Hybrid AFM / STM Lithography
We have developed a new scanning probe lithography system based on the electron exposure of an organic resist. Called the hybrid AFM / STM lithography system, it combines the key features of the atomic force microscope (AFM) and the scanning tunneling microscope (STM) by incorporating two independent feedback loops.
Force Feedback. One loop maintains a constant small force (typically 10 nN) between the tip and the electron-sensitive resist, thereby minimizing the beam spreading that limits the resolution of STM lithography. The force is detected by monitoring the cantilever deflection with a conventional optical lever beam-bounce system. A signal is sent to the piezo tube actuator to move the probe up or down to maintain the setpoint force.
Current Control. The other feedback loop maintains a constant field emission current through the resist during exposure (generally in the range of 20 pA to 1 nA). The current through the resist is measured with picoAmpere sensitivity using a commercial current preamplifier. The current is measured at the tip for lowest noise operation. The feedback circuit compares the measured current to the setpoint current. A high voltage amplifier applies the necessary voltage to the sample (generally 20-100 V) to maintain the setpoint current. Using this system, we demonstrate a minimum patterning resolution below 50 nm, nanometer alignment capabilities, and scan speeds above 1 mm/s. This system also enables continuous patterning over sample topography.
Schematic diagram of the hybrid AFM / STM lithography system containing two independent feedback loops, one to control the tip-sample force and one to control the emission current.
Advantages of Hybrid AFM / STM Lithography Mode. This hybrid AFM / STM system has advantages over either the AFM or STM used for lithography alone. In STM lithography, a fixed tip-sample bias is applied and the spacing is varied to maintain a constant current through the resist. This system, however, suffers from poor alignment capabilities (imaging may expose the resist). It is also tricky to determine the appropriate voltage bias for STM lithography. If the bias is set too high, the tip may move far from the sample resulting in beam spreading. If the bias is too low, the tip may in fact penetrate the resist in an attempt to achieve the setpoint current. These factors make STM lithography of organic resists problematic. In AFM lithography, the force between the tip and resist is held constant while a fixed voltage bias is applied to generate the field-emitted current. Constant-voltage operation is not ideal since the required voltage is a strong function of the tip and sample materials, the tip shape, and the resist thickness. Therefore if the tip shape or resist thickness changes, for instance, the dose of electrons delivered to the resist will change. This could result in non-uniform or unrepeatable patterns. These issues would also make it difficult to extend STM or AFM lithography to reliable multiple probe patterning.