Oxide Wear-Out - TDDB, CVS, CCS

Oxide Wear-out

• Time Dependent Dielectric Breakdown (TDDB)
• Constant Voltage Stress (CVS)
• Constant Current Stress (CCS)

Oxide (gate and interlayer dielectrics) failures
In normal use, a charge is continuously injected into the oxide which causes defects and “wears” it out over time. Ultra thin oxides (< 3 nm) are especially susceptible to tunneling currents causing damage in the oxide or at the oxide – silicon interface.

Oxides Fail By

• Hard Breakdowns: Current increases exponentially after a period of excessive biasing resulting in violent ruptures and thermal damage
• Soft Breakdowns: Gate leakage current and/or noise increases gradually at low voltages; thin oxides don’t store enough energy for the hard violent breakdowns thus the effects are more subtle and gradual (harder to detect)
• Slow defect generation creates a path from gate to substrate

Note: Soft breakdowns and defect generation may not result in device failure, but instead performance degradation due to higher power drains creates reliability problems.

Key failure components and conditions

• As the oxide wears, noise increases on the gate voltage. Circuit stability and operation can be affected if thresholds are exceeded.
• As the oxide wears, gate leakage currents increase. In ultra-thin oxides, it can reach an intermediate level, not enough to fail but enough to increase overall circuit power drain and reduce battery life

Test Characteristics

• Tests are long term with temperature acceleration and single stress interval
• Monitor current and noise levels of oxide. Pre and post stress testing measures gate leakage (low current capability is needed especially to identify soft breakdowns)
• Multiple sites may be done in parallel
• Darkness is required

Oxide Wear-Out (TDDB, CVS, CCS) Test Equipment

Stations
P200-300 or 8000 series manual, motorized or semiautomatic: Feature highly stable base, low noise/low current design and integrated thermal chuck connections

Chucks: Low Noise Ambient and Thermal (to accelerate testing)
High isolation design supports low current tests and promotes stability
Thermal chuck range: -65°C to +400°C for 6 and 8" (150 and 200mm); -55° to +300°C for 12" (300mm) chucks.
Thermal chuck cast-in heating and cooling elements provide thermal ramp speed and surface uniformity.
Thermal chuck special mount and design reduces expansion of surface into the probes (minimizes contact resistance changes).
Thermal chuck base “radiator” cooled to prevent heat from getting to probe station stage causing expansion / movement and reliability issues

Light and EMI Shielding: Oxide isolation structures are light sensitive
P200-300 and 8000 series stations have a number of versions of optional integrated dark and dry enclosures

• Light Tight Enclosures (LTEs) for the best light, EMI and RFI shielding
• Station base, chuck chamber and probe "Top Hats" for “local environment” dark and dry probing

Sample Contacts

• Multiple manipulators and probes can test multiple test sites at any location on the wafer and in any test site pattern. The 9000 and 8000 series probe stations offer large platen to hold many manipulators.
• Wave Manipulators with VersaTiles™ multiple probes provide super stability, long-term contact and flexibility. Wave Manipulators feature:
• “Link Arms” provide support and force for large pin counts
• 5 Axis of motion, X,Y,Z, Planarity and Theta
• Lead-screw/lead-nut Z drive for stability and consistent contact force
• Vacuum-assisted magnetic bases are stable, moveable and won’t tip if vacuum fails
• Straight and 90° attack angles for easy positioning and multi-VersaTile™ configurations
• Low noise/low current probe card holders for 8000 and P200-300 stations which are designed to support Celadon high temperature, multi-site probe cards.

Probe Contact: Probe Holder and Probe Tip
Low noise and low leakage level triaxial probes: Model 79-T and Model 83
For Soft Breakdown of oxides, low noise (femtoamp level) is critical as gate noise is the only sure indicator of irreversible oxide damage.

Low contact resistance and larger probe tip are needed to handle large current densities. 7B-10G or HCT probes HCT Probe tips reduce contact resistance by up to 7X and do not develop oxide over tip!

Integrated vibration isolation tables and light tight enclosures in combination with a vibration damping station design provide the highest degree of vibration control. Vibration control is critical to insure probe contact remains constant.

Summary

• All tests require good contact with long term stability.
• Thermal chucks are needed for accelerating failures. The H1000’s unique design is ideal.
• Individual probes are needed for small devices.
• Integrated systems of manipulators, probe holder and probes for high stability and contact integrity.
• Stations which support Wave manipulators with VersaTiles™ and Multi-site probe cards.