EE 4345Semiconductor Electronics

Design ProjectCMOS Process CMOS Process

Mohammad Butt

Ahmad Elmardini

Devices Devices

Heithem Souissi

Dina Miqdadi

Process ExtensionProcess Extension

Fares Alnajjar

Wyatt Sullivan

CMOS Process

Drawbacks of Metal-gate MOS Transistors

• Metal-gate PMOS transistors cannot maintain the minimal(+0.5V) threshold variation.

• Excess surface state charges and mobile ion contamination are two main sources of threshold variation.

• Suffer from excessive overlap capacitance.

• Parasitic capacitances Cgs and Cgd slow the transistor because they must be charged and discharged during switching.

• Aluminum is used as gate material which can erode completely causing contact spiking.

Features of Polysilicon-gate CMOS Process

• Consists of nine masking steps• Optimized to form complementary PMOS and NMOS

transistors on a common substrate• Can also fabricate some analog circuits with slight

modifications• Use (100) silicon to reduce surface state density and

improve threshold voltage control• Polysilicon-gate is doped with phosphorus to minimize

mobile ion contamination, resulting in faster switching speeds and better control of threshold voltage

Oveview of Polysilicon-gate CMOS Process

• Start with P-substrate• Grow P-type epitaxial layer on the substrate• Create N-well regions and channel stop regions• Grow gate oxide (thin oxide) and field oxide (thick

oxide)• Deposit and pattern polysilicon layer• Implant source and drain regions, substrate contacts• Create contact windows, deposit and pattern metal

layer

Epitaxial Growth• P-type substrate is doped with as much

boron as possible to minimize substrate resistivity

• Lightly doped P-type epitaxial layer is grown on substrate

• NMOS transistors are formed directly on the epi layer which serves as a backgate

P-epiP-substrate

N-well Diffusion

• The wafer is then oxidized and etched to open windows through which ion implantation deposits a controlled dose of phosphorus

• A prolonged drive creates a deep lightly doped N-type region called N-well

• Thermal oxidation covers the exposed silicon with thin layer of pad oxide

N-well Diffusion

• In N-well CMOS process, NMOS transistors occupy the epi and PMOS transistors reside in well. This process optimizes NMOS at the expense of PMOS

P-epiP-substrate

pad oxide

N-well

P-well Diffusion

• P-well CMOS process uses N+ substrate and N- epitaxial layer and a P-well. NMOS transistors are formed in P-well and PMOS transistors in the epi. This process optimizes PMOS at the expense of NMOS

N-epiN-substrate

P-wellpad oxide

Why N-well CMOS Process?

• The N-well process offers a slightly better NMOS transistor than P-well CMOS and also allows the use of a grounded substrate favored by most circuit designers.

• In a P-well process the NMOS still outperforms its counterpart because electrons are more mobile than holes

Inverse Moat

• LOCOS process is used to define field regions and moat regions

• Locally oxidized regions of the die are called ‘field regions’

• Areas protected from oxidation are called ‘moat regions’

• First a patterned nitride layer is formed by depositing nitride across entire wafer

Inverse Moat (cont.)

• A selective etch is used to remove nitride over the field regions

• The photomask used is called inverse moat mask because it codes for areas where moat is absent

P-epiP-substrate

N-wellpad oxide

photoresist

nitride

Channel Stop Implants

• Channel stop implants are required to ensure that thick-field threshold exceed the operating voltages

• It uses a boron implant followed by a patterned phosphorus implant

• Phosphorus implant counterdopes boron implant and raises NMOS thick-field threshold above maximum operating voltage

Channel Stop Implants (cont.)

• After LOCOS oxidation, a suitable etching strips away the remnants of the nitride block mask

• Curved transition region at the edges of moat results from oxidants diffusing under the edges of nitride film and is called ‘bird’s-beak’

• All photoresist is stripped off from the wafer in preparation for LOCOS oxidation

Channel Stop Implants (cont.)

• A thin layer of oxide called ‘dummy gate oxide’ is grown in moat regions to eliminate any nitride formed underneath the pad oxide due to ‘kooi effect’

P-epiP-substrate

N-wellpad oxide

nitride

photoresist Boron channel stop Phosphorus channel stop

Threshold Adjust

• Method 1:Method 1: Two separate implants to set PMOS Vt and NMOS Vt. This Method allows independent optimization of both thresholds

• Method 2:Method 2: Single Vt adjust implant to simultaneously reduce PMOS threshold and increase NMOS threshold

Threshold Adjust (cont.)

• The boron Vt adjust implant penetrates of the dummy gate oxide to dope underlying silicon

• After the Vt adjust implant, dummy gate oxide is stripped away to reveal bare silicon in moat regions

P-epiP-substrate

N-well

Boron channel stop Phosphorus channel stopField oxide

Dummy gate oxide

Boron Vt adjust implant

Polysilicon Deposition and Patterning

• Polysilicon layer used to form gate electrodes is heavily doped with phosphorus to reduce its resistivity

• The deposited polysilicon layer is patterned using polymask

P-epiP-substrate

N-well

Boron channel stop Phosphorus channel stopField oxide

Gate oxide

Boron Vt adjust implant

poly

Source/Drain Implants

• NSD implant involves application of photoresist to the wafer, followed by patternig using the NSD mask

• Heavily doped N-types regions are formed by implanting ardenic through exposed gate oxide

• Photoresist residue is stripped off and a new photoresist layer is patterned using PSD mask

Source/Drain Implants (cont.)

• Heavily doped P-type region is formed by implanting Boron through exposed gate oxide

• Photoresist is again stripped off and a brief anneal activates the implanted dopants

P-epiP-substrate

N-well

Field oxide

Gate oxide

poly NSDPSD

Contacts

• MLO (Multilevel Oxide) is deposited and the wafer is again coated with photoresist

• Contact windows are created and silicide is formed in the contact openings

• Thin film of refractory metal precedes a much thicker layer of copper doped aluminium

Protective Overcoat

• Protective overcoat is deposited over the final layer of metallization to provide mechanical protection and to prevent contamination of the die

• Selected areas of metallization are etched to attach bondwires to the integrated circuit

Devices

Transistor structure

Transistor (Bell Labs)

Transistor Layout

Gate Voltage and The Channel

Basic Transistor Parasitic

P-Channel MOSFET

N-Channel MOSFET

Layout and cross section of PNP substrate transistor

PSD/NSD Resistors

MOSFET Capacitances

Source Drain

Gate

CSB CDB

CGB

Bulk

CGS CGD

• capacitances have three origins:– The basic MOS structure

– The channel charge

– The pn-junctions depletion regions

Process Extension

Process Extension

• CMOS process extension tend to focus on improving the PMOS and NMOS transistors.

• Types:- One set seeks to provide higher operating voltages.

- Another focuses on reducing the size of the transistor.

Process ExtensionDouble-level Metal

• -It adds two steps to the process: one vias and

One for metal-2

• Interlevel oxide(ILO) is deposited between the

Two metal layers

-This provides insulation

-The planarization improves for the second

Level

Process ExtensionDouble-level Metal(cont.)

• Process extension.

• The extra processing steps increase the cost of the wafer.

• Use of extra metal layers would reduce the area required for interconnection in high density auto routed logic.

Process ExtensionDouble-level Metal(cont.)

• Advantages of more levels of metal interconnect,2,3,4,etc:

-Eases automated routing and improves power and clock distribution to modules.

-Vias are used to connect upper layers of metal to metal 1.

-”Contact cuts” are made from metal 1 to diffusion or poly.

Process ExtensionDouble-level Metal(cont.)

Process ExentionSilicidation

• Silicidation: an anneal(sintering) resulting in the formation metal-Si alloy to act as a contact.

• Used for:

1.Reducing sheet resistance.-Poly resistances between 20 and 40 ohms.

-Silicide (silicon and tantalum) used as gate material, between 1 and 5 ohms.

-Can be extended to source and drain, called.

salicide.

Process ExtensionSilicidation(cont.)

Process ExtensionLightly Doped Drain Transistors

• Designed to minimize hot-carriee effects; The reduced doping gradient between drain and channel lower electric field.

• Implementation: typical NMOS with 3mm length operate within 5-10V, PMOS with the same dimensions can withstand 15-20 V. That’s where the LDD comes in handy.

• LDD can withstand substantially higher drain-to-source voltage than the singly doped drain (SDD)devices.

Process Extension LDD(cont.)

• Use of two drain diffusions:-One forming a lightly doped drift region near

the edge of the gate.-The other forming a more heavily doped

region beneath the contact, this will reduce the drain resistance of the structure and allows the transistor to retain most of the performance of a conventional SDD device.

Process Extension LDD(cont.)

• Process to form LDD transistors:

Use of an oxide sidewall spacer to self-align the two drain diffusions, therefore, enabling precise control of the width of the drift region.

Process Extension LDD(cont.)

• Fabrication steps:1. A shallow implant self-aligned to the edges of the gate

polysilicon deposits the lightly doped drain.2. The wafer is coated with a thick layer of isotropically

deposited oxide.3. Use anisotropic dry etch to remove most of the

deposited oxide.4. A second drain implant self-aligned to the edges of the

oxide sidewall spacers to form the heavily doped portions of the LDD.

Process Extension LDD(cont.)

Advantages: Improves tolerability of high drain-to-

source voltages. Does not increase real-estate needed. Provides greater depletion regions widths,

which in turn, provides better tolerances to hot carrier degradation.

Process ExtensionLDD(cont.)

• Drawbacks:Requires additional masks to selectively

block N-S/D implantsSlightly increases drawn channel

lengths(0.5-1m)

Extended-drain, High-voltage Transistors

• Potentially can withstand in excess of 30 V.

• Uses existing masks of standard n-well poly-CMOS process.

• Drawbacks:-Inherently long channel devices.

-High overlap capacitance.

-Asymmetric.

-Higher susceptibility to oxide rupture( decreases device transconductance).

Extended-drain, High-voltage Transistors

Extended-Drain, High-voltage(cont).

Extended-Drain, High Voltage(cont).

References

• The Art of Analog Layout, by Alan Hastings

• Semiconductorglossary.com

• http://www.iue.tuwien.ac.at/publications/PhD%20Theses/puchner/node36.html

• http://www.sse.uu.se/education/acad/pdf/2000/Design_Rules.pdf

• http://vlsi.wpi.edu/webcourse/ch02/ch02.html