Source of film stress: growth of polycrystal
非平衡状态下的沉积造成的。
23
在薄膜的沉积或生长中，经常可以观测到应力的生长，这是因为薄膜在
报告提纲
24
Second – Foundation
Learning the bending mechanics
Film-on-substrate structures
1 v 12 M 1 v 6 f h f 3 Es hs Es hs2
Stoney formula
Es h Es h f 1 v 6h f 1 v 6h f R
Stoney formula: relationship between the stress and curvature
Depression
Stress engineering: bad situations
12
Delamination Fatigue Crows-feet Striae gravidarum
Stress engineering: Beneficial applications
Wrinkle and buckle are ubiquitous phenomena
Once buckled, the column’s ability to carry load is greatly reduced.
Buckling
9
Buckling
新西兰6.3级地震
10
Stressed-out behavioห้องสมุดไป่ตู้s Stress Tension Pressure
11
Anxiety
The S.S. Schenectady split apart by brittle fracture while in harbor (1944)
Fracture: utility
7
Stability failure
8
Buckling is a phenomena of compression members. Once the buckling load, or critical load, is reached, the column will instantly loose stability and “kick out” to one side.
15
Nature Materials 4, 271(2005) Nature Materials 4, 293(2005)
Foldable Electronics on a pliable substrate
Robustness against bending, stretching, folding
29
where,
elastic f
1 vf E f
f

elastic s
ts 1 vs 4t f f y 2 Es t s
1 vf E f
1 vs 4t f f misfit 0 f f E t E f s s 1 v f
25
(a) Basic structures of flexible electronics; (b) the size of thin film
Film-on-substrate structures：bending
26
The radius of curvature is adopted to denote the bending deformation of flexible electronics. The smaller shows better deformability.
16
Nature Nanotechnology 1, 201-207(2008)
Buckled Sphere
17
Cao Guoxing, Chen Xi, C. R. Li, Z. X. Cao, PRL100, 2008
Peel-Pick-Place
18
Mechanics of flexible electronics
E T misfit 1 v
Source of film stress: Epitaxial stresses
between film and substrate
22
The epitaxial stresses occurs when there is ideal continuity
Young’s modulus： ~103-105
Coefficient of heat conduction ： ~103 Coefficient of thermal expansion ： ~10-2 Soft-substrate and hard film
Source of film stress: thermal stress
4
Strength failure
Mechanical failure is determined by the weakest components of devices The crack strain of inorganic brittle material is about 1-2%，and the metal is also broken after necking The freestanding metal film is different from film-on-substrate in cracking
5
Fracture
6
Moore, G. Electronics 38, 114–117 (1965)
Fracture in microstructure
Vertical stabilizer, which separated from American Airlines Flight 587, leading to a fatal crash
外延应力产生示意图
misfit
a film asubstrate asubstrate
xx C11 C yy 12 zz C12
C12 C22 C12
C12 xx ex [100] e [010] C23 yy y C11 zz ez [001]
21
The misfit is resulted from the different coefficient of thermal expansion , so the thermal strain is generated when temperature changes
misfit film substrate T T0
Stretchability as large as 50%
Soft-hard material compound structures
of flexible electronics (Science, 2011)
20
Soft-hard material compound structures is the key technology Severe mismatch between soft and hard material, so there is coupling action due to large deformation, temperature and electrical field Soft material Vs Hard material
Chapter Flexible Electronics Device & Manufacturing
Outline
2
First – Failure Mode
Causing tremendous loss in daily life
Failure Modes
3
Columns can fail if the applied load
Stiff skin on compliant substrate: A universal model
− As a result of ageing, the soft dermis contracts, placing the system stress. − Hierarchical internal stress wrinkling by under compressive
2 s
2 s
Film-on-substrate structures：bending
薄膜和基板中的应力 基板和薄膜界面应力
− 薄膜中： − 基板中：
28
xx f
xx compression bending
tf 3t f 4t f ts xx y f f f 2 ts ts ts
13
脱氢作用dehydrogenation
Nature 419, 579(2002) Phys.Rev.Lett.90, 074302(2003)
Buckling patterns of Au on PDMS
14
Scripta materialia 50, 797(2004)
Nested Buckling of Artificial Skin
John A. Rogers: Mechanics is the future of flexible electronics
19
Features of flexible electronics: complex deformation: bend, buckle, stretch, twist Challenges: Radius of curvature as small as 1mm