Hot Electron Imaging with PixCellent CCD Systems:

Application Note


Introduction

INTEGRATED CIRCUIT (IC) design complexity continues to develop at a very great speed. As design rules permit smaller and smaller component dimensions to be achieved, it is increasingly difficult to check that both the design and the production process will meet the stringent quality requirements of IC consumers.

When an integrated circuit has a faulty junction, hot charge carriers (electrons or holes) may generate weak visible or near infrared (NIR) light. By looking carefully at an operating IC with a sensitive scientific imaging system, problem areas within the IC may be located with precision.


Method of emission microscopy

THE IC to be inspected is mounted on a probe station (an instrument that allows connection to the IC under test before it is packaged) so that the wafer is directly visible The probe station should have a microscope with a sensitive camera attached. The IC under test is operated and inspected with a camera. In some cases conventional video rate TV systems may be adequate for such tests but usually the light emission levels are very low and a much more sensitive scientific camera such as those made by PixCellent is required to achieve the necessary sensitivity and spatial resolution appropriate to modern high-density IC designs.
In its simplest form, Semiconductor Emission Microscopy consists of applying the appropriate voltages to the IC under test, recording any light emission from the IC and comparing the emission image with another image taken with reflected light to allow the identification of the troublesome components.

When emission is strong enough it is impossible to measure the colour of the emitted light and hence the temperature of the hot charge carriers by using colour or interference filters in front of the camera.

Here the technique is sometimes referred to as, Semiconductor Emission Spectroscopy or as Energy-Resolved Emission Microscopy (EREM). A knowledge of the temperature of the hot charge carriers allows insight into the mechanisms that have led to the defect encountered.
Modern IC designs often involve a number of layers of metallisation. The light from a defect under one of these layers will be blocked from passing through the silicon and being detected by the probe camera. It is possible to make small holes into the backside of the IC under test. The light emitted is absorbed strongly by the silicon in the visible, but in the NIR it is absorbed much less strongly and the defect may be visible.

Unfortunately the same mechanism that allows NIR radiation of a certain wavelength to pass through a thin layer of silicon also makes its detection in a silicon CCD less easy since it can also pass through the thin sensitive layer of the CCD.


Camera Selection Guide

THE TECHNIQUE has been used for a number of years and most commercially available systems use image intensifiers. These are far from ideal detectors in the NIR. They need to have cooled photocathodes in order to give a low photocathode dark current and may only have a lifetime of a few years.

The critical selection parameters for this application are good sensitivity in the NIR at around 1 micron wavelength and very low read-out noise even for long exposures. The overwhelming need for good IR sensitivity at first sight makes the use of thinned CCDs such as those from EEV and SITe attractive. These devices may be supplied with anti-reflection coatings optimised for the far red, giving quantum efficiencies of around 15 percent at 1 micron. Cooling the CCDs is essential if long exposure times are to be used, but cooling the CCD also worsens its far red response. This means that the CCD should be operated at as high a temperature possible consistent with a low enough dark current for the exposure or test to be carried out.

One serious limitation of some thinned CCD based systems is the fact that the dark current of SITe thinned CCDs is relatively high compared with EEV thinned CCDs and particularly when compared to front-side illuminated CCDs, even when operated in MPP mode to suppress dark current. This forces the use of coolers that go to lower temperatures than is possible with Peltier (thermoelectric) devices. Often liquid nitrogen cryostats are used, and coolers based on the much more convenient Stirling cycle cooling engines are becoming available at a much lower cost.

An alternative approach when cost is an important consideration is to use front side illuminated EEV devices. Their quantum efficiency at one micron is only slightly lower than that of the thinned devices. The EEV devices have large pixels so optical coupling efficiency is preserved but they have the great advantage of operating in super-MPP mode to give a dark current 30-300 times lower than the thinned SITe CCDs. This allows thermoelectric cooling heads to be used such as the TE3 and TE4 heads made by PixCellent . It is also possible to purchase extra thick devices with deep depletion layers to give even greater response at around 1 micron wavelength. DQE figures of as high as 35 percent at 1 micron, and 5 percent at 1.1 microns are now possible with such devices.


Typical system configurations

THE RELATIVELY long exposure times and sensitivity requirements mean that the Antares family is ideally suited to the application. Configurations that meet the requirements of these applications are:

Thinned system:

o EEV CCD 55-20 (770 x 1152 pixel) or CCD 55-30 (1242 x 1152 pixel) with AR coating for the NIR.
o High-Performance liquid nitrogen head.
o Antares electronics unit.
o IBM-PC AT link card or SUN S-bus link card.
o PixCel or UltraPlus software package.


Non-thinned system:

o EEV CCD 55-20 (770 x 1152 pixel) or CCD 55-30 (1242 x 1152 pixel) , possibly with a deep depletion layer.
o TE4 head with air or liquid recirculator.
o Antares electronics unit.
o IBM-PC AT link card or SUN S-bus link card.
o PixCel or UltraPlus software package..

Either of the systems may be mounted easily onto most standard IC probe stations using appropriate microscope/camera adapters (C-mount, Canon, Nikon, Pentax).


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Site last updated: 23 July, 1999.