Physics and Material Science of Semiconductor Nanostructures

Transcription

1 Physics and Material Science of Semiconductor Nanostructures PHYS 570P Prof. Oana Malis Course website:

2 Today Bulk semiconductor growth Single crystal techniques Nanostructure fabrication Epitaxial growth MOCVD Top down approach: Device fabrication techniques Ref. Ihn Chapter 2, 6

3 Comparison of epitaxial technques 3

4 Chemical Vapor Deposition Precursor Reactor Energy Solid Products (thin films and powders) Gas Phase products Microfabrication processes widely use CVD to deposit materials in various forms, including: monocrystalline, polycrystalline, amorphous, and epitaxial. Materials grown by CVD include: silicon, carbon fiber, carbon nanofibers, filaments, carbon nanotubes, SiO 2, silicon-germanium, tungsten, silicon carbide, silicon nitride, silicon oxynitride, titanium nitride, and various high-k dielectrics. The CVD process is also used to produce synthetic diamonds.

5 Precursor Considerations Volatility vapor pressure - simple molecules with high vapor pressure are rare determined by molecular weight and molecularity (degree polymerized) - result of structure and bonding control - temperature, valving Stability, Reactivity, and Safety bond strength, bond dissociation energy - affects process temperature and film composition (purity) thermal stability in storage and delivery into the reactor reactivity of the precursor and byproducts towards other substances (including biological objects like us) Single-Source Precursor providing more than one element into the film simpler delivery system uniform elemental distribution at atomic level possible limited composition range Common Precursors hydrides: MHx - SiH4, GeH4, AlH3(NMe3)2, NH3, PH3... halides: MXy - TiCl4, TaCl5, MoF6, WF6,... metal-organics - metal alkyls: AlMe3, AliBu3, Ti(CH2tBu)4... metal alkoxides: Ti(OiPr)4, [Cu(OtBu)]4... metal dialkylamides: Ti(NMe2)4, Cr(NEt2)4... metal diketonates: Cu(acac)2, Pt(hfac)2... metal carbonyls: Fe(CO)5, Ni(CO)4... others: complexes with alkene, allyl, cyclopentadienyl,... ligands many precursors have mixed ligands

6 Energy sources and Reactor types Thermal Energy resistive heating - tube furnace quartz tungsten halogen lamp (very good heat source) - radiant heating radio-frequency - inductive heating laser as thermal energy source Photo Energy UV-visible light laser as photo energy source

7 MOCVD (also called MOVPE) H 2 +AsH 3 RF Coils Gas Exhaust Substrate Mixing Chamber Al(CH 3 ) 3 Ga(CH 3 ) 3 H 2 Ga(CH 3 ) 3 + AsH 3 => 3CH 4 +GaAs Ref. Ihn Chapter 2

8 MOCVD (MOVPE) Metal-organic vapour phase epitaxy Thin crystalline films created at high temperatures by chemical reactions between metalorganic and anion precursors. Some typical precursors for III-V compound semiconductors: Group III Group V Dopants Ga(CH 3 ) 3 (TMG) NH 3 SiH 4 (in H 2 or He) Ga(C 2 H 5 ) 3 (TEG) AsH 3 CH 4 PH 3 Cp 2 Mg Cp 2 Fe Ga(CH 3 ) 3 + NH 3 GaN + 3 CH 4 2 Ga(C 2 H 5 ) AsH 3 2 GaAs + 6 C 2 H 6 Carrier gas usually H 2, Ar and/or N 2, used to transport metalorganic precursors to reaction chamber.

9 Carrier gas MOCVD (MOVPE) schematic Dopant 1 Filter & purifier MFC MFC MFC = mass flow controller controls the amount of gas going into the reactor Dopant 2 Group V precursor Carrier gas MFC MFC MFC MOVPE reactor Scrubber unit Gp. III precursor (in bubbler) Heater Power supply

10 Example of a real GaN reactor

11 MOCVD Precursor purity Typical atomic density in a semiconductor: 5 x atoms cm -3 Typical doping density in semiconductor devices: cm -3 To avoid unintentional doping of devices, this implies a desired purity of better than 1 (10 14 /10 22 ) = %. Best available precursors are about % pure (referred to as 7 Nines or 7N), and are very expensive. Carbon contamination from metalorganic precursors may also be a source of unintentional doping, as may contaminated system components.

12 Monitoring MOCVD growth Typical in situ monitoring uses optical techniques Example: optical reflectivity measurements on GaN: laser beam transparent GaN epilayer Sapphire substrate Beams reflected from GaN surface and GaN/sapphire interface interfere. As film grows, nature of interference oscillates between constructive and destructive, resulting in oscillating intensity of reflected light This can be used to measure growth rate.

13 Comparison MOVPE Moderate pressure (100 mbar) Higher temperatures High throughput Hydrogen normally present Greater process drift Higher growth rates Highly toxic/explosive precursors Higher impurity levels $$$ MBE Ultra-low pressure (10-11 mbar) Lower temperatures Lower throughput No hydrogen unless deliberately added Precise thickness control Lower growth rates Safer precursors (usually!) Ultra-low impurity levels $$$$

14 Other Epitaxial Techniques Liquid Phase Epitaxy - Dipping or Tipping Chemical Beam Epitaxy - Combination of MBE and MOCVD

15 Laser Assisted Vapor Deposition He H 2 SiH 4, SF 6, H 2

16 Laser Assisted Catalytic growth Fig. 2 Schematic of a nanowire growth set up using a combination of laser ablation and vapor-liquid- solid scheme Fig.1 Semiconductor (GaAs) nanowires grown using laser assisted catalytic growth [1ref]. The scale bar corresponds to 50 nm.

17 PECVD (Plasma-Enhanced CVD)

18 Top-down versus bottom-up If we want to make a very small tree we can either Get a very big piece of wood and carve it into a much smaller model tree (TOP DOWN APPROACH) Plant a seed and then control its growth to form a fullyfunctioning bonsai tree (BOTTOM UP APPROACH)

19 Top-down approach: Nano and Micro Fabrication Techniques Ref. Ihn Chapter 6

20 Main fabrication processes Lithography Photolithography Electron beam lithography Scanning probe based lithography Etching: chemical wet etch, dry etching, laser ablation, focussed ion beam nanomachining, etc. Insulator deposition: PECVD, sputter-deposition Metal deposition: thermal evaporation (mostly e- beam evaporation)

21 Example of semiconductor device fabrication: p-n junction n - Si

22 Fabrication Steps for a p-n junction Oxidation or oxide deposition -Forming a SiO 2 layer. - Works as an insulator or barrier to diffusion or implantation Litographic Process - The wafer is covered by a photosensitive film; - Radiation Exposure through a mask; - The non-polymerized regions are solved; - Thermal Treatment ( ºC) to boost the film adhering; - Attack with HF to remove the uncovered SiO 2 ; - Photosensitive film removal through a chemical solution or through an attack of oxygen plasma. Jan 2006

23 Photolithography The photolithography technique allows the manufacture of complex and dense circuits, improving this way the performance of the devices since it allows for its dimension reductions to happen. Recurring to the lithography technique, it s possible to manufacture on the same wafer both active and passive devices. The process consists in transferring a previously created pattern/model to the wafer s surface, in order to define the several regions of an integrated circuit This process has several steps, so all advances in the overall process depend on the development of each of the individual steps. Jan 2006

24 Photolithography summary Surface Preparation Coating (Spin Casting) Pre-Bake (Soft Bake) Alignment Exposure Development Post-Bake (Hard Bake) Processing Using the Photoresist as a Mask Stripping Post Processing Cleaning (Ashing) Positive and Negative photoresists Masks Some issues edge bead Cleanliness is critical cleanroom

25 Birck Nanotechnology Center Nanofabrication Cleanroom 25,252 square feet Nanofabrication cleanroom coupled with a pharmaceutical grade cleanroom Material access between cleanrooms

26 Birck Nanotechnology Center Nanofabrication Cleanroom ISO Class 3 (Class 1) 6 bays ISO Class 4 (Class 10) 5 bays ISO Class 5 (Class 100) 2 bays

27 Photolithographic Process Steps The photolithography process must be implemented in a clean room, in order to avoid dust to be deposited in the mask and wafer. 1 - Deposition of the photosensitive film Spin Coating A drop of photosensitive material is placed on the center of the wafer. The wafer is then spinned over its axis in order to promote a uniform spreading of the material. Base Material : polyisopropene Conditions: rotation speed: rpm time: 10-60s thickness: m thermal treatment : 100ºC Jan 2006

28 2- Photosensitive film exposure to radiation The transfer of the model from the mask to the wafer can be done using optic equipment (for details bigger than 0.25 m), X-rays or by an electron beam. The photosensitive film when receiving radiation can behave as: - Positive photosensitive : The images formed are the same as the ones in the mask. - Negative photosensitive: The images formed are complementary to the ones in the mask. The radiation exposed regions become insoluble and therefore can not be removed. Jan 2006

29 Positive Photosensitive : It s built by 3 components: a resin, a photosensitive element and an organic solvent. The region exposed to radiation changes its chemical structure becoming soluble. The broken connections between the molecules allow an easy removal. Negative Photosensitive : Polymers are combined with a photosensitive element. The region exposed to radiation become insoluble due to the cross connections formed between the molecules. The high molecular weight of the molecules prevents their removal. Jan 2006

30 Tools for Photolithography

31 Exposure Response Curves and their transversal sections Photosensitive film fully soluble in developer for an energy equaling E T Photosensitive film fully insoluble for an energy equaling 2E T Jan 2006

32 3- Etching/Writing/Recording Process The next step is the etching process. That must allow the removal of material in the regions where the photosensitive film doesn t exist. SiO 2 is selectively attacked, whereas the substrate remains unaltered Jan 2006

33 The simplest recording process is chemical recording: - It involves a chemical reaction followed by the removal of reaction elements. - The elements used for chemical attacks are mostly acids (HF, HNO 3, H 4 C 2 O 2, H 2 SO 4 ). E.g. : SiO 2 + 6HF -> H 2 SiF 6 + 2H 2 O Water can be used as diluent for this attacker Jan 2006

34 The Attack depends on the orientation degree: Some solutions are more easily solved in some specific crystallographic planes. The material used in the attack should attack only one layer at a time and should be self-limitative. Chemical recording is simple and cheap, however it s neither compatible with submicrometric technologies nor permits an anisotropic attack. Jan 2006

35 4 Photoresist Removal After the recording, the photosensitive film must be removed Usually, the film removal is made by chemical attack (acetone) Jan 2006

36 Manufacture Steps of a p-n junction Oxidation -Forming a SiO 2 layer. - Works as an insulator or barrier to diffusion or implantation Litographic Process - The wafer is covered by a photosensitive film; - Radiation Exposure through a mask; - The non-polymerized regions are solved; - Thermical Treatment ( ºC) to boost the film adhering; - Attack with HF to remove the uncovered SiO 2 ; - Photosensitive film removal through a chemical solution or through an attack of oxygen plasma. Jan 2006

37 In order to form active elements in integrated circuits it s necessary to selectively introduce dopants in the substrate; The surface is exposed to an high ion dopant concentration, that are incorporated in the semiconductor crystal lattice; The SiO 2 layer is a barrier to diffusion and to the impurity implantation. Jan 2006

38 Doping techniques: - Diffusion: Thermal Treatment in an oven at ºC on rich doping environment (e.g. Phosphorus or boron) - Ion Implantation : the doping atoms are ionized and accelerated against the surface in order to be implanted in the substrate Jan 2006

39 Metallization Ohmic Contact Formation, through a phased chemical or physical vapor deposition of a metallic film. A lithographic process is used in order to define contact zones. Metallization of the backside of the semiconductor. Thermal Treatment at ~500ºC in order to get low contact resistance between metallic layers and the semiconductor. Jan 2006

40 Other patterning techniques

41 J. Phys. Chem. B, Vol. 105, No. 24, 2001 Nanosphere Lithography

42 Two photon polymerization JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 21, NO. 3, MARCH October 2004 / Vol. 12, No. 21 / OPTICS EXPRESS 5221

43 Interference Lithography Appl. Phys. Lett., Vol. 82, No. 8, 24 February 2003 H. Smith s group at MIT J. Appl. Phys., Vol. 82, No. 1, 1 July 1997

44 Plasmon based Phase mask Dip Pen Nano imprinting Soft Lithography Photonic alignment Other Techniques