# Presentation: 334311

## Degradation and Breakdown of 

- 2Center for Reliability Engineering, University of Maryland

- Degradation and Breakdown of
- Ultra-thin Silicon Dioxide by
- Electron and Hole Injection
- Eric M. Vogel1
- M. Edelstein1, J. Suehle1, D. Heh2, and J. Bernstein2

## E vs. 1/E Lifetime Extrapolation - Neither are Valid

**E vs. 1/E Lifetime Extrapolation - *****Neither are Valid***

## Motivation #1

**Motivation #1**
- Determining physical model for oxide breakdown (Vg < ~ 7.5 V)

***Anode Hole ***
_***Injection***_* *

***Trap Creation/***
_***Hydrogen Release***_

_***Energetic Carrier Models***_

_***Electric Field Models***_

**Motivation #2 **
- Comprehensive and self-consistent understanding of MOSFET degradation and breakdown that includes hot-carrier and uniform tunneling stress conditions

- (Thermochemical-E)

## Charge-to-Breakdown (Qbd) for CVS

-   For CVS, the Qbd is independent of substrate bias.
-   For ultra-thin oxides (< ~3.0 nm), Qbd versus Vg  is approximately independent of thickness.

## Time-to-Breakdown (tbd) versus Vg for CVS

- Time-to-Breakdown (tbd) versus Vg for CVS

-   At a given Vg, the tbd increases with increasing thickness due to the smaller current flowing in a thicker oxide at a given gate voltage.

## Time-to-Breakdown (tbd) versus Eox for CVS

- Time-to-Breakdown (tbd) versus Eox for CVS

-   At a given Eox, the tbd decreases with increasing thickness because a larger gate voltage is required for a thinner oxide to obtain a given electric field.

## Experimental for SHE/CVS

-   N-channel MOSFETs
-   <100> silicon, Na = 2x1017 cm-3
-   tox = 2.0 nm to 3.4 nm
-   n+ polysilicon
-   Constant Voltage Stress (CVS),
-     Vg > 0, Vs = Vd = Vb = 0
-   Substrate Hot Electron (SHE) Stress,
-     Vg > 0, Vs = Vd = 0, Vb < 0, Vinj < Vb
-   Stress Induced Leakage Current (SILC)
-     and sinusoidal Charge Pumping (CP)
-     used to monitor electrically active defects.

*** ******SHE set-up***

## SHE Gate Current Density Characteristics

-   For low gate voltages, the SHE current dominates the gate current and is dependent on the injector bias.
-   For higher gate voltages, the normal tunneling current dominates the gate current characteristics.

## SHE Band-diagram

-   The gate current is the sum of the SHE current and the tunnel current.
-   The SHE current has an energy distribution at the interface determined mainly by the substrate bias, and a density that can be controlled using the injector bias.
-   The tunneling carriers have energy in the silicon that corresponds approximately to the bottom of the conduction band and a density that is determined by the gate bias.

## tbd versus Vg for CVS and SHE

-   The tbd versus Vg characteristics are inversely proportional to the gate current density.

- tbd versus Vg for CVS and SHE

## Representative Dit Build-Up

-   For thin oxides (<~ 3.0 nm) the number of interface states at breakdown measured using charge-pumping is approximately independent of thickness and stress condition (CVS vs. SHE).

## Summary SHE and CVS Reliability

-  The results confirm that energetic electrons are responsible for degradation and breakdown of ultra-thin silicon dioxide.

***Anode Hole ***
_***Injection***_* *

***Trap Creation/***
_***Hydrogen Release***_

_***Energetic Carrier Models***_

## Experimental for SHH/CVS

-   P-channel MOSFETs
-   <100> silicon, Nd = 2x1017 cm-3
-   tox = 2.0 nm and 3.0 nm
-   p+ polysilicon
-   Constant Voltage Stress (CVS),
-     Vg <> 0, Vs = Vd = Vb = 0
-   Substrate Hot Hole (SHH) Stress,
-     Vg < 0, Vs = Vd = 0, Vb > 0, Vinj > Vb
-   Stress Induced Leakage Current (SILC)
-     and sinusoidal Charge Pumping (CP)
-     used to monitor electrically active defects.

*** ******SHH set-up***

## Substrate Hot Hole (SHH) Injection vs. 

- Anode Hole Injection (AHI)

**SHHI on a p-channel MOSFET**
- (Vg<0, Vb>0, Vinj>Vb)

**Proposed AHI on a **
**n-channel MOSFET**
- (Vg > 0, Vb=0)

## Gate Voltage Dependence of CP Defects 

- Produced by SHH Stress

-   For SHH stress, the number
- of defects produced per hole
- injected is independent of gate
- voltage (oxide field).
-   The Nbd for SHH stress is
- much greater than the Nbd for
- CVS.
-   SILC and CP show similar
- results.

-   *Breakdown occurred after the*
*final defect measurement shown*

## Temperature Dependence of Defect Generation by SHHs

-   Defect generation by ***SHHs*** is
***decreased ***at higher temperatures.
-   Defect generation by ***CVS*** is
***increased*** at higher temperatures.

## Temperature Dependence of Breakdown by SHHs

-   Breakdown by SHHs is observed
- to have a very weak dependence
- on temperature as compared to
- breakdown by CVS.
-   This is because defect generation
- by the trapping of holes has a very
- weak temperature dependence.

## Effect of Trapped Holes on CVS Breakdown

*  *These results show that prior
- injection of holes does not result
- in a reduction of subsequent CVS
- Qbd.
-   This again illustrates the
- ineffectiveness of defects
- generated by holes to cause
- breakdown.
-   This suggests that the recently
- theorized1 “hole-catalyzed
- thermochemical electric field
- model” is incorrect.

- 1J. W. McPherson et al., J. Appl.
- Phys. **88**, 5351 (2000).

## Calculation of Ratio of Holes to Electrons

- 1) Measure the tunnel gate current: Itun (Vb=0,Vg)
- 2) For high Vinj, measure the SHH gate current at the Vb of interest: Ishh (Vb>0, Vg, Vinj=high)
- 3) For high Vinj, measure the hot hole current impinging the interface: Ids (Vb>0, Vg, Vinj=high)
- 4) For high Vinj, calculate the transmission coefficient for hot holes:
- 	T  [Ishh (Vb>0, Vg, Vinj=high) - Itun (Vb=0,Vg)]/Ids (Vb>0, Vg)
- 	*Assume that T is independent of injector bias and time.
- 6) Calculate the ratio of holes to electrons for any
- injector bias:
- 	R  [T x Ids (Vb>0, Vg, Vinj)]/ Itun (Vb=0,Vg)

## Bulk Defect Generation Measured Using SILC

-  The rate of bulk defect
- generation as a function of hole
- charge injected is observed to
- be similar even though the ratio
- of hole current to tunneling
- current is different by two orders
- magnitude.

## Effect of Hole to Electron Ratio on Subsequent Breakdown

-  SHHs were injected to ~2 C/cm2
- with two different hole to electron
- ratios.
*** ***The number of defects at
- ~2 C/cm2 was on the order of the
- CVS Nbd.
-  Subsequent CVS breakdown
- measurements were performed.
-  The breakdown susceptibility
- was not effected by the hole to
- electron ratio.

## Summary

-  The energetic electron fluence entering the anode is responsible for
- the degradation and breakdown of ultra-thin silicon dioxide under
- constant voltage tunneling stress.
-  Hole trapping alone can not explain the breakdown of ultra-thin
- silicon dioxide under tunneling stress conditions.
-  The interaction of tunneling electrons with defects created by holes
- is not a viable mechanism for explaining the catastrophic breakdown
- of ultra-thin silicon dioxide.
-  The results imply that hydrogen release from the anode may be
- responsible for the degradation and breakdown of ultra-thin silicon
- dioxide.