Introduction

Measuring leakage currents on semiconductor wafers and within devices has always been a challenge for analytical probing equipment, as well as the supporting instrumentation. With market demands for higher density ICs, lower power, improved reliability, and features like programmability, semiconductor makers engineer processes to push leakages and device "off currents" ever lower. When these currents are less than a picoamp, direct measurement can be very difficult. Though large cross-section process control structures can "scale up" some currents, space limitations and/or special requirements (e.g. failure analysis and spot defect investigations) often render this technique impractical. An alternative method, "magnifying" leakage via high temperatures, demands good temperature control and a low noise environment.

Triaxial Chuck

Until recently, probing currents in the femtoamp range required special equipment and expertise. Often, valuable data simply was not collected. Enhanced instruments, such as the Hewlett Packard 4156A semiconductor parameter analyzer, attack part of this problem. But to be fully useful for analytical probing, probing equipment enhancements are also needed. Specifically, triaxial shielding techniques (e.g. separate force-sense, driven guard and shield paths) employed within the HP 4156A must be extended to the probe station's wafer test area. Parts of this extension must withstand "hot chuck" temperatures to enable testing at high temperatures.

Signatone Corporation now offers two products, the TRX Probe and Tri-Temp Hot Chuck, which address these needs. Combined with other Signatone products, such as the QuieTemp temperature controller, probe stations, manipulators and a dark box, a complete solution to low current probing requirements is now available. This application note describes results achieved with these products, which show little or no degradation in measurement capability up to chuck temperatures of 300oC. Descriptions of equipment employed, as well as setup and application tips, are included to aid in obtaining good results, quickly and easily. This information should help bring the full capabilities of these new products to bear on your analytical probing tasks.

Results

Three sets of tests demonstrate the low noise and leakage characteristics of the TRX Probe and Tri-Temp Hot Chuck: First, the leakage and noise contribution of the TRX Probe are demonstrated. Next, the noise and equivalent resistance of the Tri-Temp Hot Chuck are measured. Last, to show a typical application, sweeps of an N-MOS transistor drain current, from subthreshold through saturation, were repeated between 20oC and 300oC. Tests were performed employing the practices outlined under "Setup Tips" below. Of greatest importance were proper use of the dark box, implementation of solid, low impedance electrical connections, and compliance with the HP 4156A recommended operating procedures.

First, the four high resolution source-measure units (HRSMU's) of the HP 4156A were connected to the source, drain, gate and substrate pads of a transistor on the test wafer. After confirming all connections via a sweep, the drain probe tip was lift ed, just breaking contact with the wafer, and the gate voltage was swept again. TRX Probe noise and leakage, at room temperature and 250oC, appear in Figures 1 and 2. This represents the combined noise of the probe, setup and parameter analyzer. The total magnitude approximates +1 fA at both temperatures, at or below the 1 fA resolution limit of the analyzer. Both tests were made with QuieTemp controller power on, after temperature stability was achieved. Per recommended procedures, sweeps were performed after an "offset cancellation" (instrument zero reset) and completed quickly to minimize drift. (As described later, long sweep times can result in greater drift and noise, but in all cases the noise and leakage contribution of the TRX Probe remained negligible.)

The wafer back side often provides an essential connection which must be biased and shielded from noise to achieve meaningful results. At other times, measurement of current flowing from the backside (e.g. substrate current) is needed. The Tri-Temp Hot Chuck accommodates both needs by providing a triaxially isolated and shielded wafer platen, while also resolving the thermal conductivity, thermal expansion and mechanical stability issues. To evaluate the Tri-Temp Hot Chuck, substrate-to-ground noise and leakage of a wafer were measured as the substrate was swept from - 40 to +40 volts and back. As before, measurements were made with the thermal chuck power on. Results at 20oC (Figure 3) show peak "noise" currents near 20 fA, with average noise below 10 fA. (These small currents are thought due to relatively larger "driven guard" loading effects in the Tri-Temp Hot Chuck, and low level noise emanating from the nearby heating element.) Results of a similar -40 to +40 volt sweep, performed at 300oC, appear in Figure 4. Peak-to-peak noise remained about the same, but a minor resistive leakage is observed. This sub-80 fA change for a voltage change of 80 volts, shows the equivalent chuck- to-ground resistance exceeds 1015 ohms (i.e. > 1000 TW !), defining the Tri- Temp Hot Chuck as a "best of class" product.

Last, TRX Probes were connected to transistor source, drain, gate and substrate pads, with the substrate probe guard also connected to the Tri-Temp Hot Chuck guard terminal. Channel length and width of this N-MOS device were 1.0 m and 50 m respectively. A -0.5 volt substrate bias assured that a resident protection device (between gate and substrate) would not interfere with subthreshold measurements. With the drain-to-source voltage programmed to 0.2 volts, the gate voltage was swept from -0.5 to 1.5 volts. Resulting drain current plots, at wafer chuck temperatures of 20oC, 50oC, 100oC, 150oC, 200oC, 250oC and 300oC, appear in Figure 5. Note the measured 1-3 fA off currents at 20oC are "as predicted" by semilog extrapolation of 50-150oC off currents! Though some measured currents are below the specified +20 fA accuracy limits of the analyzer, this data further reveals the low noise and leakage capabilities of the TRX Probe and Tri-Temp Hot Chuck, and their ability to facilitate ultra-low current measurements across a wide temperature range.

Equipment

The essentials for making "on wafer" femtoamp current measurements are first, an instrument capable of resolving low currents, and second, methods of maintaining a low noise environment up to and surrounding the wafer chuck. Last, noting it would take 100 seconds for 10 femtoamps to charge 1 pF through 1 volt, we see capacitances cannot be ignored. To achieve tolerable measurement times, triaxial "driven guard" methodologies must be employed to nullify the effects of interconnect and device capacitances. The equipment describes below provides all elements of this recipe.

Probe Station, Dark Box & QuieTemp Controller

A solid environment for low current probing was provided by mounting a Signatone S1160 probe station and manipulators, in a metal dark box. This box shields the wafer from ambient light, which dramatically alters leakages. It also extends the electrical "shield" to entirely enclosed the test site. Proper use of dark box is essential for accurate, repeatable results. TRX Probes were mounted to standard manipulators, with their cables connected to the internal side of triaxial bulkhead connectors, mounted through the wall of the dark box. The Tri-Temp Hot Chuck was mounted atop the probe stations X-Y table, with all cabling and cooling tubes routed through a rear service port to an external QuieTemp temperature controller and recirculating chiller.

The Parameter Analyzer

Fixturing and other setup extensions rarely enhance, but often degrade, measurement accuracy, repeatability and noise. Using a high quality instrument is recommended, as results must always be interpreted in the context of the instrument's limitations. These evaluations were made using the HP 4156A parameter analyzer. This versatile tool is similar to its predecessor, the HP 4145, but it boasts extended capabilities. Key to these evaluations were the new low current capabilities, including a 10 pA measurement range, with 1 fA resolution and +20 fA accuracy specifications. Recommended operating procedures, including appropriate hold and delay time settings, integration periods, and frequent "offset cancellation" were used to maximize performance. Via repeating and cross- checking measurements, the analyzer was observed to stay well within these specifications. The analyzer's rear mounted HRSMU connectors were connected to the external side of the dark box "through wall" connectors via manufacturer supplied triaxial cables.

TRX Probe & Tri-Temp Hot Chuck

Signatone's TRX Probe and Tri-Temp Hot Chuck were developed to address the challenges of low current, high temperature probing. Incorporating low noise, triaxial techniques throughout, TRX Probes mate with conventional BNC style triaxial connectors, and are easily mounted to several models of Signatone manipulators. Probe tips are replaceable for easy maintenance and to permit tailored tip geometries for specific needs. Materials have been selected for their low noise and high temperature characteristics, providing compatibility with chuck temperatures to 300oC. The Tri-Temp Hot Chuck also incorporates low noise, high temperature materials. The design incorporates an isolated wafer platen, with underlying "driven guard" and "shield" layers, a full implementation of triaxial methodology. In use, the wafer platen is connectedto the source-measure terminal of one HRSMU, while the Tri-Temp Hot Chuck "guard" is tied to the driven guard of the same HRSMU. The Tri-Temp Hot Chuck "shield" is grounded to the dark box frame, connecting it to the HRSMU shield via the "through wall" connectors.

Setup & Measurement Tips

Though low current measurements can now be relatively straight forward, applying several of the precautions and techniques below can speed the process. When currents are very small, long settling times and extensive measurement averaging are necessary. Since analyzer sweeps can take minutes each, getting correct results "the first time" saves a lot of time. Three groups of "tips", addressing equipment setup, use of the HP 4156A, and a few device issues, appear below.

Basic Setup Issues

Eliminate all light from the dark box; even dim light increases leakage by orders of magnitude!
Inspect the box and seal all light leaks.
Employ tape over metal foil to block both light and electrical noise.
Fully close the door, engaging the light locks, for each measurement.
Make sure microscope lights are fully off.
Assure no light leaks in via the fiber optic "light pipes" of an external light source
Provide good, low impedance grounds; even millivolts of ground noise will degrade results:
Pick one point (e.g. a "through wall" connector lug) as a ground Mecca; avoid ground loops.
Check all grounds with a meter; impedances should be a few hundred milliohms or less.
Reduce or eliminate "noise" sources inside the dark box:
Use grounded shielding around hot chuck power cables and thermocouples exiting the box.
Remove or shield insulator sleeves which could generate or store static electricity.
Avoid rubbing or bending cables or insulators which could induce static charge.
Test after the chuck temperature has stabilized; this minimizes heating element noise.
Periodically clean the chuck with alcohol; contamination can introduce unstable leakages.

Using the HP 4156A

The HP 4156A is a sophisticated digital instrument, featuring periodic auto calibration, as well as built-in filtering and integration techniques Several recommendations for making ultra-low current measurements are found i the users manuals. Key points from the manuals, and a few lessons learne while performing the above tests are reiterated below:

Providing the recommended environmental conditions (23oC + 5oC temperature and 5-60% relative humidity) and 40 minute plus warm up period minimizes drift.

To achieve 1 fA resolution, set the HRSMU to "auto-range" or "fixed" on the 10 pA range.

Set the integration period to "long" to average out noise and maximize accuracy.

Increase "hold" and "delay" times until sweeping in both directions achieves similar results. Hysteresis in "double sweeps" usually implied times were too short. The above N-MOS drain current sweeps were achieved with a 1 second hold and 0.2 second delay times. High voltage sweeps of the Tri-Temp Hot Chuck employed 15 second hold and 1 second delay times.

Perform a "cancellation offset" directly before each low current measurement. Temporarily lift the probe tip to null out residual device leakages. Note, under the specified conditions, this assures the specified +20 fA accuracy, but not that ever femtoamp resolved in the prior sweep will be nulled.

By experimentation, find minimum delay and hold times needed for good results. Use these settings to minimize measurement time, thereby reducing the opportunity for instrument drift. To illustrate, compare the sweeps of Figure 1 with Figure 6 below. Settings were similar except that the latter plot contains 12 times as many steps. The resultant 6 minute (vs. 30 second) sweep time reveals the additional drift and noise encountered as execution time increases. (Note, in both cases the HP 4156A was well within its spec.)

Some Device Issues

If results are inconsistent, be aware device or wafer level issues may be the root cause:

To evaluate dual well CMOS, control both well potentials to avoid small currents which can flow as floating structures charge and discharge.

Bias active (amplifying) devices to keep gains low, minimizing the possibilities of oscillation. Note analyzer filtering and integration (averaging) functions could easily mask this condition.

Over stressed or ESD damaged devices can exhibit unstable leakages; again filtering and integration could also mask this problem.