1. Valery Ray Particle Beam Systems & Technology, Methuen, USA vray@partbeamsystech.com Fluorocarbon Precursor for High Aspect Ratio Via Milling in Focused Ion Beam Modification of Integrated Circuits PBS&T

2. 5/20/2015 ISTFA 2004 2 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.

3. 5/20/2015 ISTFA 2004 3 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.

4. 5/20/2015 ISTFA 2004 4 Enhancements of FIB GAE Precursors Precursor Gas Si SiO 2 WAl Cl 2 ~ 8 ~ 1 8 ~ 16 Br 2 ~ 6 ~ 1 6 ~ 10 XeF 2 800 ~ 2800 7 ~ 10 8 ~ 10 ~ 1

5. 5/20/2015 ISTFA 2004 5 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.

6. 5/20/2015 ISTFA 2004 6 TFA – Molecular Structure F F F C C HO O Reactive GroupAttachment Group

7. 5/20/2015 ISTFA 2004 7 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  There is a small excess of carbon.

8. 5/20/2015 ISTFA 2004 8 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  There is a large excess of carbon.

9. 5/20/2015 ISTFA 2004 9 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. 6.2μm 0.68μm 0.67μm

10. 5/20/2015 ISTFA 2004 10 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.

11. 5/20/2015 ISTFA 2004 11 GAE Recipe Development: Yield Equation Yield = ------- = ------------ AR + AS Removed Atoms Incident Ions Jt D 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

12. 5/20/2015 ISTFA 2004 12 GAE Recipe Development: Two Phases of GAE Within Pixel t D = t AR + t AS t D → t AR, and t AS → 0 For effective GAE

13. 5/20/2015 ISTFA 2004 13 GAE Recipe Development: Reactive Yield vs. Mill Parameters Effect on Reactive Yield AR Pixel Refresh 1~ 10mSec Pixel Overlap ~ 0 PixelDwell 0.2μSec ParameterChange And Limit

14. 5/20/2015 ISTFA 2004 14 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.

15. 5/20/2015 ISTFA 2004 15 GAE Recipe Development: Timing of Pixels within Raster t Raster = t Refresh = Σt Di i=0 n Raster time equivalent to refresh time provides most efficient GAE.

16. 5/20/2015 ISTFA 2004 16 GAE Recipe Development: Gas Refresh Defines Number of Pixels N = ------------------- t Refresh t D = 0.2 μSec Shortest pixel dwell, available in modern FIB systems, is close to 0.2 μSec.

17. 5/20/2015 ISTFA 2004 17 GAE Recipe Development: Via Size “L” Defines Pixel Distance dX = dY = -------------------- L (Sqrt (N) - 1) Dwell points are desirable on the edges of the via.

18. 5/20/2015 ISTFA 2004 18 GAE Recipe Development: Pixel Distance Defines Beam Size  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. D Beam = dX = dY For uniform orthogonal raster:

19. 5/20/2015 ISTFA 2004 19 GAE Recipe Development: Numerical Example 2 μm via in Si milled with Cl 2, t Refresh = 1 mSec; N = 1000μSec / 0.2 μSec = 5000 pixels / raster; dX = dY = 2μm / (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

20. 5/20/2015 ISTFA 2004 20 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?

21. 5/20/2015 ISTFA 2004 21 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.

22. 5/20/2015 ISTFA 2004 22 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.

23. 5/20/2015 ISTFA 2004 23 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.