Synthesis of Silicon nanowires for sensornanowires for sensorapplications

Benefits of silicon nanowires SiNWs• High surface / volume ratio: surface phenomena predominate • Possibility of surface functionalization for biological applications • Development of na

Anne Claire Salaün

Nanowires Team

Laurent Pichon (Pr),Laurent Pichon (Pr), Régis Rogel (Ass.Pr), Anne-Claire Salaün (Ass Pr)

IETR - Institut d’Electronique et de Télécommunications de Rennes FRANCE

Rennes

Competences and know-how

Synthesis of Si related materials (thin films, nano-objects)

Sensors Microsystems and systems Electronic on plastic substrates

Si & Ge low temperature technology on flexible substrate

Actuation and detection techniques C

Chemical and biological detection

Context

• Owing to their physical and electrical properties, silicon nanowires represent

• a promising material with strong potential

• large variety of applications in future nanoelectronic devices

• Fabrication of innovative devices based on silicon nanowires with remarkable electronic properties

N t i i th l b

• New topic in the lab

Our objectives

• Synthesis of silicon nanowires

• Study and optimization of their electrical performances

• Fabrication of electronic devices (resistors, field-effect transistors)

• Development of innovative micro sensors (chemical biological)

• Development of innovative micro-sensors (chemical, biological)

Benefits of silicon nanowires (SiNWs)

• High surface / volume ratio: surface phenomena predominate

• Possibility of surface functionalization for biological applications

• Development of nanosensors with high sensitivityDevelopment of nanosensors with high sensitivity

• Compatibility with CMOS technologies

Silicon nanowires synthesis: 2 approaches

Starts from bulk materials and scales down

the patterned areas

Growth technique from molecular precursors using

• Chemical species detection

Silicon nanowires: top down approach

• Synthesis method

• Fabrication of silicon nanowires based resistors

• Fabrication of nanowires Thin Film Transistors (SiNW TFT)

• Electrical characterization

• Chemical species detection

• Nanowire synthesis is achieved by a vapor-liquid-solid (VLS) process

Bottom-up approach

A droplet of a liquid catalyst is put in contact with gaseous precursor molecules

A droplet of a liquid catalyst is put in contact with gaseous precursor molecules

SiNWs are synthesized using gold (Au) as metal catalyst and silane (SiH4) as precursor gas

in a hot wall LPCVD reactor

SiNW synthesis : Bottom-up

The diameter of the nanowire, grown by the VLS process, is given by the diameter of the gold nanoparticles

Nanowire

Au

• Gold deposited by thermal evaporation (thickness below 5 nm),

• Growth in LPCVD reactor with silane at 480°C (pressure: 40Pa)

• Diameter : about 100 nm, various orientation, length can exceed 10 µm

Advantage of the VLS method:

• Possibility of selective doping (n- or p-type) of the nanowires, by controlled injection

of the dopant precursor gas

• High quality single crystalline nanowires with well-controlled composition and

electronic properties

Challenges:

• Controlling diameter and orientation of the nanowires which impede the success ofControlling diameter and orientation of the nanowires which impede the success of the fabrication of nanowire arrays with high degree of reproducibility

SiNW synthesis : Bottom-up

Highly in-situ doped

polysilicon

Fabrication of nanowires based resistors

First mask

Definition of the comb shape

• Different interdigitated structures can be achieved, varying number

of teeth and local deposition of ld

gold

5,0x10-9

1,0x10-8

Au thickness : 5nm

SiNW synthesis : Bottom-up

Thin Film Transistors with Nanowires

Output characteristics

Source

Drain

p1,5x10-7

Source

1,0x10-7

VG=0V VG=10V

• Surface sensitive to charges in ambiance

Silicon Nanowire as gas sensor

• Main interest of SiNWs rests on their high surface that can be sensitive to charges

 ammonia and smoke may act as chemical gates:

species act as electrons donor (reducing agents) at the SiNWs surface.

 positively charged gas molecules binded on SiNWs surface can modulate the conductance

• Chemical species detection

Silicon nanowires: top down approach

• Synthesis method

• Fabrication of silicon nanowires based resistors

• Fabrication of nanowires Thin Film Transistors (SiNW TFT)

• Electrical characterization

• Chemical species detection

Top-down approach

• starts from bulk materials and scales down the patterned areas

• Advantages

• high yield high uniformity and well aligned production of nanowires

• high-yield, high-uniformity, and well-aligned production of nanowires

• this approach is more promising for mass production of highly uniformed nanowire arrays and nanowire-based devices

Sidewall spacer formation technique

• This technique uses anisotropic dry etching

• Low cost (does not require the use of high cost lithographic techniques)

• Compatible with conventional CMOS technology

• Benefits of excellent homogeneity and reproducibility of conformal CVD processes

SiNW synthesis : Top-down

Fabrication of silicon nanowires by the sidewall spacers formation technique

RIE etching

Si l N+

Undoped polysilicon

Si poly N Silicon oxide

APCVD oxide

Monocristalline substrate

Optmization of the steepness:

dry etching parameters (pressure, power)

SiNW synthesis : Top-down

Fabrication of Nanowires resistors

-2 ,0 x1 0-8

Current versus voltage

Thermal oxide

Silicon nanowires

Silicon Nanowire as gas sensor

 Ammonia and smoke may act as chemical gates: donor of electrons

 Carrier transport strongly depends on structural nanowires defects (polysilicon grain boundaries)

SiNW synthesis : Top-down

3,5x10-8

g Vgs = 1V Vgs = 2V Vgs = 3V Vgs = 4V

0 0 5,0x10 -9

1E-11 1E-10

1E 11

Conclusions and future directions

Nanowires synthesis

• Feasibility of silicon nanowires for both synthesis methods explored

• Electrical behavior show good potential for electronic devices

• Curvature radius has to be lowered to observe the nanometric size effects on the electrical behaviour

Future directions

• Detection of chemical and biological species (areas of healthcare, life sciences)

• Nanowires: new opportunities in this interdisciplinary area

•  diameters comparable to those of the biological/chemical species being sensed diameters comparable to those of the biological/chemical species being sensed.

• Devices based on nanowires: ultrasensitive electrical sensors for the detection

of biological and chemical species

• ability to bind analytes on their surface

• - ability to bind analytes on their surface

• - direct electrical detection (without using labels)

• - low concentrations of DNA, proteins or viruses

• - rapid analysis of these speciesp y p

Conclusions and future directions

• The nanowire surface can be modified with a variety of linker molecules

• Specific sensing achieved by linking a recognition group to the surface of the

• Significant signal changes with the binding of molecules

• SiNWs development could significantly impact areas of electronics, genomics, biomedical diagnostics drug discovery

The nanowire surface can be modified with a variety of linker molecules (bioaffinitive agents): functionalization

• Significant signal changes with the binding of molecules

• SiNWs development could significantly impact areas of electronics, genomics, biomedical diagnostics drug discovery

biomedical diagnostics, drug discovery