Equipment refines soft-error detection in chips

Equipment refines soft-error detection in chips
By David Lammers, EE Times
May 27, 2002 (5:26 a.m. EST)
URL: http://www.eetimes.com/story/OEG20020524S0073

AUSTIN, Texas — The detection of soft errors in silicon chips is attracting attention on several fronts, as the struggle intensifies to prevent glitches caused by the naturally occurring bombardment of alpha particles from packaging materials, and neutrons from cosmic rays.

While hard errors affect the silicon itself, soft errors, which occur more frequently, alter the stored charge in memory cells and logic, leaving the physical circuit intact. Soft-error rates (SER) are worsening as voltages and capacitance in CMOS memory and logic circuits scale, said Rob Baumann, a reliability scientist at Texas Instruments Inc. (Dallas).

The International Sematech consortium's committee on soft errors, which Baumann chairs, has recommended more stringent guidelines for the detection of alpha particles emitted by packaging materials and solder bumps. Alpha particle emissions, from thorium and uranium impurities in those materials, account for r oughly half of all soft errors, even when pure materials are used, Baumann said.

Gas-propagation counter technology underpins current alpha detection equipment and detects alpha emission levels of between 0.001 and 0.0005 alpha particles/hr-cm². The existing equipment takes about two weeks to examine the materials under test and return data. Gas-propagation counter equipment will likely continue its reign as the mainstay for some time, but a new generation of equipment is needed to meet International Sematech's new goal of measuring 0.0001 alpha particles per centimeter squared.

Particle detector

A tool that meets Sematech's goal may emerge from an alliance being finalized between a small research firm, Inovatia Laboratories LLC (Fayette, Mo.), and Alpha Sciences Inc. (Milpitas, Calif.), the leading equipment vendor in the field.

The Inovatia approach digests, or dissolves, the sample material during a 22-hour period, turning a chip and its package into a liquid. An organic ligand — a gas designed to recognize a particular portion of the periodic table — detects the alpha particles. When a particle is detected, the system creates a flash of visible light, and a photocell counts the flashes. The system can use a much larger sample, thereby reducing the testing time, said Rhys Thomas, a chemist and managing partner at Inovatia.

Alpha Sciences said it intends to take the prototype system that Inovatia developed and design a commercial-grade system that would meet the measurement and cost metrics established recently by Sematech's SER committee.

Those goals include a detection limit of 0.0001 alpha particles/hr-cm² from 1 - 10 MeV, over a sample area of 1,500 cm² or less, with a run-time of less than one week, at a cost of less than $50,000, Baumann said.

The development project was supported by a $100,000 grant from the Department of Commerce, which identified the need for a new type of alpha-particle det ection equipment, said J.B. Waggoner, Inovatia's director of business development.

The gas-propagation system made by Alpha Sciences is able to test multiple wafers in a toaster-like system, said president Mike Tucker. The Inovatia system would be able to test much larger quantities of sample materials. Many companies would prefer to reduce the testing time by working with these larger samples, even though this can add to testing costs, Tucker said.

IRoC's testing service

The alpha particles from packaging account for a majority of SERs at sea level, but cosmic rays account for another source of soft errors. The neutrons from cosmic rays produce ionizing effects that create free electron-hole pairs in silicon.

A suite of services that will perform neutron radiation testing of CMOS ICs on an outsourcing basis will be introduced in the coming week by iRoC Technologies Corp. (Santa Clara, Calif.). The bundle of services, called Ser-test, includes simulation software an d a programmable test board, based on an FPGA array, which detects neutron-induced errors while a device is bombarded with neutrons.

Eric Dupont, iRoC's president, said the company provides an "IP infrastructure" service, using equipment at iRoC's facility, to support semiconductor companies and foundries under time-to-market pressures.