Ann N. Campbell

Micromilling of metal alloys with focused ion beam-fabricated tools

Abstract This work combines focused ion beam sputtering and ultra-precision machining as a first step in fabricating metal alloy microcomponents. Micro-end mills having ∼25 μm diameters are made by sputtering cobalt M42 high-speed steel and C2 micrograin tungsten carbide tool blanks. A 20 keV focused gallium ion beam is used to define a number of cutting edges and tool end clearance. Cutting edge radii of curvature are less than or equal to 0.1 μm. Micro-end mill tools having 2, 4 and 5 cutting edges successfully machine millimeter long trenches in 6061-T4 aluminum, brass, 4340 steel and polymethyl methacrylate. Machined trench widths are approximately equal to the tool diameters, and surface roughnesses (Ra) at the bottom of micromachined features are ∼200 nm. Microtools are robust and operate for more than 6 h without fracture. Results from ultra-precision machining aluminum alloy at feed rates as high as 50 mm/minute and an axial depth of 1.0 μm are included.

Relationship between texture and electromigration lifetime in sputtered Al-1% Si thin films

The relationship among the grain structure, texture, and electromigration lifetime of four Al-1% silicon metallizations produced under similar sputtering conditions was explored. The grain sizes and distributions were similar and the grain structure was near-bamboo for all metallizations. All metallizations exhibited a near-(111) fiber texture, as determined by the pole figure technique. Differences in electromigration behavior were noted. Three of the metallizations exhibited a bimodal failure distribution while the fourth was monomodal and had the longest electromigration lifetime. The electromigration lifetime was directly related to the strength of the (111) fiber texture in the metallization as anticipated. However, whereas the grain size distribution has an effect on the electromigration lifetime when metallization lines are several grains wide, the electromigration lifetime of these near-bamboo metallizations appeared independent of the grain structure. It was also observed that a number of failures occurred in the 8 μm interconnect supplying the 5 μm wide test lines. This apparently reflects an increased susceptibility of the wider interconnect lines to electromigration damage.

Effects of focused ion beam irradiation on MOS transistors

The effects of irradiation from a focused ion beam (FIB) system on MOS transistors are reported systematically for the first time. Three MOS transistor technologies, with 0.5, 1, and 3 /spl mu/m minimum feature sizes and with gate oxide thicknesses ranging from 11 to 50 nm, were analyzed. Significant shifts in transistor parameters (such as threshold voltage, transconductance, and mobility) were observed following irradiation with a 30 keV Ga/sup +/ focused ion beam with ion doses varying by over 5 orders of magnitude. The apparent damage mechanism (which involved the creation of interface traps, oxide trapped charge, or both) and extent of damage were different for each of the three technologies investigated.

Failure Analysis of Worn Surface Micromachined Microengines

Failure analysis tools have been applied to analyze failing polysilicon microengines. These devices were stressed to failure under accelerated conditions in both oxidizing and non-oxidizing environments. The dominant failure mechanism of these microengines was identified as wear of rubbing surfaces. This often results in either seized microengines or microengines with broken pin joints. Analysis of these failed polysilicon devices found that wear debris was produced in both oxidizing and non-oxidizing environments. By varying the relative percent humidity (%RH), we observed an increase in the amount of wear debris with decreasing humidity. Plan view imaging under scanning electron microscopy revealed build-up of wear debris on the surface of microengines. Focused ion beam (FIB) cross sections revealed the location and build-up of wear debris within the microengine. Seized regions were also observed in the pin joint area using FIB processing. By using transmission electron microscopy in conjunction with energy dispersive x- ray spectroscopy and electron energy loss spectroscopy, we were able to identify wear debris produced in low (1.8% RH, medium and high (39% RH) humidities.

Internal current probing of integrated circuits using magnetic force microscopy

A model for the magnetic force microscopy (MFM) imaging of IC currents is presented. MFM signal generation is described and the ability to analyze current direction and magnitude with a sensitivity of approximately 1 mA DC and approximately 1 mu A AC is demonstrated. Experimental results are a significant improvement over the 100 mA AC resolution previously reported using an electron beam to detect IC currents.<<ETX>>

Magnetic force microscopy/current contrast imaging: a new technique for internal current probing of ICs

Abstract This invited paper describes recently reported work on the application of magnetic force microscopy (MFM) to image currents in IC conductors [1]. A computer model for MFM imaging of IC currents and experimental results demonstrating the ability to determine current direction and magnitude with a resolution of ∽ 1 mA dc and ∽ 1 μA ac are presented. The physics of MFM signal generation and applications to current imaging and measurement are described.

Structure and properties of Al-1% Si thin films on Si as a function of gas impurities during DC magnetron-sputtered deposition

Thin films of Al-l%Si were sputter deposited on Si under a variety of pressures of atmospheric impurity gases. The effect of the impurity gases (oxygen, nitrogen, and water), and deposition temperature (15° and 300° C), on the microstructure and properties of the aluminum thin films were studied. The gas pressures introduced during deposition varied from 5 × 10s−6 Torr (6.7 × 10s−4 Pa) to 1 × 10s-10 Torr (1 × 10s−8 Pa). The thin films were investigated by transmission electron microscopy and were found to have a columnar microstructure with an even distribution of silicon precipitates. Both the grain size and silicon precipitate size increased at the higher deposition temperature. A smaller grain size was found in samples that were deposited under the higher impurity gas pressures tested. The specular reflectance of the films was found to be dependent upon the amount of impurity gases present during deposition, the greater the partial pressure the greater the surface roughness. This study also investigated the possibility of using a Resistivity Ratio (RR) measurement to evaluate the grain size of the Al thin films. It has been previously observed that a small Al grain size has poor electromigration resistance. This study found that the correlation between Al grain size and RR values was good indicating that RR tests may be used as a quick, non-destructive measure of film quality.

OBIC analysis of stressed, thermally-isolated polysilicon resistors

High gain Optical Beam Induced Current (OBIC) imaging has been used for the first time to examine the internal structural effects of electrical stress on thermally-isolated polysilicon resistors. The resistors are examined over a wide range of current densities, producing Joule heating up to /spl sim/1200/spl deg/C. Throughout this current density range, the OBIC images indicate a clustering of dopant under dc stress and a more uniform distribution under ac conditions. The OBIC images also reveal areas that are precursors to catastrophic resistor failure. In addition to OBIC imaging, conventional electrical measurements were performed, examining the polysilicon resistance degradation and time-to-failure as a function of electrical stress. The electrical measurements show a monotonic increase in polysilicon resistor lifetime with frequency (up to 2 kHz) when subjected to a bipolar ac stress. The enhanced lifetime was observed even under high temperature (from Joule heating) stress conditions previously reported to be electromigration-free. The dopant redistribution indicated by the OBIC images is consistent with an electromigration stress experienced by the polysilicon resistors. The implications for thermally-isolated polysilicon resistor reliability are examined briefly.

Non-invasive current and voltage imaging techniques for integrated circuits using scanning probe microscopy. Final report, LDRD Project FY93 and FY94

This report describes the first practical, non-invasive technique for detecting and imaging currents internal to operating integrated circuits (ICs). This technique is based on magnetic force microscopy and was developed under Sandia National Laboratories` LDRD (Laboratory Directed Research and Development) program during FY 93 and FY 94. LDRD funds were also used to explore a related technique, charge force microscopy, for voltage probing of ICs. This report describes the technical work performed under this LDRD as well as the outcomes of the project in terms of publications and awards, intellectual property and licensing, synergistic work, potential future work, hiring of additional permanent staff, and benefits to DOE`s defense programs (DP).