stand alone flash memories

8/18/2019 Stand Alone Flash Memories

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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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Jurriaan Schmitz, Semiconductor Components ;

+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


8/72Jurriaan Schmitz, Semiconductor Components

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


11/72Jurriaan Schmitz, Semiconductor Components 11

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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Jurriaan Schmitz, Semiconductor Components 1%

M0SF#T operation 5-er basic7

C

V

depletionaccumulation

Vfb

inversion

VT


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Jurriaan Schmitz, Semiconductor Components ()

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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Jurriaan Schmitz, Semiconductor Components (&

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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Jurriaan Schmitz, Semiconductor Components (<

+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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Jurriaan Schmitz, Semiconductor Components (

'rra addressin


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Jurriaan Schmitz, Semiconductor Components (%

9arer memories: cut into bloc4s


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Jurriaan Schmitz, Semiconductor Components *)


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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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Jurriaan Schmitz, Semiconductor Components *(

Band diaram 5o-ersimpli!iedA7


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Jurriaan Schmitz, Semiconductor Components **

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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Jurriaan Schmitz, Semiconductor Components *&

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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Jurriaan Schmitz, Semiconductor Components *<

Proram6erase mechanisms


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Jurriaan Schmitz, Semiconductor Components *=

Flash proram and erase methods


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Jurriaan Schmitz, Semiconductor Components *

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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Jurriaan Schmitz, Semiconductor Components *%

Prorammin: Channel 3ot #lectron "nIection


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Jurriaan Schmitz, Semiconductor Components &)

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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Jurriaan Schmitz, Semiconductor Components &1

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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Jurriaan Schmitz, Semiconductor Components &(

ni!orm or drainside F+ tunnelin

+onuni!orm: onl !or erasinH less demandin !or the dielectric


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Jurriaan Schmitz, Semiconductor Components &*

'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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Jurriaan Schmitz, Semiconductor Components &<

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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Jurriaan Schmitz, Semiconductor Components &

( )

−⋅⋅−⋅−+=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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Jurriaan Schmitz, Semiconductor Components ;1

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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Jurriaan Schmitz, Semiconductor Components ;<

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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stand alone flash memories

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