Evaluation/Analysis of Reliability
What is reliability?
- Reliability is generally defined as the lifetime that how long the product or component can be used safely without failure.
- Semiconductor devices and semiconductor chips should satisfy the various reliability specifications.
- Exact evaluation and analysis of reliability characteristics are high necessary for exact estimation of device lifetime.
Necessitates of reliability evaluation
- General semiconductor chip is consisted of tens of millions or several billions of devices.
- If one device among the billions of devices goes wrong, the semiconductor chip would not operate properly.
- Moreover, the system which includes the semiconductor chip will not operate properly.
- Reliability spec. of semiconductor device is much more severe than general electronic products and components.
- Therefore, accurate evaluation of diverse reliability items is a key for exact estimation of reliability lifetime.
Test and Analysis Items
- Hot Carrier, BTI(Bias Temperature Instability), TDDB (Time Dependent Dielectric Breakdown) and others
- Voltage stress and/or current stress is applied for the passive devices such as capacitor, resistor and inductor.
- Electro-migration(EM) and stress migration(EM) for back-end-of-line process.
Extraction of Hot Carrier Lifetime
- High energy carriers named hot carrier are generated by high electric field near the drain edge of MSOFETs.
- Hot carrier transfers its energy to silicon lattice through phonon emission and breaks bond at the interface between Si and SiO2
- A portion of hot carriers become trapped inside the gate dielectric (SiO2)
- Trapping of hot carriers and breakdown of bond generate oxide trapped charge and interface charge and results in the decrease of carrier mobility.
- Degradation of device performance due to the increase of Threshold voltage (VT), decrease of Transconductance(gm) and Drain current (ID)
- Measurements of the variation ratio of above three device parameters as a function of stress time and extraction of lifetime at the operation voltage
Extraction of NBTI(Negative Bias Temperature Instability) Lifetime
- Two BTI stresses : PBTI of NMOS and NBTI of PMOS
- NBTI reliability is normally evaluated in SiO2 based CMOS technology because PMOS degradation by NBTI is much more severe than NMOS degradation by PBTI.
- Although Negative Bias Temperature Instability (NBTI) phenomenon is one of the key reliability issue, its physical mechanism is not fully understood.
- It is believed that NBTI is controlled by the electro-chemical reaction.
- Holes in PMOSFET channel react with silicon bond(Si-H, Si-D, etc) at the interface and donor type interface state and positive fixed charge.
- NBTI stress is performed at high temperature because the electro-chemical reaction is dependent on the electric field and stress temperature.
- Generation of Interface state and positive fixed charge induce the increase of threshold voltage (VT)
- Variation of threshold voltage is commonly used to evaluate the degradation of MOSFET by NBTI stress.
Evaluation of TDDB (Time Dependent Dielectric Breakdown) Lifetime
- Reliability of gate dielectric which determines the performance of MOSFET or Capacitor is one of key items in developing MOSFET devices.
- Fabrication of MOSFET or Capacitor is meaningless if the reliability of dielectric does not meet the spec.
- Analysis of Time Dependent Dielectric Breakdown(TDDB) of gate dielectric is one of the key reliability factor.
- Dielectric breakdown greatly affects the dielectric reliability as the thickness of MOS gate dielectric becomes thinner due to the scale down of MOSFET
- TDDB reliability is getting more and more important as the high-k dielectric is adopted for the gate dielectric or capacitor dielectric.
Gate leakage current versus stress time under constant voltage stress(CVS). At first, gate leakage increases due to the hole trapping and then decreases due to the electron trapping. [Ref : Journal of Applied Physics, Vol. 85, No. 11, 1 June 1999]
Applicable devices and products
- Semiconductor devices (Si MOSFET, III-V MOSFET, Power MOSFET, etc..)
- Capacitors (Discrete capacitor, 3-D capacitor, etc..)
- Bio device/sensor
- MEMS device/sensor