Integrated Voltage Controller

Manufacturing of GMR Thin Films

Model-based Control

Guided by the results from the integrated model, careful measurements were made at NVE to detect correlations between the process parameters (performance variables) and the critical device properties, viz, the GMR ratio, saturation magnetic field strength (h_sat), and the sheet resistance (rho_s). The data showed that three of the four variables were relatively well-controlled, with the integrated target bias voltage correlating with variations in wafer hsat, as shown in the figure to the right.

The bias voltage is the DC component of the voltage between the sputter target and ground. Data collected at 200 Hz showed that there was significant variation (>1%) in the bias voltage from layer-to-layer for each wafer, and from wafer-to-wafer. The plot below shows the bias voltage for the three CuAgAu layers in a single wafer. Most of this variation occurs when the plasma is ignited.

A controller was designed and implemented at NVE that compensates for the RF bias voltage fluctuations by adjusting the plasma on-time so as to regulate the time-integrated voltage (for all layers of the same material deposited). This approach keeps the total RF energy input into the plasma constant. A schematic representation of the controller is shown on the top right. The controller computes the time-integral of the voltage in real-time, and shuts off RF power when the reference value of this integral is reached. In effect, one is controlling the cumulative number of atoms deposited on the substrate.

Results obtained with the controller at NVE are shown in the figure below. The plots show the three performance variables (average of measurements at thirteen different locations on each wafer), without and with voltage control.

As a result of voltage control, the standard deviation in the average GMR from wafer-to-wafer was reduced by 65% (down to 0.16% from 0.42%). The standard deviation in the sheet resistance was reduced by 52% (from 0.15W down to 0.07W). The saturation magnetic field strength, hsat, showed a step jump after the nineteenth wafer was processed. The standard deviation of these two groups (wafers 1-19, and wafers 20-27) showed an average decrease of 46% with the group without voltage control. (The step-jump in hsat results from a relatively large change in bias voltage whose effect can be eliminated using a modified form of this controller).

These figures show significant improvement in wafer-to-wafer repeatability resulting from the use of SC’s controller. For further details, including within-wafer thickness uniformity improvement, and our embedded controllers, please visit our website.

SC Solutions provides custom embedded control

SC Solutions designs and delivers high performance controllers for advanced manufacturing processes using modern model-based control design techniques. In addition, SC Solutions provides customized physical models for equipment simulation that assist process modifications and fault detection. We use state-of-the-art automatic code generation tools to produce source code for controller implementation. The resulting code can be rapidly targeted to SC’s embedded controller platform, or can be delivered as a turn-key hardware system with custom hardware, software, and user interfaces.