stand alone flash memories
TRANSCRIPT
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Jurriaan Schmitz, Semiconductor Components 1
Flash memories
Based on:
Roberto Bez et al., ST Microelectronics
Proceedins o! the "###, $ol. %1 no. &, 'pril ())*.
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Jurriaan Schmitz, Semiconductor Components (
Contents
+on-olatile memories /hat are +$M
method o! operation
#PR0M, ##PR0M, and Flash
Reliabilit concerns
retention
endurance
Scalin
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Jurriaan Schmitz, Semiconductor Components *
+on$olatile Memories
' non-olatile memor is a memor that can hold itsin!ormation /ithout the need !or an e2ternal -oltae
suppl. The data can be electricall cleared and re/ritten
#2amples: Manetic Core
3arddis4
0TP: onetime prorammable 5diodes6!uses7
#PR0M: electricall prorammable R0M
##PR0M: electricall erasable and prorammable R0M
Flash
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Jurriaan Schmitz, Semiconductor Components &
"C memor classi!ication
SR'M 8R'M R0M
#PR0M
PR0M
##PR0M
F9'S3 ##PR0M
$olatile memories9ose data /hen po/er do/n +on-olatile memorieseep data /ithout po/er suppl
Standalone -ersusembedded memories
This lecture: standalone
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+on-olatile memor comparison
Floatin
a teme
mories
Comparison: later toda
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Jurriaan Schmitz, Semiconductor Components <
Retention -s. alterabilit
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3o/ does a Flash memor cell /or4>
?3o/ does a M0S transistor /or4>
?@hat is a semiconductor>
See: collee 3al!eleiderde-icesAA
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Semiconductor essentials: properties
Metallic conductor:
tpicall 1 or ( !reel mo-in electrons per atom
Semiconductor:
tpicall 1 !reel mo-in electron per 1)%1)1= atoms
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Semiconductor essentials resisti-it
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Semiconductors in the periodic table
II III IV V VI
Be B C N O
Mg Al Si P S
Zn Ga Ge As Se
#lemental semiconductors:C, Si, e 5all roup "$7
Compound semiconductors:
"""$: a's, a+?""$": Dn0, DnS,?
roup""" and roup$
atoms are Edopants
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Semiconductor essentials: impurities
Small impurities can dramaticall chane conducti-it: G sliht phosphorous contamination in silicon i-es
man e2tra !ree electrons in the material 5one per P
atomA7
G sliht aluminum contamination i-es man e2traholes 5one per 'l atom7
P Al
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5silicon lattice is o! course *8A7
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Silicon dopants
II III IV V VI
Be B C N O
Mg Al Si P S
Zn Ga Ge As Se
In
Boron most /idel usedas ptpe dopantH
Phosphorous and arsenic
both used /idel as ntpe
dopant
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Semiconductor essentials: n and p tpe
ntpe doped semiconductor
e.. silicon /ith phosphorus impuritelectrons determine conducti-it
ptpe doped semiconductor
e.. silicon /ith 'l impuritholes determine conducti-it
pn Iunction:
current can onl !lo/ one /aA
Semiconductor diode
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The !ield e!!ect
+ + + + + + + +accumulation
depletion
in-ersion
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The M0S transistor
S0RC# 8R'"+
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Jurriaan Schmitz, Semiconductor Components 1=
' M0S transistor laout
source drainatesource drainate
5cross section7
5cross section7
5top -ie/7
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Jurriaan Schmitz, Semiconductor Components 1
Free electronFree hole
NMOS
K K K
PMOS
Conducts at +VGB
NMOS + PMOS = CMOS
Conducts at -VGB
+M0S and PM0S transistors
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M0SF#T operation 5-er basic7
C
V
depletionaccumulation
Vfb
inversion
VT
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Current throuh the M0S transistor
"
$s
M0S transistor simplistic
$T
"
$s
M0S transistor real
$T
inversionChannel chare: L 5$s G $T7
Channel current: " 5$s G $T7
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Jurriaan Schmitz, Semiconductor Components (1
Concept o! the !loatinate memor cell
M0S transistor: 1 !i2ed threshold -oltaeFlash memor cell: $T can be chaned b proram6erase
"d
$s
M0S transistor Floatin ate transistor
"d
$s
prorammin
$T
erasin
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Jurriaan Schmitz, Semiconductor Components ((
http:66///*pub.amd.com6products6n-d6mirrorbit6!lash.htm
Floatin ate animation
http://www3pub.amd.com/products/nvd/mirrorbit/flash.htmhttp://www3pub.amd.com/products/nvd/mirrorbit/flash.htm
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Jurriaan Schmitz, Semiconductor Components (*
Floatin ate transistor: principle
$T is shi!ted b inIectin electrons into the !loatin ateH"t is shi!ted bac4 b remo-in these electrons aain.
CM0S compatible technoloA
ControlateFloatin
ate
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Channel chare in !loatin ate transistors
Control ate
Floatin ate
silicon
Control ate
Floatin ate
unprorammed prorammed
To obtain the same channel chare, the prorammed ate needs a
hiher controlate -oltae than the unprorammed ate
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Jurriaan Schmitz, Semiconductor Components (;
9oic E) and E1
"d
$s
N$T O L6Cpp
$read
E1 "read QQ )E) "read O )
Readin a bit means:1. 'ppl $read on the control ate
(. Measure drain current "d o! the
!loatinate transistor
@hen cells are placed in a matri2:
Control
ate
lines
drain lines
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+0R or +'+8 addressin
+0R +'+8
less contacts more compact
@ord O control ateH bit O drain
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Jurriaan Schmitz, Semiconductor Components (=
+'+8 -ersus +0R
1)2 better endurance
Fast read 51)) ns7
Slo/ /rite 51) s7
sed !or Code
Smaller cell size
Slo/ read 51 s7
Faster /rite 51 s7
sed !or 8ata
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'rra addressin
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9arer memories: cut into bloc4s
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Jurriaan Schmitz, Semiconductor Components *1
Prorammin and erasin the !loatin ate
Control
ateFloatin
ate
Floatin ate
Control ateSi0(
Si*+&
Polsilicon
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Band diaram 5o-ersimpli!iedA7
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Proram6erase o! a !loatin ate transistor
Floatin ate is surrounded b insulatin material.3o/ to dri-e chare in and out o! it>
"nIection6eIection mechanisms:
G Fo/ler+ordheim tunnelin 5F+7
G Channel 3ot #lectron "nIection 5C3#7
G "rradiation 5most common: $, !or #PR0Ms7
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Conduction throuh Si0(
$8
$8ominant current components: "ntrinsic Uuantummechanical conduction
Fo/ler+ordheim tunnelin
8irect Tunnelin
8e!ectrelated: Trapassisted tunnelin
5-ia a molecular de!ect7
Current throuh lare de!ects
5e.. pinholes7 "ntrinsic current is de!ined b eometr V materials
8e!ectrelated current can be suppressed b enineerin $B
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Jurriaan Schmitz, Semiconductor Components *;
ate o2ide conduction e2ample
( 1 ) 1 ( * & ;1)
1&1)
1*
1)1(
1)11
1)1)
1)%
1)
1)=
1)<
1);
1)&1)*
$ 5$7
W " C
W 5 ' 7
3ardbrea4do/n
nstressed o2ide
S"9C
So!t
brea4do/n
4 nm oxide
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Proram6erase mechanisms
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Flash proram and erase methods
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C3#: 3ot electron prorammin
Pincho!! → hih electric !ields near drain → hot carrier inIection throuh Si0(+ote: X 1Y o! the electrons /ill reach the !loatin ate → po/erine!!icient
3ot holes
3ot electrons
3ole substrate current
Field → 4inetic ener → o-ercome the barrier
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Prorammin: Channel 3ot #lectron "nIection
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C3#: properties
@or4s onl to create a positi-e $T shi!t 3ih po/er consumption: *)) Z'6cell
5most electrons et to the drain: lost e!!ort7
Moderate prorammin -oltaes
Ris4: hot carriers can damae materials G Ma lead to !i2ed chare, inter!ace traps, bul4 traps
G Results in deradation o! the cell 5see later7
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Fo/ler+ordheim tunnelin
ni!orm tunnelin throuh entire dielectric is possible $Tshi!t can be positi-e as /ell as neati-e
Can be used !or proram and erase
ReUuires hih -oltae and hih capacitances
9ittle po/er needed 51) n'6cell7
Ris4s o! this techniUue:
G Chare trappin in o2ide
G Stressinduced lea4ae current
G 8e!ectrelated o2ide brea4do/n
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ni!orm or drainside F+ tunnelin
+onuni!orm: onl !or erasinH less demandin !or the dielectric
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'lternati-e: tunnel throuh interpol o2ide
5erasin, combined /ith C3# proram7
9ess demandin !or the tunnel o2ide
There!ore less S"9C and better retentionMore demandin !or interpol o2ide
ses hih -oltae and lo/ po/er
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Jurriaan Schmitz, Semiconductor Components &&
+0R and +'+8 !lash technolo
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Jurriaan Schmitz, Semiconductor Components &;
BR#'
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Flash reliabilit issues and scalin
Flash reliabilit concerns: The reular reliabilit concerns o! CM0S
G 02ide brea4do/n
G "nterconnect problems 5electromiration7
G ? Speci!ic !or Flash:
G Retention
G #ndurance
Scalin: Can /e ma4e the !lash cell more compact>
G 8ominant problem: scalin the dielectrics
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Jurriaan Schmitz, Semiconductor Components &=
Reliabilit issues
Speci!ic problems in non-olatile memories:Fast prorammin and erasin 51)
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( )
−⋅⋅−⋅−+=kT
E t V V V t V athththth expexp)0()( 00 ν
Retention (herinneringsvermogen)
Ability to retain valid data for a prolonged period of timeunder storage conditions (non-volatile).
Sinle Cell:
time be!ore chane o! ).1Y chane in stored data /hile notunder electrical stress"ntrinsic retention
'rra o! Cells:
retention o! the /orst cell in the arra be!ore and a!ter
cclinde!ect related O #2trinsic retentionE'lzheimers 9a/:
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Jurriaan Schmitz, Semiconductor Components &%
Retention
Chare loss due to: detrappin o! electrons6holeso2ide de!ects
mobile ions
contamination
'ccelerated test at hih T #a o! the dominant process
$irin de-ices re-eal insulatin properties o! dielectric
Stressed de-ices 5a!ter proram6erase ccles7: retention \
3ih T /or4s as ba4eout
MaIor retention hazard: stressinduced lea4ae current
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Jurriaan Schmitz, Semiconductor Components ;)
Retention
Problem: not a sinle cell, but embedded in a matri28urin prorammin o! one cell, all neihbours are also
e2posed to the same hih prorammin -oltae
F+tunnellin can then induce chare loss5lea4in a/a o! in!ormation6data7
cell !loatin ate capacitance 1!F
loss o! 1!L causes $T shi!t o! 1$
Chare loss rate !or 1) ear retention:
Less than 5 electrons per day!!
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1
1.;
(
(.;
*
*.;
&
&.;;
;.;
<
1 1) 1)) 1))) 1))))
storage time [hours]
T h r e s h o l d
v o l t a g e [ V
]
+(o anneal, 1(; C
control, 1(; C
+(o anneal, (;) C
control, (;) C
calculated
't (;) ]C decrease starts a!ter 1)h#2trapolation leads to conclusion that the
li!etime at room temperature Q1) ears
?usin /hich model>>>>
#2ample o! retention stud
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Jurriaan Schmitz, Semiconductor Components ;(
1. Test at di!!erent temperatures
(. 8etermine acti-ation ener 5assumin 'rrhenius7
*. 5"denti! mechanism7
' more thorouh stud
Time until $T has
shi!ted b ;)) m$
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Jurriaan Schmitz, Semiconductor Components ;*
8ata retention prohibits tunnel o2ide scalin
Tunnel o2ide
thic4ness
Time !or ()Y
chare loss
&.; nm &.& minutes
; nm 1 da
< nm ^ < ears
7-8 nm is the bare minimum
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Jurriaan Schmitz, Semiconductor Components ;&
Retention: summar
Retention O the abilit to hold on to the chare 9oss Q ; electrons per da is 4illin in the lon run
Mostl limited b de!ects in the tunnel o2ide
Retention can be compromised /ith error correction
For thin o2ides X = nm, the retention o! Flash isintrinsicall insu!!icient
To test retention, measure at di!!erent T and !ield
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Jurriaan Schmitz, Semiconductor Components ;;
#ndurance (uithoudingsvermogen)
Ability to perform even after a large number of program/erase cycles
Sho/stoppers:
02ide brea4do/n 9oss o! memor /indo/
Shi!t in operatin marin
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C V
AQn
fg
injbd
pe
∆
=
#ndurance: o2ide brea4do/n
' dielectric /ill brea4 do/n /hen a certain amount o!chare has crossed it: this amount is LB8.
Tpical !or ood Si0( material: LB8 O 1) C6cm(.
Simple relation:
npe the number o! proram6erase ccles until brea4do/n N$! the shi!t bet/een the E) and E1 state
ood enineerin i-es a rip on LB8 then, no problem
# d i d l i
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Jurriaan Schmitz, Semiconductor Components ;=
#ndurance: /indo/ closin
Proram6erase ccles
Fi2ed chares appear
in the tunnel o2ide a!ter
proram6erase ccles
# d hi!t i ti i
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Jurriaan Schmitz, Semiconductor Components ;
Proram6erase ccles
#ndurance: shi!t in operatin marin
$T,erase increases due to electron trappin in interpoldielectric 5normal7
Simultaneous $T,proram increases indicates chare trappin
in the ate o2ide
# d l
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Jurriaan Schmitz, Semiconductor Components ;%
Proram6erase ccles
#ndurance: e2ample
' simple modi!ication o! the tunnel dielectric @indo/ closure is retarded /ith more than an order o!
manitude
# d h i ti
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"TRS ())=: lonterm -ision on +$M
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