Chapter Three

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Nov 2, 2013 (3 years and 7 months ago)

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Spencer/Ghausi,
Introduction to Electronic Circuit Design
, 1e,
©2003, Pearson Education, Inc.

Chapter 3, slide
1


Introduction

to

Electronic Circuit Design

Richard R. Spencer

Mohammed S. Ghausi

Spencer/Ghausi,
Introduction to Electronic Circuit Design
, 1e,
©2003, Pearson Education, Inc.

Chapter 3, slide
2


Figure 3
-
1

CMOS inverter

Spencer/Ghausi,
Introduction to Electronic Circuit Design
, 1e,
©2003, Pearson Education, Inc.

Chapter 3, slide
3


Figure 3
-
2

Cross section of the final CMOS integrated circuit. The PMOS
transistor is shown on the left and the NMOS device is on the right.
Remember that the bulk, or substrate, of each device is tied to its source (not
shown in this cross section)

Spencer/Ghausi,
Introduction to Electronic Circuit Design
, 1e,
©2003, Pearson Education, Inc.

Chapter 3, slide
4


Figure 3
-
4

Mask #1 patterns the photoresist. The Si
3
N
4

layer is removed by
dry etching where it is not protected by the photoresist.

Following initial cleaning, an SiO
2

layer is thermally grown on
the silicon substrate. An Si
3
N
4

layer is then deposited by
LPCVD. Photoresist is spun on the wafer to prepare for the first
masking operation. The result of the first masking operation is
shown below in Figure 3
-
4.

Spencer/Ghausi,
Introduction to Electronic Circuit Design
, 1e,
©2003, Pearson Education, Inc.

Chapter 3, slide
5


Figure 3
-
6

Photoresist is used to mask the regions where PMOS devices will
be built using Mask #2. A boron implant provides the doping for the
P

wells
for the NMOS devices.

After the photoresist is stripped, the field oxide is grown. Then
the Si
3
N
4

layer is stripped off and a new layer of photoresist is
spun on prior to Mask #2 being used. The result of using Mask
#2 is shown in Figure 3
-
6 below.

Spencer/Ghausi,
Introduction to Electronic Circuit Design
, 1e,
©2003, Pearson Education, Inc.

Chapter 3, slide
6


Figure 3
-
7

Photoresist is used to mask the
regions where NMOS devices will be built using
Mask #3. A phosphorus implant provides the
doping for the
N

wells for the PMOS devices.

The wells are then driven in by
further high temperature
processing.

Spencer/Ghausi,
Introduction to Electronic Circuit Design
, 1e,
©2003, Pearson Education, Inc.

Chapter 3, slide
7


Figure 3
-
9

After photoresist is spun onto the
wafer, Mask #4 is used to define the NMOS
transistors. A boron implant adjusts the
N
-
channel
V
th
.

Spencer/Ghausi,
Introduction to Electronic Circuit Design
, 1e,
©2003, Pearson Education, Inc.

Chapter 3, slide
8


Figure 3
-
10

After photoresist is spun onto the
wafer, Mask #5 is used to define the PMOS
transistors. An arsenic implant adjusts the
P
-
channel
V
th
.

Spencer/Ghausi,
Introduction to Electronic Circuit Design
, 1e,
©2003, Pearson Education, Inc.

Chapter 3, slide
9


Figure 3
-
13

Photoresist is applied, and Mask #6 is
used to define the regions where MOS gates are
located. The polysilicon layer is then etched by
means of plasma etching.

After Mask #5, the thin oxide is etched back to bare silicon and a
new gate oxide is grown. A layer of polysilicon is deposited and
implanted with phosphorous to make it conductive. Then, Mask
#6 is used to define the gates as shown in Figure 3
-
13 below.

Spencer/Ghausi,
Introduction to Electronic Circuit Design
, 1e,
©2003, Pearson Education, Inc.

Chapter 3, slide
10


Figure 3
-
14

Mask #7 is used to cover the
PMOS devices. A phosphorus implant is
used to form the tip or extension (LDD)
regions in the NMOS devices.

After removal of the patterned resist and spinning on a new
layer of photoresist, Mask #7 is used to produce the structure
shown in Figure 3
-
14 below.

Spencer/Ghausi,
Introduction to Electronic Circuit Design
, 1e,
©2003, Pearson Education, Inc.

Chapter 3, slide
11


Figure 3
-
15

Mask #8 is used to cover the
NMOS devices. A boron implant is used to
form the tip or extension (LDD) regions in
the PMOS devices.

After removal of the patterned resist and spinning on a new
layer of photoresist, Mask #8 is used to produce the structure
shown in Figure 3
-
15 below.

Spencer/Ghausi,
Introduction to Electronic Circuit Design
, 1e,
©2003, Pearson Education, Inc.

Chapter 3, slide
12


Figure 3
-
18

After a thin “screen”
oxide is grown, photoresist is applied
and Mask #9 is used to protect the
PMOS transistors. An arsenic implant
then forms the NMOS source and
drain regions.

After removal of the patterned photoresist, SiO2 is deposited and
anisotropically etched to leave sidewall spacers along the edges
of the polysilicon.

Spencer/Ghausi,
Introduction to Electronic Circuit Design
, 1e,
©2003, Pearson Education, Inc.

Chapter 3, slide
13


Figure 3
-
19

After photoresist is applied, Mask
#10 is used to protect the NMOS transistors. A
boron implant then forms the PMOS source
and drain regions.

Spencer/Ghausi,
Introduction to Electronic Circuit Design
, 1e,
©2003, Pearson Education, Inc.

Chapter 3, slide
14


Figure 3
-
24

Photoresist is applied and Mask #11 is used to define the regions
where TiN local interconnects will be used. The TiN is then etched.

After Mask #10, a high
-
temperature drive in activates all the
implanted dopants and diffuses junctions to their final depths.
An unmasked then removes the oxide from the source and drain
regions and from the top of the polysilicon. Titanium is then
sputtered onto the surface and is reacted in an N
2

ambient to
form TiS
2

where it contacts silicon or polysilicon and TiN
elsewhere.

Spencer/Ghausi,
Introduction to Electronic Circuit Design
, 1e,
©2003, Pearson Education, Inc.

Chapter 3, slide
15


Figure 3
-
27

Photoresist is spun onto the wafer.
Mask #12 is used to define the contact holes.
The deposited SiO
2

layer is then etched to allow
connections to the silicon, polysilicon, and local
interconnect regions.

After Mask #11 is used, the photoresist is stripped, a conformal
oxide layer is deposited by LPCVD, and chemical
-
mechanical
polishing (CMP) is used to planarize the surface.

Spencer/Ghausi,
Introduction to Electronic Circuit Design
, 1e,
©2003, Pearson Education, Inc.

Chapter 3, slide
16


Figure 3
-
29

Aluminum is deposited on the wafer
by sputtering. Photoresist is spun onto the wafer,
and Mask #13 is used to define the first level of
metal. The Al is then plasma etched.

A thin TiN barrier
-
adhesion layer is deposited on the wafer by
sputtering, followed by deposition of a W layer by CVD.

Spencer/Ghausi,
Introduction to Electronic Circuit Design
, 1e,
©2003, Pearson Education, Inc.

Chapter 3, slide
17


Figure 3
-
30

The steps to form the second level
of Al interconnect follow those in Figures 3
-
25
through 3
-
29. Mask #14 is used to define via
holes between metal 2 and metal 1. Mask #15 is
used to define metal 2. The last step in the
process is the deposition of a final passivation
layer, usually Si
3
N
4

deposited by PECVD. The
last mask (#16) is used to open holes in this
mask over the bonding pads.

Spencer/Ghausi,
Introduction to Electronic Circuit Design
, 1e,
©2003, Pearson Education, Inc.

Chapter 3, slide
18


Figure 3
-
31

Junction
-
isolated bipolar transistors. (a) A vertical
npn

transistor. (b) A lateral
pnp

transistor.