Baonian Guo

TEAMS depth profiles in semiconductors

Abstract Accelerator Mass Spectrometry (AMS) is routinely used to measure abundance ratios of long-lived radioisotopes such as 14C, 36Cl and 129I to their stable isotopes at levels as low as 1 × 10−15. Secondary Ion Mass Spectrometry (SIMS) is one of the most sensitive techniques for the determination of impurity depth profiles in semiconductors. Trace Element Accelerator Mass Spectrometry (TEAMS) is the combination of these two techniques, applied to the measurement of very low levels of stable elements in a matrix that may be quite different from the element being detected. TEAMS offers the possibility of detection limits of the order of tens of ppt for certain impurities in silicon, which is substantially better than SIMS. In general TEAMS data is subject to the same constraints as SIMS, the big improvement arising from the elimination of molecular interferences which bedevil SIMS. The IBMAL at University of North Texas (UNT) has a dedicated facility for TEAMS measurements. A detailed description of the laboratory and TEAMS apparatus will be presented along with recent TEAMS depth profiles from a variety of implantations in semiconductors.

The high-energy heavy ion nuclear microprobe at the University of North Texas

Abstract The high-energy, heavy ion, microprobe recently installed at the University of North Texas (UNT) has a demagnification factor of ∼60. It has a probe-forming lens system with a new Russian quadruplet configuration. The microprobe is installed on a 3 MV NEC 9SDH-2 Pelletron tandem accelerator, which has ultra stable high energy for heavy ions ( ΔE / E ∼10 −4 ). Sputter and RF sources produce a variety of ions for microprobe applications. A resolution of ∼2 μm has been achieved for 2.0 MeV protons, 4.0 MeV C ions and 9.0 MeV α-particles with a current of about 50–100 pA. Materials characterization and failure analysis of microelectronics are discussed. Current limitations and future improvements to the system are outlined.

Low-level copper concentration measurements in silicon wafers using trace-element accelerator mass spectrometry

Accelerator mass spectrometry (AMS) is now widely used in over 30 laboratories throughout the world to measure ratios of the abundances of long-lived radioisotopes such as 10Be, 14C, 36Cl, and 127I to their stable isotopes at levels as low as 10−16. Trace-element AMS (TEAMS) is an application of AMS to the measurement of very low levels of stable isotope impurities. Copper concentrations as low as 1 part per billion have been measured in silicon wafers. In this letter, we demonstrate the use of TEAMS to measure previously unknown copper concentration depth profiles in As-implanted Si wafers at a few parts per billion. To verify the TEAMS technique, the samples from the same wafer were measured with secondary ion mass spectrometry, which showed the same profiles, albeit plateauing out at a concentration level six times higher than the TEAMS measurement. The ability to measure at these levels is especially significant in light of the recent moves towards the use of copper interconnects in place of aluminum ...

Diffusion-time-resolved ion-beam-induced charge collection from stripe-like test junctions induced by heavy-ion microbeams

Abstract To design more radiation-tolerant integrated circuits (ICs), it is necessary to design and test accurate models of ionizing-radiation-induced charge collection dynamics. A new technique, diffusion-time-resolved ion-beam-induced charge collection (DTRIBICC), is used to measure the average arrival time of the diffused charge, which is related to the average time of the arrival carrier density at the junction. Specially designed stripe-like test junctions are studied using a 12 MeV carbon microbeam with a spot size of ∼1 μm. The relative arrival time of ion-generated charge and the collected charge are measured using a multiple parameter data acquisition system. A 2-D device simulation code, MEDICI, is used to calculate the charge collection dynamics on the stripe-like test junctions. The simulations compare well with experimental microbeam measurements. The results show the importance of the diffused charge collection by junctions, which is especially significant for single-event upsets (SEUs) and multiple-event upsets (MEUs) in electronic devices. The charge sharing results also indicate that stripe-like junctions may be used as position-sensitive detectors with a resolution of ∼0.1 μm.

Heavy ion microbeam studies of diffusion time resolved charge collection from p-n junctions

The knowledge of (diffusion, drift, and funneling assisted) charge collection within electronic devices is essential to design radiation hardened Integrated Circuits (ICs). In the present work, diffusion time resolved charge collection studies were performed on stripe-like junctions using 12 MeV carbon and 28 MeV silicon microbeams and MEDICI simulation calculations. The relative average arrival time of the diffused charge on the junctions was measured along with the amount of charge collection by the junctions. The average arrival time of the diffused charge is related to the first moment (or the average time) of the arrival carrier density on the junction. The experimental results and MEDICI (a 2D-device simulator) calculations support this interpretation. These results show the importance of the diffusive charge collection by junctions, which is especially significant in accounting for Single Event Upsets (SEUs) and Multiple Bit Upset (MBUs) in digital devices.

High sensitivity impurity measurements in semiconductors using trace-element accelerator mass spectrometry (TEAMS)

Trace-Element Accelerator Mass Spectrometry (TEAMS) is the extension of conventional radioisotope Accelerator Mass Spectrometry to the measurement of very low-levels of stable isotopes. The primary application of TEAMS is in the field of materials analysis, particularly semiconductors, where it offers the potential for measuring trace impurities at concentration levels as low as a few parts per trillion (ppt). The Ion Beam Modification and Analysis Laboratory at the the University of North Texas has built a dedicated facility for TEAMS measurements. This paper describes recent modifications and improvements to the system and shows results of some recent measurements.

The recent progress of the high-energy heavy ion nuclear microprobe at the University of North Texas

The paper reports the recent progress of a high-energy, heavy ion nuclear microprobe facility established at the University of North Texas. The microprobe system is installed on a 3MV NEC 9SDH-2 Pelletron tandem accelerator. A high demagnification factor (∼60) has been achieved with the system, using a probe-forming lens system (from MARC, Melbourne, Australia) with the new Russian quadruplet configuration. The spatial resolution of 2–3 μm has been achieved for 4.0 MeV carbon ions or 9.0 MeV alpha particles with a beam current of ∼50–100 pA. Better spatial resolution (approaching one μm) is achievable when an extremely low beam current (100–2000 ions/sec) is used in the applications of IBICC and IBIL. Applications of the analytical techniques with the nuclear microprobe are outlined and discussed.

Materials Analysis using Trace Element Accelerator Mass Spectrometry (TEAMS)

The Ion Beam Modification and Analysis Laboratory (IBMAL) at the University of North Texas has set up a dedicated Trace-Element Accelerator Mass Spectrometry (TEAMS) system for low-level impurity measurements. TEAMS has previously shown the ability to measure impurity copper levels in silicon wafers with a better sensitivity than Secondary Ion Mass Spectrometry (SIMS), one of the most sensitive measurement techniques. TEAMS is especially suited for bulk measurements of low levels of impurities in materials. A discussion of some of the issues involved in TEAMS measurements is presented along with the results of impurity iron measurements in silicon.

Ion induced electron emission from Si and photoresist surfaces

Ion induced electron emission (IIEE) from solid surfaces is one of the fundamental processes with ion beam applications. The different IIEE yields from different surfaces such as Si, SiO2, metals and photoresist (PR) may cause charging and damage the gate oxide in ion implantation. IIEE yields with B+ and Si+ beams were measured for several kinds of PR materials, bare and oxide wafers. Although the target chamber pressure was always in or below the low 10−7 Torr range, the IIEE yield from PR surfaces was found to be a function of implant dose with the most dramatic change in the beginning of implantation. For other materials such as Si and SiO2, the IIEE yield is independent of implant dose after the initial variation due to surface contamination.

Ion beam induced charge collection (IBICC) from integrated circuit test structures using a 10 MeV carbon microbeam

As future sizes of Integrated Circuits (ICs) continue to shrink the sensitivity of these devices, particularly SRAMs and DRAMs, to natural radiation is increasing. In this paper, the Ion Beam Induced Charge Collection (IBICC) technique is utilized to simulate neutron-induced Si recoil effects in ICS. The IBICC measurements, conducted at the Sandia National Laboratories employed a 10 MeV carbon microbeam with 1pm diameter spot to scan test structures on specifically designed ICS. With the aid of layout information, an analysis of the charge collection efficiency from different test areas is presented. In the present work a 10 MeV Carbon high-resolution microbeam was used to demonstrate the differential charge collection efficiency in ICS with the aid of the IC design Information. When ions strike outside the FET, the charge was only measured on the outer ring, and decreased with strike distance from this diode. When ions directly strike the inner and ring diodes, the collected charge was localized to these diodes. The charge for ions striking the gate region was shared between the inner and ring diodes. I The IBICC measurements directly confirmed the interpretations made in the earlier work.