Post on 23-Dec-2014
description
N MOS Fabrication Process
Sun Tiefeng: 2008054238Bhuvan Adhikari 2009002702
INTRODUCTION
Photolithography Etching NMOS Fabrication process
• Gate• Source and Drain• Vias• Metal Contacts
Part 1 What is photolithography What is etchingPart 2 How photolithography and
etching are used when we make MOSFET
What is photo-What is photo-lithographylithographyPhotolithography a process used in micro-
fabrication to pattern parts of a thin film or the bulk of a substrate.
It uses light to transfer a geometric pattern from a photo-mask to a light-sensitive chemical "photo-resist", or simply "resist," on the substrate.
TYPES OF RESIST TYPES OF RESIST
• Positive resiststhe resist is exposed with UV light wherever the underlying material is to be removed.
•Negative resists :the negative resist remains on the surface wherever it is exposed
What is etching What is etching To produce circuit features, the
resist patterns must be transferred into the underlying layers comprising the device.
The pattern transfer is accomplish by an etching process that selectively removes unmasked portions of a layer
The two types of etching The two types of etching processesprocesses
Wet etch processes
can lead to undercutting
result in an isotropic etch profile
the vertical etch rate is
equal to the horizontal etch rate
The two types of etching The two types of etching processesprocesses
Dry etching One of the most widely
used
Use plasma generated free radicals
Remove material only from the area dictated by photo-resist pattern
Provide the ability to control sidewall anisotropic
etching
N-MOS Fabrication ProcessN-MOS Fabrication Process
Fig. (1) Pure Si single crystal
Si-substrate
Fig. (2) P-type impurity is lightly doped
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
N-MOS Fabrication ProcessN-MOS Fabrication Process
Fig. (3) SiO2 Deposited over si surface
Fig. (4) Photoresist is deposited over SiO2 layer
Thick SiO2
(1 µm)
Photoresist
Thick SiO2
(1 µm)- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
N-MOS Fabrication ProcessN-MOS Fabrication Process
Fig. (5) Photoresist layer is exposed to UV Light through a
mask
Photoresist
Thick SiO2
(1 µm)
UV Light
Mask-1
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - --- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
Mask-1 is used to expose the SiO2
where S, D and G is to be formed.
N-MOS Fabrication ProcessN-MOS Fabrication Process
Fig. (6) Developer removes unpolymerised photoresist. It will cause no effect on Si surface
Polymerised Photoresist
Thick SiO2
(1 µm)
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - --- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
N-MOS Fabrication ProcessN-MOS Fabrication Process
Fig. (7) Etching [HF acid is used] will remove SiO2 layer which is in direct contact with etching solution
Thick SiO2
(1 µm)- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - --- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
N-MOS Fabrication ProcessN-MOS Fabrication Process
Fig. (7) unpolymerised photoresist is also etched away [using H2SO4]
Thick SiO2
(1 µm)- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - --- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
N-MOS Fabrication ProcessN-MOS Fabrication Process
Fig. (8) A thin layer of SiO2 grown over the entire chip surface
Thick SiO2
(1 µm)
Thin SiO2
(0.1 µm)
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - --- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
N-MOS Fabrication ProcessN-MOS Fabrication Process
Fig. (9) A thin layer of polysilicon is grown over the entire chip surface to form GATE
Thick SiO2
(1 µm)
Thin SiO2
(0.1 µm)
Polysilicon layer (1 – 2 µm)
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - --- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
N-MOS Fabrication ProcessN-MOS Fabrication Process
Fig. (10) A layer of photoresist is grown over polysilicon layer
Thick SiO2
(1 µm)
Thin SiO2
(0.1 µm)
Polysilicon layer
Photoresist
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - --- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
N-MOS Fabrication ProcessN-MOS Fabrication Process
Fig. (11) Photoresist is exposed to UV Light
UV Light
Mask-2
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - --- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
Mask-2 is used to deposit Polysilicon to form gate.
N-MOS Fabrication ProcessN-MOS Fabrication Process
Fig. (12) Etching will remove that portion of Thin SiO2 which is not exposed to UV light
Thick SiO2
(1 µm)
Thin SiO2
(0.1 µm)
Polysilicon
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - --- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
N-MOS Fabrication ProcessN-MOS Fabrication Process
Fig. (13) Polymerised photoresist is also stripped away
Thick SiO2
(1 µm)
Thin SiO2
(0.1 µm)
Polysilicon used as GATE (1 – 2 µm)
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - --- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
N-MOS Fabrication ProcessN-MOS Fabrication Process
Fig. (14) n+ Doping to form SOURCE and DRAIN
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - --- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
Thick SiO2
(1 µm)
Thin SiO2
(0.1 µm)
GATE
- - - - - - n+
- - - - - - n+
SOURCE
DRAIN
N-MOS Fabrication ProcessN-MOS Fabrication ProcessStep - MetallizationStep - Metallization
Fig. (15) A thick layer of SiO2 (1 µm) is again grown.
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - --- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
Thick SiO2
(1 µm)
- - - - - - n+
- - - - - - n+ Thick SiO2
(1 µm)
N-MOS Fabrication ProcessN-MOS Fabrication ProcessStep - MetallizationStep - Metallization
Fig. (16) Photoresist is grown over thick SiO2. Selected areas of the poly GATE and SOURCE and DRAIN are exposed where contact cuts are to be made
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - --- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
Thick SiO2
(1 µm)
- - - - - - n+
- - - - - - n+ Thick SiO2
(1 µm)
Photoresist
Mask-3
UV Light
Mask-3 is used to make contact cuts for S, D and G.
N-MOS Fabrication ProcessN-MOS Fabrication ProcessStep - MetallizationStep - Metallization
Fig. (17) The region of photoresist which is not exposed by UV light will become soft. This unpolymerised photoresist and SiO2 below it are etched away.
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - --- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
Thick SiO2
(1 µm)
- - - - - - n+
- - - - - - n+ Thick SiO2
(1 µm)
Photoresist
Mask-3
N-MOS Fabrication ProcessN-MOS Fabrication ProcessStep - MetallizationStep - Metallization
Fig. (18) The contact cuts are formed for S, D and G (hardened photoresist is stripped away).
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - --- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
Thick SiO2
(1 µm)
- - - - - - n+
- - - - - - n+ Thick SiO2
(1 µm)
Photoresist
Mask-3
N-MOS Fabrication ProcessN-MOS Fabrication ProcessStep - MetallizationStep - Metallization
Fig. (19) Metal (aluminium) is deposited over the surface of whole chip (1 µm thickness).
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - --- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
Thick SiO2
(1 µm)
- - - - - - n+
- - - - - - n+ Thick SiO2
(1 µm)
Metal (1µm)
N-MOS Fabrication ProcessN-MOS Fabrication ProcessStep - MetallizationStep - Metallization
Fig. (20) Photoresist is deposited over the metal.
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - --- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
Thick SiO2
(1 µm)
- - - - - - n+
- - - - - - n+ Thick SiO2
(1 µm)
Metal (1µm)
Photoresist
N-MOS Fabrication ProcessN-MOS Fabrication ProcessStep - MetallizationStep - Metallization
Fig. (21) UV Light is passed through Mask-4 (with a aim of removing all metal other than metal in contact-cuts).
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - --- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
Thick SiO2
(1 µm)
- - - - - - n+
- - - - - - n+ Thick SiO2
(1 µm)
Metal (1µm)
Photoresist
UV Light
Mask-4
Mask-4 is used to deposit metal in contact cuts of S, D and G.
N-MOS Fabrication ProcessN-MOS Fabrication ProcessStep - MetallizationStep - Metallization
Fig. (22) Photoresist and metal which is not exposed to UV light are etched away.
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - --- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
Thick SiO2
(1 µm)
- - - - - - n+
- - - - - - n+ Thick SiO2
(1 µm)
Metal (1µm)
Photoresist
Mask-4
N-MOS Fabrication ProcessN-MOS Fabrication ProcessStep - MetallizationStep - Metallization
Fig. (23) Final n-MOS Transistor
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - --- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
- - - - - - n+
- - - - - - n+
SOURCE
DRAIN GATE
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