1. Thin film technology, reliability

2. Failure modes Film discontinuity surface Film morphology change
thin film 1
surface
interface 2
interface 1
Film discontinuity
Film morphology change
Film-film adhesion loss
Film-substrate adhesion loss
Interfacial reactions
Transport of matter across interfaces

3. Contamination filmfilm
Sputtered W thin film from 99.95% purity target contains sizable amounts of sodium (Na).
During MOS transistor use this sodium will diffuse into silicon dioxide, and cause transistor instability because its mobile charge changes flat band voltage.
substrate
thin film 1
thin film 2

4. Bathtub curve

5. Reliability Reliability can be taken to mean: performance as designed
resistance to failure
avoiding unexpected failure
It is about long term performance in the field.

6. Yield Yield is success rate during fabrication.
Yield is the quotient “good outcomes/total”, defined at different stages of manufacturing.
Chip yield: good chips/all chips
Fab yield: good product wafers/all wafers
(development/monitoring wafers constitute a sizable fraction of all wafers in circulation, e.g. 5-15%)

7. Yield vs. reliability Yield and reliability are connected:
Poor process control leads to low yields and more failing devices.
Devices with poorly controlled properties are subject to larger variation and more failures.

8. Causes of failure
The film is inherently incapable (e.g. too thin, too porous)
The film is overstressed (e.g. temperature, oxidizing ambient)
Too much variability (e.g. drifts in deposition process, or variation across wafer or across batch)
Wearout (e.g. electromigration due to high current density)

9. Causes of failure (2)
Random events: particle attachement, or corrosion couple formed
Errors: faulty part escapes inspection, or incorrect use
Incorrect specifications: e.g. accelerated tests misinterpreted
Sneaks: individual elements OK, but system fails because it goes into condition not foreseen, e.g. combined high temperature and overvoltage

10. Stress and strain 0.2% Murarka
Elastic/Linear region, E = σ/ε (Hooke !)
Slope of stress-strain gives Young’s modulus
Plastic I: permanent deformation
Yield strength: 0.2% shifted curve intersects stress-strain curve
Maximum stress = ultimate strength
Plastic II: necking occurs
For aluminum thin films:
Young’s modulus: 74 GPa
Ultimate strength: 176 MPa
Yield strength: 124 MPa
Ultimate strength
Elastic limit = yield strength
0.2%
Murarka

11. Polysilicon stress-strain
Brittle at 540oC
Ductile at 890oC

12. Ti-Al reaction effect
Ti3Al precipitates (shown by arrows) strengthen the material, but have only a minor effect on resistivity.
10 nm Ti/500 nm Al
Annealed at 550oC.
Lee & Bravman

13. Stress evolution
Much more extensive relaxation takes place in the Al thin films than in bulk.
Thin film grain size, typically ~1 µm vs. ~ 100 µm for bulk, would enable much more extensive grain boundary sliding and hence greater stress relaxation.
Hoo-Jeong Lee,a) Guido Cornella, and John C. Bravman: Appl. Phys. Lett., Vol. 76, p. 3415
King Tu, p. 131

14. Stress relaxation (1)
King Tu, p. 133

15. Stress relaxation (2)
Aluminum hillocks causing nitride passivation cracking
(it could have been nitride stresses causing nitride cracking !)
Murarka p. 227

16. Hillock generation Compressive stress is relieved by hillock growth.
If surface is free (as in high vacuum), the surface acts as a sink for vacancies and extra atoms are uniformly distributed over the surface.
Oxide covered surface breaks randomly and hillocks are formed.
Hillocks are same size as microlithographic structures and film thicknesses.
 Hillocks can short two neighboring lines or two films.

17. Grain boundary vs. bulk diffusion
Grain boundaries and dislocations are paths of rapid diffusion (below 0.65 Tm).
Impurities and dopants are easily trapped and precipitated at defects and grain boundaries  fast diffusion paths blocked  improved resistance against processes that rely on them.
Murarka p. 93

18. Stress voiding In order to relieve stress, vacancies diffuse.
Grain boundary diffusion vs. bulk diffusion
Murarka p. 84

19. Electromigration Electron collisions move atoms. Voids formed.
Happens when current density is high.
King Tu, p. 240

20. Mean time to failure (MTF) due to electromigration
where A is a constant dependent on wire geometry and metal microstructure, J is the current density and Ea the activation energy. The factor n is not known very accurately, but n=1.7 is used for aluminum. For aluminum thin films Ea is of the order of eV, whereas for bulk aluminum it is eV. As a general trend the higher the activation energy, the better the electromigration resistance .

21. Alloy composition effect
Mean time to failure of 2.5 µm wide pure Al,
Al (0.5 wt % Cu) Al(2 wt% Cu)
lines at different temperatures with
1 MA/cm2 current density
Hu, C.-K. et al: Electromigration of Al(Cu) two-level structures: effect of Cu kinetics of damage formation, J.Appl.Phys. 74 (1993), p. 969

22. Incubation time
Incubation time before resistance increase sets in (measured at 255C)
Hu, C.-K. et al: Electromigration and stress-induced voiding in fine Al- and Al-alloy thin- film lines, IBM J.Res.Dev. 39 (1995), p. 465

23. Bamboo structure 3-grain boiundaries No 3-grain boundaries
Murarka p. 99

24. Bamboo structure (2)

25. Surface diffusion Copper cannot be plasma etched.
Copper metallization (damascene)
is done by etching oxide, depositing a barrier layer, filling by electroplating and removing excess copper by CMP. Free copper surface is created by CMP.
Copper and oxide do not adhere well, and the copper top surface is a fast diffusion path.
Electromigration mechanism of Al and Cu is thus different. OX Cu

26. Thermomigration Eutectic formed in the solder ball.
Constant temperature annealing:
No effect.
Selected bumps powered, others not, but thermal gradient established.
Thermal gradient drives Sn towards the hot end (Si-side); and
Pb to cool end (Cu-side) in the NON-POWERED BUMPS.

27. Failure of MEMS, creep
Creep, time-dependent increase in strain, under constant stress and temperature
No creep below Tm =0.3
Tm =0.3 corresponds to 5oC for Al !
In Finnish: viruminen

28. Failure of MEMS, fatigue
Fatigue, yield strength reduction due to cyclic stress
Work hardening, overstress above yield limit
Nickel films (LIGA MEMS) shown.
Endurance limits:
70 µm == 260 MPa
270 µm == 190 MPa
Materials Science and Engineering: A371, 2004, pp. 256–266

29. Fatigue of nickel

30. Corrosion E.g. aluminum is prone to corrosion.
H2O + e-  (1/2) H2 + OH-
This hydroxyl will react with aluminum.
Residues of chlorine from plasma etching of aluminum will react with ambient water, and form HCl, which reacts with aluminum.
Copper surface is easily oxidized, but copper diffuses thru oxide, and reacts at the surface with the environmental agent.

31. Stress corrosion cracking
“The cause of failure due to cyclic load is not fully understood: stress corrosion cracking depends on surface oxide stressed so that underlying silicon is exposed, but data shows that the number of cycles is determinant.”

32. Anodic oxidation
Anodic oxidation, a room temperature process, in humid environments: polysilicon oxidizes:
Si + 2 OH- + 2h+  SiO2 + H2
p. 198

33. Whisker growth due to compressive stress
Cu + Sn  Cu6Sn5
reaction takes place at room temperature.
King Tu, p. 324

34. Whisker formation: chemical driving force
Cu + Sn  Cu6Sn5
reaction takes place at room temperature.
This reaction keeps going as long as there is unreacted copper and tin.
Volume increase leads to compressive stress.
Compressive stress leads to whisker growth.
Tin oxide constrains the volume, and the surfaces do not act as sources or sinks of vacancies.

35. Purple plague: AuAl2
King Tu, p. 171

36. Purple plague 2: AuAl2
King Tu, p. 172

38. Oxide defects Na+ silicon substrate Oxide defects (left to right):-
+ + +
silicon substrate
Oxide defects (left to right):
Na+ mobile charge,
thinning,
fixed charge,
surface and interface microroughness,
pinhole,
void,
interface charge,
particle,
stacking fault.

39. Oxide defect distribution
A-mode failures are gross defects: pinholes and voids.
B-mode failures are more benign and more subtle, like oxide thinning, trapped charges or metal contamination induced defects.
C-mode failures are intrinsic to oxide structure, but can be affected by nanoscopic defects like increased surface and interface roughness.

40. Oxide defects A-mode failures are seen as yield loss;
B-mode failures as reliability problems in accelerated testing or in the field.
C-mode failures become important in the end of product life.

41. Failure of MEMS, electrical
Dielectric charging: high electric fields across (thin) dielectrics leads to some charge being trapped in insulator (and no leakage path exists)
-drift as charge slowly
builds up
-latch-up, when potential
is strong enough
to prevent movement hD ha
Capacitive RF switch

42. MEMS mirror
The black dots between and under electrodes represent trapped charge in the dielectric, as well as slowly moving mobile charges on the surface of the dielectric. The substrate is grounded while the electrodes can be grounded or held at a fixed potential.
Mirror and electrode wafers are fabricated separately and then assembled using polyimide attachment and polyimide spacers.
p. 198

43. Charging tilts a mirror
Comparison of charging induced tilt angle drift for an SOI MEMS micromirror without a Charge Dissipation Layer (CDL) on the electrodes (upper curve), and with a 40-nm-thick Co-Fe-O charge dissipating layer on the electrodes (lower curve)
with approximate composition

44. Dielectric breakdown Wear:
charges and defects cumulate in dielectric, and at threshold, current can flow.
This current causes Joule heating and creates local damage.
In MEMS devices electrical properties are often a compromised because of mechanical requirements or processing issues. Low stress nitride SiNx has lower breakdown field and faster KOH etch rate.
MEMS voltages >> CMOS !
Hartzell: MEMS Reliability, p. 144

45. Failure analysis Risks: How failure occurs ? Consequences ? Severity
Occurence
Detection
How failure occurs ?
Consequences ?
Safeguards ?
Actions ?