Larry Levit

Electrical breakdown and ESD phenomena for devices with nanometer-to-micron gaps

The current vs. voltage and electrical breakdown behavior for devices with micron and sub-micron gaps between conductors is studied. The limitations of the well-known but often-misinterpreted Paschen curve are discussed. The little-known modified Paschen curve, that includes field emission effects so important in understanding breakdown behavior for devices with sub-micron gaps, is described. Current vs. voltage measurements across metal-air-metal, metal-insulator-metal and metal-insulator-air-insulator-metal gaps with gaps ranging from 4 nm to 4 μm are reported. The breakdown voltage for an air gap of 0.9 μm was found to be 150 V, far below the Paschen curve minimum breakdown limit, and field emission behavior was confirmed via the Fowler-Nordheim plot. Metal-insulator-metal gaps with a diamond-like carbon thin-film with a thickness of 4 nm had a breakdown voltage of only 1V. SEM and AFM analysis show that the breakdown damage is crater-like and through the carbon layer. Other characterization of the damage caused by breakdown is presented. Tribocharging, electrostatic induction, and other ESD-related phenomena, are discussed for several devices with sub-micron gaps. It is concluded that devices with sub-micron gaps can face a serious challenge due to electrical breakdown during manufacturing, handling and operation. These devices include photolithographic reticles, magnetic recording heads, MEMS and field emission displays.

Biased-plate characterization of pulsed DC ionizers☆

The operation of a pulsed DC air ionization system was studied in a side-by-side comparison using a charged-plate monitor (CPM) and a biased-plate monitor (BPM). When the pulse period is short compared to the decay time, both methods give equivalent results. When the pulse is not short, however, the CPM suffers from a random error on the order of the pulse period. In addition, the CPM method is quite sensitive to the motion of persons and other objects in its vicinity. Neither of these errors occurs with the BPM, which therefore provides a significantly faster way of adjusting pulsed DC ionizers.

A study of the mechanisms for ESD damage to reticles

Reticles were exposed to the fringing field from an electrode biased to a high voltage. The reticles in the study included special reticles intended to benchmark the ESD hazard of a photobay and production reticles of a variety of feature sizes. It was found that without any electrical contact between the reticle and the electrode, reticle damage could be done. A wide bandwidth transient-EMI sensing antenna revealed that the reticle sparked when a voltage as low as 2000 V was applied to the electrode. The tests showed that the ESD threshold of reticles with smaller feature sizes was lower than for reticles with larger feature sizes. Reticles were scanned under a microscope for reticle damage. It was found that when the voltage was ramped to 17 kV and returned to zero, damage to the reticle was observed. When a voltage of 7.5 kV was applied once, no damage was observed but when it was applied 100 times, reticle damage was observed. This study confirms that ESD damage is done to a reticle by charged objects in the vicinity of the reticle in contrast with the prevailing belief that reticle damage is done only by charged reticles. The study also showed that reticles can be sufficiently damaged to cause printing errors due to the accumulated damage caused by repeated low level exposure to the fringing field of a charged object in the vicinity of the reticle.

Measurement of ionizer performance-a new approach

The ANSI/EOS/ESDS 3.1-1991 standard utilizes a charge plate monitor (CPM) to measure ionizer performance. The CPM measures positive and negative discharge times (DT+ and DT-) and offset voltage on a 200 mm/spl times/200 mm plate, not on real parts. While the CPM produces consistent measurements that are a good metric for ionizer performance, it is not necessarily the best way to measure ionizer performance on other objects. This paper analyzes an instrument and a protocol that produces measurements that correlate with CPM balance and discharge times and represent physical quantities which can project to the response of various objects in the manufacturing line. These parameters are air resistance and offset voltage. The study shows that the ionized air acts as an ohmic medium. The instrument under study, a biased plate monitor (BPM) is a biased HV plate with a nano-ammeter to measure the ion current it intercepts. The calculated discharge time from BPM as compared with the CPM measurement is excellent (<10%). The biased plate monitor (BPM) has the advantage that its readout is immediate and does not require a wait to obtain the discharge time. The biased plate also can record the offset voltage. Measurements were made over the range of 0 to 70 V of offset and compared with a CPM measurement. The results showed excellent matching.

Measurement technique developed to evaluate transient EMI in a photo bay with and without air ionization

A new technique for quantifying the magnitude and the rate of ESD-induced transient electromagnetic interference (ESD-EMI) was developed, and is presented along with results of measurements of the rate of such EMI with and without ionization in Photo Bay 2 at International SEMATECH. The rate of ESD-EMI events recorded in the bay was 30 times greater with the ionizers off than with them on. The technique used a very high sampling rate (4 GS/sec) digitizing oscilloscope, with a wide bandwidth (>1.5 GHz), and a histogram technique to acquire a spectrum of amplitudes of the transient events.

Transporting FOUPs as a driver for ESD-induced EMI

FOUPs are constructed from insulative plastics. As such, they take on charges of multiple kilovolts and carry large electric fields with them. Because of the large size of the FOUP, appreciable fields extend for a distance of >30 cm from the FOUP and can reach many objects withing the cleanroom. When the fields intersect an ungrounded or poorly grounded conductor, metal-to-metal discharges can occur. These discharges, in turn radiate nanosecond pulses (transient EMI) which can disrupt robotics. This paper details the measurement of such transients in an operating 300 mm fab. The results show that EMI is created by manual handling of FOUPs but that charged FOUPs moving in an overhead track are a much greater cause of EMI with the potential for robotic difficulties.