50. 7. Deposition Deposition systems may be divided into two groups:
a) Chemical Vapor Deposition (CVD) systems
Which rely on the chemical reaction of the constituents of a vapor phase at the substrate surface to deposit a solid film on this surface.
b) Physical Vapor Deposition (PVD) systems
Which directly deposit the source material onto a given substrate in a “line-of-site impingement type deposition”.
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51. 7. Deposition a) CVD: Summary of transport and reaction processes
© L.M. Landsberger
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52. 7. Deposition a) CVD: Common film types & sample chemistries
Polysilicon:
Silicon Nitride:
Silicon Dioxide:
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53. 7. Deposition a) CVD: Chemistry & system selection
Factors to be considered:
deposition temperature (ex: depositing over metals?)
film quality (tensile/compressive; grain boundary size, etc.)
growth rate
deposition system compatibility
throughput
Tensile
Compressive
Stress in deposited films
Deposited polysilicon grain size
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54. Cold-wall induction type with tilted susceptor
7. Deposition
a) CVD: Atmospheric Systems (APCVD)
Cold-wall induction type with tilted susceptor
Poly Si deposition: Note the use of a liquid source in this example (SiCl4)
Barrel type
Induction heating: RF energy couples with the graphite susceptor, thereby heating it rather than heating the process gases and substrates themselves (improved contamination control)
Rotating pancake type
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55. 7. Deposition a) CVD: Low-Pressure Systems (LPCVD)
One disadvantage of APCVD systems is that the diffusion, D, of the reacting species to the surface of the substrate is a limiting step. Thus, few wafers can be processed at the same time (mass-transport limited). Since “D” is inversely proportional to pressure, This issue may be overcome by lowering the pressure within the reaction chamber. 1 Torr, D increases by a factor of In this case, the diffusion of the species through the boundary layer through to the reaction site is no longer a limiting step and the system is said to be surface reaction limited.
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56. 7. Deposition a) Overview of CVD process types (Madou, p. 109)
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57. 7. Deposition b) Physical Vapor Deposition (PVD) systems
In these types of thin film deposition systems, the source materials to be deposited take on a variety of forms:
Solid
Liquid
Vapor
In the case of PVD systems, the materials to be deposited are physically deposited using a variety of methods including:
Thermal Evaporation
Sputtering
Etc. (Laser Ablation, Molecular Beam Epitaxy)
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58. 7. Deposition a) Physical Vapor Deposition (PVD) systems - continued
The range of materials that may be deposited using these methods include:
Metals such as: Al Cu Au Ag
etc.
Compound & hard materials such as: Cr
TiN
CrN
AlCuSi
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59. Thermal Evaporation – Resistive Heating Thermal Evaporation – e-Beam
7. Deposition
a.1) Thermal Evaporation - General
The material to be deposited is placed in a crucible within a high-vacuum chamber.
After the chamber is pumped down, the source is heated via (typically) resistive or e-beam heating. The material is heated to its boiling point such that it sublimates onto all exposed surfaces in the vacuum chamber.
The amount of material deposited is controlled via a thickness monitor which is placed within the deposition chamber.
The source material must be of high purity.
Vacuum levels are on the order of 10-5 to 10-7 Torr.
Thermal Evaporation – Resistive Heating
Thermal Evaporation – e-Beam
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60. 7. Deposition a.1) Thermal Evaporation - drawbacks
Resistive heating is the simplest method of evaporating metals such as Al or Au, but it is also the “dirtiest” in that contaminants which find their way onto the filament tend to be evaporated along with the metal.
The purity issue can be addressed via e-beam evaporation since the cooled, non-molten high-purity material to be deposited acts as a crucible during the process (see schematic on previous slide).
In the case of resistive heating, temperature uniformity across the filament is difficult to control and therefore, evaporation uniformity onto the substrates may be a problem. This is not an issue with e-beam evaporation
E-beam evaporation may cause surface damage due to ionizing radiation and/or X-rays voltages above 10kV, the incident electron beam will give rise to X-ray emission).
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61. 7. Deposition a.1) Thermal Evaporation - drawbacks
The different components of certain alloys such as NiCr have different evaporation rates, hence, the composition of the deposited material will not be the same as that of the starting material. Thus, thermal evaporation does not lend itself well to the deposition of all alloys.
The previous problem may be tackled via the use of multi- pocket e-beam systems.
Due to the point-source nature of the material being evaporated, shadowing effects may hamper the uniformity of the deposited layer over steps existing on the substrate (bad step coverage) in both the e-beam & resistive heating cases.
Step Coverage Issues
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62. 7. Deposition a.2) Sputtering – principle of operation
A solid slab (ie., target) of the material to be deposited is placed in a vacuum chamber along with the substrate on which the deposition is to take place.
The target is grounded.
Argon gas is introduced into the chamber and ionized to a positive charge.
The Ar ions bombard the target and cause the target atoms to scatter, with some of them landing on the substrate.
The plasma is composed of the Ar atoms, Ar ions, the sputtered material, gas atoms and electrons generated by the sputtering process.
Allows the deposition of a large assortment of materials on any type of substrate
Sputtering
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63. 7. Deposition a.2) Sputtering – advantages/disadvantages M.J. Madou
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64. 8. Dry Etching
Sample reactions
In general, any material that forms a volatile fluoride or chloride can be plasma etched.
Typical setup
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65. 8. Dry Etching
M.J. Madou
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