370 On-chip Probes for Silicon Defectivity Ranking and Mapping A. Zanchi * , F. Zappa * , M. Ghioni * , and A.P. Morrison + * Dip. Elettronica e Informazione - Politecnico di Milano - Piazza L. Da Vinci, 32 – I-20133 Milano (Italy) Phone: +39-02-23996149 Fax: +39-02-2367604 E-mail: zappa@elet.polimi.it + Dept. Electrical Engineering and Microelectronics - University College Cork - Cork (Ireland) ABSTRACT We present process probes useful to investigate the process- dependent quality of p-n junctions in semiconductors. The probes are sensitive to the presence of thermal generation centers, which ignite macroscopic current avalanches. Since the carrier generation events are promoted by the presence of localized imperfections such as dislocations, stacking faults, etc., the avalanche ignition rate represents a suitable figure of merit for ranking the overall process cleanliness. In particular, by using these probes we report a non- uniform distribution of lattice defects within certain junctions. This phenomenon has been verified by means of standard etching and infrared optical inspection. Some technological hints are finally provided, capable of reducing the defectivity and improving the fabrication of microelectronic devices. INTRODUCTION The quality of semiconductor technological processing is one of the paramount issues for the modern microelectronics. The current leakage of devices and the quality of shallower junctions can put in jeopardy VLSI electronic systems, as they imply boosted power consumption and performance degradation. Though major attention has been traditionally paid to the leakage ascribed to peripheral defectivity, the bulk impurities also play a role, for example in flicker-noise-related issues [1]. Therefore, besides the struggle to purifying the starting silicon ingot, a research for successive low- damage, defect-free technological steps has always been pursued. Purpose of our work was to rank the pureness of fabrication processes by means of simple probe devices, whose characteristics are especially sensitive to the quality of the technology. We employed basic junctions and biased them above the breakdown voltage, in order to exploit the avalanche multiplication phenomena, as described in the following section. Briefly, when a carrier pair is generated within the junction depleted region, such an event is signaled via the output of a milliamp current pulse. This basic idea is derived from ultra-high sensitivity photodetectors, in which the absorption of a single photon in the junction's depleted region, causes the turn on of the avalanche [2]. As photodetectors, their performance is compromised by unwanted current ignitions not due to photons, the so-called “dark” counting rate. On the contrary, from our application's standpoint, the thermal- induced counting rate is useful to assess the generation rate in the semiconductor, while photon-assisted carrier generation must be inhibited. Instead of blinding the probes by overlaid metal layers, we preferred to leave the junction uncovered and to operate in the dark. The silicon defects distribution had often been considered uniform over the junction. Instead, by means of the proposed probes, we show that this is generally not true. We derive how the defect distribution varies according to the diameter of the junction, and progressively increases towards the edges. The finding is consistent with other published data (see [3]). Moreover, the increase in the defect density near the junction boundary is confirmed by the data collected using different experimental techniques, ranging from curve-tracer analysis to infrared optical inspection. Eventually, the exploitation of alternative probe geometries underlines the benefits of the defect gettering, while suggesting a straightforward way to improve the performance of any device, especially those with reverse-operated junctions. PROBE’S OPERATING PRINCIPLE The most convenient operating regime for the defectivity probes is the Geiger mode, much like in Single-Photon Avalanche Diodes (SPADs, [2]). Rather than providing a reverse current proportional to the photon flux, like in conventional Avalanche Photodiodes (APDs), the proposed probes output a standard current pulse, synchronous to the avalanche ignition. Fig. 1 shows a circular probe, with the schematic cross section and top views. Thanks to its simple layout, such a device can be included in most microelectronic technologies, virtually with no additional masks. The cathode of the diode is a large n + diffusion (diameter ranging from 5 up to 200μm) into a p epi-layer, surrounded by a collecting p + sinker that conveys the current to the anode metallization. Alternatively, current can be collected directly from the bottom of the wafer, avoiding the need for sinker diffusion, contact opening and anode metal layer. The high-field active region is defined by a p + deep enrichment diffusion within the shallow n + diffusion. Note that all of these layers can be found in most standard process flows, especially bipolar, with no additional overhead on the thermal budget. Fig. 2 sketches the vertical doping profiles in the active region, used throughout this work, as available in the BiCMOS technology under investigation. metal enrichmentp shallow n+ substrate p+ O xide p+ p+ epi p inspection area inspection area Anode m etal C athode m eta l p+ FIGURE 1.CROSS SECTION (TOP) AND TOP VIEWS (BOTTOM) OF A CIRCULAR PROBE WITH THE SUGGESTED TECHNOLOGICAL LAYERS Reprint from “Proceedings. 38th IEEE International Reliability Physics Symposium, 2000”, Pages: 370 -376, 2000