THIN GATE OXIDE Growth and Reliability The main

THIN GATE OXIDE Growth and Reliability

The main character of our story: The MOSFET q No other human artifact has been fabricated in larger numbers (except perhaps nails? ) q “…some consider it one of the most important technological breakthroughs in human history…” (Wikipedia, the source of all human knowledge)

Scaling q Shrink dimensions maintaining aspect-ratio q Must shrink electrostatic features as well (depletion regions→ doping level and profiles) ↔ 1 μm 1980 → 1985 → 1988 → 1991 → 1994 → 1999 → 2003 … LGATE = 10 μm 5 μm 2. 5 μm 1. 3 μm 0. 63 μm 0. 25 μm 130 nm ….

4 LECTURE OBJECTIVES • Understand the importance and requirement of thin gate oxide • Able to describe the technique to grow high quality thin oxides • Understand the nature of Mode-A, B &C of breakdown failure • Able to calculate the oxide strength, τ BD and QBD

5 MOSFET: Electronics Building block

6 Gate Dielectric Scaling

7 Gate Dielectric Trend

8 Gate Dielectric Manfacturability

9 Gate Oxide

10 Scaling from 90 nm to 22 nm and innovations that enable the scaling

11 Oxide Charges and Traps

12 Oxide Charges and Trap

Gate Oxide 13 L GATE Gate Oxide n+ ID P-well n+

Gate Oxide Requirements • Possible to grow thin oxide precisely and uniformly across the wafer • Adequate reliability characteristics under operating conditions in terms of strength (Breakdown Voltage), Reliability of operation over specified time (τ BD, QBD) and resistance to hot-carrier degradation)

15 Desired Gate Oxide Characteristics • The thickness closely match the specification in MOSFET design • Uniform across the wafer, from wafer to wafer, run to run. • Small interface charge • High dielectric strength • Long lifetime under normal operating conditions • High resistance to hot-carrier damage

16 Thermal Oxidation

17 Rapid Thermal Annealing

18 Furnace vs RTP Annealing

19 Technology of Thin Oxide Growth 1. Oxidation furnace Vertical furnace is more favorable compared to horizontal. 3 reasons: • Wafer/wafer holders make no contact with oxidation tube during loading, growth and unloading (fewer particles generated). • Lighter boat material can be used, as a result, better heat and gas distributions (result in better oxide uniformity). • Precise control of wafer to wafer spacing.

20 Technology of Thin Oxide Growth 2. Control of Growth Rates • Slow growth rates required to reproducibly grow thin oxides with precise thickness. • Grow in dry O 2 at atmospheric pressure at lower temperatures (800 -900 C). • Growth at reduced total pressure, or reduced O 2 partial pressure

21 Technology of Thin Oxide Growth Typical Thin Gate Oxide Process 1. Carried out in vertical furnace 2. Prior to oxide growth; 1. Grow and strip sacrificial oxide (to remove defect in silicon layer) 2. Cleaning procedure (normally generic of RCA cleaning) 3. Loaded by robotic wafer handling at specific insertion rates ~ 15 cm per minute for 150 mm wafer, 10 cm per minute for 200 mm wafer. Furnace temperature ~ 650 – 700 C.

22 Technology of Thin Oxide Growth Typical Thin Gate Oxide Process 4. Furnace temperature is ramped up to the growth temperature. During temperature ramp up, N 2 or Ar is purged to prevent unwanted growth. 5. Temperature stabilisation around 5 min before O 2 is released. 6. Dry oxidation at atmospheric pressure at temperature ~ 800900 C. Some companies use dry/wet/dry to control thermal budget. Others use HCl, TCA , DCE as chlorine source.

23 Technology of Thin Oxide Growth Typical Thin Gate Oxide Process 7. Wafers subjected to Post Oxidation Annealing (POA) in N 2 at ~100 C. The purpose is to minimise the interface trap density by neutralising the dangling bond of Si with H atoms. This will improve the τ BD and QBD. 8. Furnace temperature is ramped down, and wafer is unloaded at certain rate.

24 Gate Oxide Characterisations 1. Gate oxide Strength a) Breakdown voltage, then calculate the breakdown electric field. 2. Gate Oxide Reliability a) τ BD – time to breakdown b) QBD – charge to breakdown

Gate Oxide Strength Definition – The maximum electric field strength that can be applied to the oxide before it breaks down. Unit MV/cm. Test procedure – Ramped Voltage Test using MOS capacitor

26 Gate Oxide Strength T 1 GATE Gate Oxide substrate T 2 • A voltage that is increase linearly with time (ramp voltage, ∆V / ∆t) is applied between the gate and substrate of MOS test structure until oxide breakdown. • Breakdown is defined at the voltage value VBD at which a sudden voltage drop across the oxide is observed. • Ramp voltage between T 1 and T 2, measure current. • The sudden voltage drops at the electrode is detected using a sensing resistor.

27 Ramp-voltage Oxide Breakdown set-up

28 Gate Oxide Strength Ig V t VBD V • Take the voltage at the voltage drop as the breakdown voltage (VBD) • Calculate the oxide strength by VBD / oxide thickness

29 Example Thin oxide of 50 nm is stressed under voltage ramp test until it breaks at 1 V. Calculate the oxide strength. Oxide strength = Breakdown voltage / oxide thickness = 1 V / 5 e-6 cm = 0. 2 e 6 V/cm = 0. 2 MV / cm

30 Mode of Breakdown Failure A histogram is usually used to plot the results of the ramped-voltage tests (oxide strength) of a group of oxide samples (normally up to 1000 capacitor tested to characterized certain oxidation process recipe), with the no of group that failed within each voltage interval (within each 1 MV/cm) Breakdown Frequency Mode-B (2 -6 MV/cm) Mode-C (8 -12 MV/cm) Mode-A (<1 MV/cm) 5 10 Breakdown Field, MV/cm

31 Mode-A • Fail instantly upon the application of a small gate bias. • Its believed that this oxide may experienced a gross defect such as pinholes and may already be shorted before the application of low strength field.

32 Mode-B • Fail at electric field of intermediate strength (2 -6 MV/cm) • Contain weak spots that do not produce instant shorting, but may give rise to early failures of ICs under normal operating conditions. • Majority of oxide breakdown failure in sub-micron CMOS fall under this category. Mostly due to the defects exist in the oxide.

33 Mode-B • Defects include; • Sodium contamination – originated from W furnace filament, chemicals • Metal contamination – from substrate, other processes • Surface roughness at Si-Si. O 2 interface from etching or cleaning procedures – promote localize weak spot • Non-uniformity of oxide growth • Crystalline defect – defect originated during crystal growth

34 Mode-C • Can withstand the highest electric fields (8 -12 MV/cm) • Failure mechanism always referred as intrinsic failure. • Generally assumed as defect-free oxide. • Several models proposed to explain the intrinsic breakdown

35 Mode-C • Models to explain the intrinsic breakdown • Holes generation and trapping model. Electrons are injected into the conduction band of oxide by FN tunneling. In the oxide, these electrons are accelerated towards the gate, and generate electronhole pair in the oxide. These generated holes are trapped at the localised areas and in return trap positive oxide charge. This will increase the positive charge at certain point in the oxide, causing the tunneling current density to increase there up to critical point where the breakdown occurs. • Wolters Electron Lattice-Damage Model

36 Gate Oxide Reliability Gate oxide strength is not directly relevant to the normal device operation, since what is really needed is how long the thin oxide will survive at lower field strength. The measurement of oxide performance at lower electric field is called Time-Dependent Dielectric Breakdown (TDDB). • 1) Time to Breakdown under Constant-Voltage Stressing (τ BD) • 2) Time to Breakdown under Constant-Current Stressing (τ BD) • Charge to Breakdown (QBD)

37 Gate Oxide Reliability Time to Breakdown under Constant-Voltage Stressing Electric field in the oxide is held constant (voltage is held constant) during the stress test. The length of time, τ BD elapsed until breakdown occurs is then measured.

38 Gate Oxide Reliability Time to Breakdown under Constant-Current Stressing. - An alternative to constant-voltage test. Current is injected into the oxide by Fowler- Nordheim tunneling, and this value Jinj is held constant. Voltage and time are recorded until breakdown occurs.

39 Fowler-Nordheim Tunneling

40 Charge to Breakdown In a constant-current test; - QBD is charge-to-breakdown. - The product of current density and t. BD QBD = Jinj. τBD In a constant voltage test;

41 Gate Oxide Reliability Emperical Model to Estimate Oxide Reliability Applicable to Mode-B and Mode-C Failures 1. QBD for intrinsic oxide is approximately constant for small FN tunneling current. 2. Qp (hole charge to breakdown) also constant. Qp = Jpα τ BD Where at 300 °C sec , G=350 MV/cm

42 Example Calculate the time-to-breakdown, τ BD at 300 K of 8 nm defect free gate oxide, use in CMOS technology with Vcc 5. 0 V. τ BD (300 K)= 1 e-11 (sec) exp (350 e 6. Tox / Vox) = 1 e-11 exp (350 e 6. 8 e-7 / 5. 0) = 2 e 13 sec

43 HOMEWORK Calculate the minimum thickness of a defect free oxide that could be used in a MOSFET that is to operate at 5. 5 V for 10 years at 150 C (423 K) without suffering oxide breakdown. Given G(423 K)= 283 MV/cm and Τ 0 (423 K)=0. 75 e-11 sec. Comment on reliability of the MOSFET to actually survive 10 years without breakdown. tox min = 87. 82 Å <- This is the answer