Transcript Fluorocarbon Precursor for High Aspect Ratio Via Milling
PBS&T
Fluorocarbon Precursor for High Aspect Ratio Via Milling in Focused Ion Beam Modification of Integrated Circuits Valery Ray Particle Beam Systems & Technology, Methuen, USA [email protected]
Purpose
Introduce Trifluoroacetic Acid (TFA) as a precursor for Gas Assisted Etching (GAE) of dielectrics in Focused Ion Beam (FIB) system.
Demonstrate viability of TFA precursor for High Aspect Ratio (HAR) via milling.
Provide starting-point guidelines for GAE recipe development with newly introduced precursor gases. 4/28/2020 ISTFA 2004 2
Outline
Overview of precursors for Focused Ion Beam Gas Assisted Etching (FIB GAE).
Trifluoroacetic Acid and FIB GAE of dielectrics.
Considerations for development of etching recipes.
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Enhancements of FIB GAE Precursors Al Precursor Gas Cl 2 Si ~ 8 Br 2 XeF 2 ~ 6 4/28/2020 SiO ~ 1 ~ 1 2 W ~ 1 ~ 1 800 ~ 2800 7 ~ 10 ISTFA 2004 8 ~ 10 8 ~ 16 6 ~ 10 ~ 1 4
4/28/2020 TFA – Physical Properties
At room temperature: thick, heavy liquid;
Critical Vapor Pressure: 101 ± 7 Torr;
Boiling temperature: 72 ˚C (162˚F);
Easy delivery by existing GAE apparatus.
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TFA – Molecular Structure 4/28/2020 Reactive Group F F C C Attachment Group O O H F ISTFA 2004 6
SiO2 Etching by TFA Proposed by Dr. Clive Chandler, US Patent 6,211,527
C
2
F
3
O
2
H + SiO
2
+ FIB SiF
4
+ CO
2
+ H
2
O
4/28/2020 There is a small excess of carbon.
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TFA – Etch Inhibition on Metals and Si Proposed by Dr. Clive Chandler, US Patent 6,211,527
C
2
F
3
O
2
H + Si + FIB SiF
4
+ CO
2
+ H
2
O
4/28/2020 There is a large excess of carbon.
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TFA – High Aspect Ratio Via Good via profile control for SiO 2 milling.
Very small amount of carbon is generated during the milling and facilitates sensitive endpoint.
Milling of Si and metals is inhibited by carbon deposition. 4/28/2020 ISTFA 2004 6.2
μm 0.68
μm 0.67
μm 9
GAE Recipe Development: Dose Enhancement and Milling Rate
FIB GAE theory (K. Edinger, JVST B 18(6), 2000, and Microelectron. Eng. 57 – 58, 2001) is dealing with yield enhancement.
Milling rate is also important in industrial applications of FIB.
Maximal rate recipes required for effective High Aspect Ratio via milling.
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GAE Recipe Development: Yield Equation Removed Atoms AR + AS
Yield = ------- = ------------
Incident Jt D Ions AR ( Atoms Reacted ) – FAST , parameter-sensitive, not limited by aspect ratio .
AS ( Atoms Spattered ) – SLOW, limited by aspect ratio J - Ion Beam Current Density t D – Time of beam dwell within the pixel 4/28/2020 ISTFA 2004 11
GAE Recipe Development: Two Phases of GAE Within Pixel
t
D
= t
AR
+ t
AS For effective GAE 4/28/2020
t
D
→ t
AR
, and t
AS → 0 ISTFA 2004 12
GAE Recipe Development: Reactive Yield vs. Mill Parameters Parameter Change And Limit Effect on Reactive Yield AR Pixel Dwell 0.2
μSec Pixel Overlap ~ 0 Pixel Refresh 1~ 10mSec 4/28/2020 ISTFA 2004 13
GAE Recipe Development: Gas Refresh Within Pixel
FIB GAE process is localized within a pixel.
Replenishment of gas begins when ion beam moves away from the pixel.
Therefore pixel refresh time is a critical parameter for gas replenishment.
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GAE Recipe Development: Timing of Pixels within Raster
t
Raster
= t
Refresh n
= Σt
Di i=0 4/28/2020 Raster time equivalent to refresh time provides most efficient GAE.
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GAE Recipe Development: Gas Refresh Defines Number of Pixels
t
Refresh
N = ------------------ t
D = 0.2 μSec 4/28/2020 Shortest pixel dwell, available in modern FIB systems, is close to 0.2 μSec.
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GAE Recipe Development: Via Size “L” Defines Pixel Distance
L dX = dY = ------------------- (Sqrt (N) - 1)
Dwell points are desirable on the edges of the via.
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GAE Recipe Development: Pixel Distance Defines Beam Size For uniform orthogonal raster:
D
Beam
= dX = dY
Beam diameter equivalent to pixel distance ensures minimal overlap and maximal yield.
Corresponding current value is controlled by the FIB system; diffused beam is desirable.
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GAE Recipe Development: Numerical Example 2 μm via in Si milled with Cl 2 , t Refresh = 1 mSec; N = 1000 μSec / 0.2 μSec = 5000 pixels / raster; dX = dY = 2 μm / (Sqrt(5000) – 1) = ~ 30 nm; Corresponding beam current depends on FIB system, diffused beam is desirable; Extra refresh for milling of UHAR vias, extra beam current for surface micromachinning 4/28/2020 ISTFA 2004 19
GAE Recipe Development: Equipment Limitations
Research system behavior (scanning and blanking) during GAE raster to understand limitations imposed by the hardware!
Pixel dwell shorter then 0.2 μSec?
Beam diffusion (de-focus) control?
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GAE Recipe Development: Raster Generation Techniques
Line-interlaced scanning – improved pixel refresh;
Varied scan direction – improved floor uniformity;
Non-uniform and non-orthogonal rasters;
Lissajous-like patterns.
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Summary
Trifluoroacetic Acid is introduced as a viable GAE precursor for FIB milling of dielectrics.
Good HAR via profile control with TFA and sensitive endpoint on Si is demonstrated.
Starting-point considerations for development of GAE recipes are discussed.
Further research on maximizing the rate of FIB GAE milling is needed.
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Acknowledgements
Author would like to thank Mr. Nicholas Antoniou, Dr. Clive Chandler, Dr. Tom Gannon, Mr. Alex Krechmer, and Mr. Andrew Saxonis from FEI Company for contributions to the abstract of this presentation.
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