1. Plasma-only etching strategy for PS-b-PMMA nanostructures
P.Bézard, G. Cunge, O. Joubert, E. Latu-Romain – LTM, CNRS
X. Chevalier – Arkema
R. Tiron, A. Gharbi, M. Argoud – CEA LETI
Philippe Bézard- PS-b-PMMA dry etching

2. Block copolymer (BCP) as an alternative lithography
Courtesy X. Chevalier & A. Gharbi
PS-b-PMMA self assembly in vertical PMMA cylinders configuration
(1) Surface neutralization with brush (PS-r-PMMA)
Spin coating
of brush
Bake of
Brush layer
Washing of
brush layer
(2) Self assembly of BCP
(PS-b-PMMA)
of BCP
Bake for
Self assembly
Acetic acid bath
Application:
Contact shrink
Main issue: CD control
X.Chevalier et al., Proc of SPIE 2011, 7970
Philippe Bézard- PS-b-PMMA dry etching 2
M. Darnon - SPIE San Francisco
M. Darnon - SPIE San Francisco 2

3. Philippe Bézard- PS-b-PMMA dry etching
Pattern transfer
PMMA residues etching
Brush etching
acetic acid treatment
Si (100)
Hard mask
PS-r-PMMA
Si (100)
Hard mask
PS-r-PMMA
Si (100)
Hard mask
PS-r-PMMA
Si (100)
Hard mask
Plasma-only etching
Si (100)
Si etching
Hard mask etching
Specific plasma etching steps must be developped to ensure nano-holes etching with a good CD control.
Philippe Bézard- PS-b-PMMA dry etching 3

4. Philippe Bézard- PS-b-PMMA dry etching
Equipment/ Experiment description
Mass Spectrometer / Ion flux probe
LED
for UVAS
(Cl2)
D2 lamp
for VUVAS
(HBr, Br2)
Quasi in-situ
XPS
We are using an industrial ICP reactor from AMAT designed to etch 300 mm wafers but modified to host plasma and surface diagnostics. System equipped with pulsed RF generators / Operation at constant peak power. Typical pulsing frequency: 1-10 kHz
Philippe Bézard- PS-b-PMMA dry etching

5. Philippe Bézard- PS-b-PMMA dry etching
PMMA removal
Why developing an « all-dry » strategy?
Philippe Bézard- PS-b-PMMA dry etching

6. Philippe Bézard- PS-b-PMMA dry etching
PS mask fabrication: acid acetic treatment
AFM
Before acetic ac.
After acetic ac.
phase
Covered by thin PS films or still partly filled with PMMA, many holes are not entirely opened.
SEM top-view of PS-b-PMMA after acetic acid Si
Acid acetic
rinsing
PMMA PS
brush
The acetic acid treatment does not eliminate the PMMA but rather pushes it to the surface  can plasma etching be used to open the holes?
Philippe Bézard- PS-b-PMMA dry etching

7. Philippe Bézard- PS-b-PMMA dry etching
Impact of N2/H2 plasma on PMMA
H2 based plasmas are widely used to etch PMMA  good initial chemistry
MIR of C=O bond
XPS C1s
H-C-O
C=O
PMMA
H2 plasmas reduce C=O and C-O bonds and hydrogenate carbone dangling bonds
 PMMA etching rate is driven by the flux of H atoms
Philippe Bézard- PS-b-PMMA dry etching

8. Philippe Bézard- PS-b-PMMA dry etching
Impact of N2/H2 plasma on PS
MIR of C-H bonds
PS is etched slowly compared to PMMA (typical selectivity 5-8).
However, PS aromatic cycles are destroyed by the H2 plasmas
 PS etching resistance is lowered after the plasma etching of PMMA residues.
Example of highly damaged PS after transfer
High PMMA/PS selectivity is expected at low energy / high H atom flux
N2/H2 plasma at high ICP power, low bias power, high pressure
Philippe Bézard- PS-b-PMMA dry etching

9. PMMA removal: optimizing H2 plasma after acetic acid
Plasma conditions adjusted to etch PMMA selectively towards PS  low ion energy
After low ion energy plasma
CD dispersion > 3 nm !
CDbias = +2 nm (PS hole diameter increase)
Philippe Bézard- PS-b-PMMA dry etching

10. Philippe Bézard- PS-b-PMMA dry etching
PMMA removal: acetic acid - microloading
Amount of PMMA residues is fluctuating from hole to hole
The lateral etch rate of a PS hole filled by PMMA is delayed / hole without PMMA
 loss of CD control (CD microloading) H
Acetic acid + plasma do not allow a satisfying CD control  what about plasma alone ?
Philippe Bézard- PS-b-PMMA dry etching

11. Philippe Bézard- PS-b-PMMA dry etching
PMMA removal: “All-dry” strategy – conditions
PS vertical etch rate must be minimized:
Low ion energy
Low ion energy
High ion energy
H atoms are responsible
for PMMA etching
High source power (ICP)
High pressure
N2 addition
Low radical flux
High radical flux
An « all-dry » process needs low energy ions with high atomic hydrogen flux.
Philippe Bézard- PS-b-PMMA dry etching

12. Philippe Bézard- PS-b-PMMA dry etching
PMMA removal: “All-dry” strategy – Results
SEM cross section of PS-b-PMMA after « all-dry » removal
SEM top view of PS-b-PMMA after « all-dry » removal
 ~35 nm remaining of initial ~40nm-thick PS mask (selectivity > 8)
CD dispersion ~2.5nm (1σ): already too high
Philippe Bézard- PS-b-PMMA dry etching

13. Philippe Bézard- PS-b-PMMA dry etching
PMMA removal: “All-dry” strategy – Defects and CD dispersion
SEM top view of PS-b-PMMA after « all-dry » removal
CD dispersion due to defects in the BCP organisation + initial CD dispersion is enhanced by plasma
 The «all-dry » opening of the PS mask is promising if lower defect density in BCP can be achieved
Philippe Bézard- PS-b-PMMA dry etching

14. Brush opening after « all-dry » Philippe Bézard- PS-b-PMMA dry etching

15. Philippe Bézard- PS-b-PMMA dry etching
Brush opening : the issue of charging effects
Ion + e - +
Due to differential charging of the features, the ion energy and flux are reduced
etch stop issues
This effect becomes stronger when Aspect Ratio increases
Solution: increase ion energy and/or prevent charging by pulsing the plasma
Philippe Bézard- PS-b-PMMA dry etching

16. Philippe Bézard- PS-b-PMMA dry etching
Example: brush opening after acetic acid
CW plasma with bias
(100 eV ions energy)
Synchonized pulsed plasma:
Ion energy > 100 eV and no charging effects
CW plasma without bias
(15 eV ions energy)
Energetic ion bombardment is mandatory to open the ultrathin brush layer in H2 plasmas
Can we use same synchronized pulsed plasma to open brush after the « all dry » strategy ?
Philippe Bézard- PS-b-PMMA dry etching

17. Philippe Bézard- PS-b-PMMA dry etching
Brush opening after “all-dry” development
SEM cross section after « all-dry » development and a H2/N2
synchronously pulsed with high bias power - optimised
Brush is not opened and PS mask is considerably damaged !!!
need for an alternative plasma chemistry to open the brush with fragile PS mask
Philippe Bézard- PS-b-PMMA dry etching

18. Philippe Bézard- PS-b-PMMA dry etching
Brush opening after “all-dry” development– CF4?
CF4 creates a thin fluorocarbon layer on top of PS:
XPS doesn’t detect any C-O (due to PMMA) after
brush opening step
XPS study of PS after brush opening step
SEM cross section after brush opening step
SEM top view of PS after brush opening step
Etching is controlled by ions -> operations at high source power low pressure
(1000Ws 5mT) to maximize the ion flux.
CF4 does etch brush layer but leads to striations
Philippe Bézard- PS-b-PMMA dry etching

19. Philippe Bézard- PS-b-PMMA dry etching
Brush opening after “all-dry” development – Avoid striations
Reducing
Ion energy
SEM observations after brush opening step
SEM observations after brush opening step
Brush layer can be opened with CF4 CW ICP plasma with lower bias power
Philippe Bézard- PS-b-PMMA dry etching

20. Philippe Bézard- PS-b-PMMA dry etching
Brush opening after “all-dry” PMMA removal – Results
SEM cross section of PS-b-PMMA after brush opening step
SEM top view of marked Silicon after brush opening
Silicon is marked as proof of efficient brush opening for each hole (PS is stripped with O2 plasma before observation).
>30nm copolymer mask remaining (due to CFx deposition)
Striations are not transfered into Silicon
Delayed etching for some holes/ loss of CD control.
Philippe Bézard- PS-b-PMMA dry etching

21. Philippe Bézard- PS-b-PMMA dry etching
PS in PMMA cylinders?
After plasma treatment, PS has been thinned down to <10nm thickness
PS instead of PMMA. PMMA cylinder didn’t organise well here
Some « PMMA cylinders » are in fact mostly PS!
Since H2/N2 plasma etches PS very slowly, there is a delay in the etching of those
defects, leading to partial transfer in the underlayer and CD control loss.
Philippe Bézard- PS-b-PMMA dry etching

22. Philippe Bézard- PS-b-PMMA dry etching
Conclusions (1/2)
Low ion energy
High pressure
High source power
H2 N2 chemistry
200 sccm H2 10 sccm N2 40mTorr 1200Ws 0Wb CW 20sec
PROCESS WORKS WELL BUT ENHANCES BCP DEFECTS!
Philippe Bézard- PS-b-PMMA dry etching

23. DEFECTS HAVE TO BE CONTROLLED FOR AN ALL DRY DEVELOPMENT
Conclusions (2/2)
Low/moderate ion energy
Low pressure (5-10mT)
High source power
CF4 chemistry
70 sccm CF4 5mTorr 1000Ws 50Wb CW 10sec
BUT: partial transfer due to PS in PMMA cylinders.
BRUSH OPENED WITH NO STRIATION IN UNDERLAYER
DEFECTS HAVE TO BE CONTROLLED
FOR AN ALL DRY DEVELOPMENT
Philippe Bézard- PS-b-PMMA dry etching

24. Philippe Bézard- PS-b-PMMA dry etching
Thank you for your attention!
This work has been partially supported by the LabEx Minos ANR-10-LABX-55-01
Philippe Bézard- PS-b-PMMA dry etching