity of the interferometer. During operation, the plasma density sensor is installed on the chamber wall where the microstrip transmission line is immersed in plasma and a microwave is launched from one end of the line and exits through the other end. As in conventional microwave interferometers, the plasma density is determined by the phase shift of the microwave propagating through the transmission line. 3D electromagnetic numerical simulations, where plasma is treated as a dielectric medium having a plasma permittivity determined by plasma density and microwave frequency, were employed to determine the phase shift/plasma density relation of this sensor. The sensor is designed to operate at 2.4 GHz microwave frequency, with a compact size and materials that are compatible with most plasma processing tools. Measurement results show that plasma density measured by the sensor, although placed at the chamber wall, does reflect the variations of the plasma density near the chamber center. In the real-time plasma etch process, the dependence of plasma densities on source powers and pressures measured by the sensor is also consistent with the results of ion current on the wafer electrode obtained from an impedance meter.

Effects of in situ N-2 plasma treatment on etch of HfO2 in inductively coupled Cl-2/N-2 plasmas

The etch selectivity of HfO2 to Si reported to date is poor. To improve the selectivity, one needs to either increase the etch rate of HfO2 or decrease the etch rate of Si. In this work, the authors investigate the etch selectivity of HfO2 in Cl2∕N2 plasmas. In particular, the effects of in situ N2 plasma treatment of HfO2 and Si were investigated. The silicon substrate was exposed to nitrogen plasma and was nitrided, which was confirmed by x-ray photoelectron spectroscopy. The nitrided Si etching was suppressed in Cl2∕N2 plasmas. The effectiveness of nitridation was studied with varying the plasma power, bias power, pressure, and N2 plasma exposure time. The results show that the etch resistance increased with increased power and decreased pressure. A minimum exposure time was required to obtain etch resistant property. The applied bias power increased the etch rate of Si substrate, so it should not be used during N2 plasma treatment. Fortunately, the etch rate of HfO2 was increased by the nitridation pr...