April 12, 2019

ASD 2019

John Tolle and Joe Margetis

SELECTIVE EPITAXIAL DEPOSITION

April 12, 2019 | ASD 2019

TALK OVERVIEW

› Epitaxial reactor introduction› Mechanism for Selectivity› Naturally selective process –no HCl needed› Selective Epitaxial Growth –co-flow process› Making use of the nucleation delay› Cyclic Deposition and Etch

April 12, 2019 | ASD 2019

SELECTIVE EPITAXIAL GROWTH

› The first mention of selective epitaxial growth was in 1962 (~57 years ago!) by Joyce and Baldrey at Texas Instruments using high temperature depositions of SiCl4 on oxide masked areas

› This growth approach gradually took of with several other researchers exploring the possibilities for device applications Reduced temperature deposition of Si and Ge using SiH4 and GeH4 Addition of HCl to the growth process

April 12, 2019 | ASD 2019

EPITAXIAL SYSTEM

› Vacuum load locks and reduced pressure transfer chamber to remove atmospheric contamination/moisture

› Preclean to remove the oxide and/ormodify the surface from the areas in which the epi will be grown

› Growth is conducted in the epi chamber

ASM Intrepid ES system

April 12, 2019 | ASD 2019

EPITAXIAL CHAMBER

› The environment of the epi chamber is continually purged with either purified N2 or H2

› The temperature of the wafer is precisely controlled using thermocouples and monitored with pyrometers

› Gas flow through the chamber and across the wafer is carefully controlled and distributed based on the process optimization

› Wafer is rotated to further improve thermal uniformity and mitigate any gas depletion effects

Operational RangeTemperature < 1200 oCPressure ~ 5 to 760 TorrDepositions times ~30s to 1hr

April 12, 2019 | ASD 2019

PRECURSOR CHOICE

› Precursor choice is driven by the compositional target as well as the thermal budget› Selectivity would be secondary so we sometimes just have to adjust the processes

space (low pressure) or add HCl to processArrhenius Plot for Si and Ge Deposition RateArrhenius Plot for Si Deposition Rate

April 12, 2019 | ASD 2019

PATTERN WAFERS

› On advanced node pattern wafers there are several different types of materials exposed to the reactive gases of the epi process Exposed Si or SiGe –places that the epi is supposed to grow Oxide Nitride Other possible materials such as SiOC, SiOCN, polysilicon….

April 12, 2019 | ASD 2019

MECHANISM FOR SELECTIVITY

› The selectivity difference between oxide and nitride can be explained by the following- More stable passivation of SiO2 by H compared to nitride Higher number of surface bonds possible on the nitride surface

› The interaction of the reactive species with the dielectric surface dictates process selectivity

silicon

Dielectric

Claassen and Bloem J. Electrochem Soc. 1981

N2 H2

April 12, 2019 | ASD 2019

ADDITION OF HCL TO PROCESS CHEMISTRY

› The addition of HCl to the process flow enables selective processes

› Addition of HCl to the process chemistry has its own baggage Reduced growth rate –throughput impact Compositional change –reduction of dopant atoms via chlorinated species (PCl3, AsCl3, BCl3) Potential impact on uniformity –thickness and composition

Borland and Drowley Solid State Tech. 1985

950 oC

900 oC

1000 oC

950 oC, 133 slm H2

DCS 330 sccm

77 slm H2

• Power Devices

April 12, 2019 | ASD 2019

NATURALLY SELECTIVE PROCESS –LOW TEMPERATURE GE

Low temperature (< 400 oC) deposition of Ge using GeH4/H2 –no HCl

›There are a couple of ways to explain the select -H passivation of the oxide surface and/or the formation an desorption of Ge-O

Ge

Slight adjustment in deposition pressure (~2Torr delta) shifts process to fully selective

April 12, 2019 | ASD 2019

MAKE USE OF NUCLEATION DELAY

Evaluating process knobs that influence the nucleation delay

› Driving force for deposition pi is the input partial pressure

peq is the equilibrium partial pressure of the growth precursor i.e.

SiH2 for SiH4 or GeH2 for GeH4.

› Our compositional target has high Ge improves the selectivity due to increases nucleation barriers on SiO2 and Si3N4

Minimize pi, low precursor flow and/or low total pressure

Low temperature

silicon

oxideepi

silicon

oxideepi

silicon

oxideepi

time

April 12, 2019 | ASD 2019

CYCLIC DEPOSITION AND ETCH

› In some cases it is very difficult to achieve a targeted composition that is selective Thermal budget limitations imposed by previous processing restrictions and/or compositional

› By utilizing a cyclic deposition and etch (CDE) approach the limitation associated with a selective process are separated Use of very high growth rates to suppress unwanted surface diffusion

Unwanted interaction of HCl with desired species –trying to incorporate high levels of P or As

› Additional benefits Low temperatures processes enabled by reactive gases (Si3H8, Si2H6, Ge2H6 and Cl2)

Tunable selectivity through etch time adjustments -useful for filling complicated structures

Growth Selective (Etch)+

Selective Epitaxial Growth

Net Selective Process

April 12, 2019 | ASD 2019

IMPROVEMENT OF LT EPI USING CDE

Cyclic Deposition & Etch (CDE) process (developed by ASM)

– Non-selective deposition– In-situ Selective etch-back of amorphous

and defective material

– Repeat cycle

EpiSiB

“Defective”SiB-SiB

Single step deposition LT CDE deposition

Demonstration of the improvement in material quality deposited at low temperature (< 450oC) by incorporating CDE processBauer ECS Transactions 2010

April 12, 2019 | ASD 2019

SUMMARY

Described the different methods in which selectivity is achieved in the epitaxial process› Chemistry/process conditions naturally selective process› Addition of HCl in a co-flow process› Exploiting the nucleation delay for relatively thin layers› Cyclic deposition and Etch process

April 12, 2019 | ASD 2019

THANK YOU