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InMRAM 2013 JP.Nozieres

inMRAMinMRAM2013

MRAM

PROCESSING


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- MTJ used as a variable resistance

- Resistance compatible with CMOS (~ k)

- End-of-back-end process- No trade-off with logic process

-Easy / cheap to embedd

- 0 to 3 add-masks

- No HV required

- Front-end contamination ?

- Low-T BE process (T


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Microcontamination procedure

Table values are for blanket film.

Etch reduces values by 30-80% onpatterned wafer

SEZ backside clean post deposition

and etch steps Monitoring of tools pre/post MRAM

wafers using TXRF

ElementElement ConcentrationConcentration

((g/cm2)g/cm2)BB


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90nmCMOS

MRAM

0.29m2BitCell

90nm front end CMOS.

Cladded Cu M5 / M6 lines

MRAM DEMO CHIPS

Freescale / Everspin

4Mb Toggle MRAM


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MRAM DEMO CHIPS

Hynix 32 Mb planar STT

14F


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PROCESS SCHEMATIC - STT MRAM

6

Low-T

Back end MTJ

M0

Bit line

Cell

Strap

Pads

Regular

Front end

Non-standard

Process

Standard

Process


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Low-TBack end

V1

M0

M4 Bit-Line

V3

M2 Bit-Line

VM2

MM1

MM2 - Strap

V4

V2

M1

STI

Pads

Regular

Front end

7

MTJNon-standard

Process

Standard

Process

Shallow via

Cladded line

PROCESS SCHEMATIC FIELD MRAM


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Decreases write current by ~2x

(improved power, electromigration)

Lower cladded line easy to process

Simple replacement of liner material

with ferromagnetic material (e.g. NiFe)

Upper cladded line more complex multi-step process to preserve film

continuity and prevent ferromagnetic

material at the bottom of the bit line.

Beware of changes in wire resistivity

and inductance !

FIELD MRAM - CLADDED LINES

NiFe


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FRONT END BUILD-UP

M2M2

oxide

SiNx

V2

Process Cu CMP

Target Oxide surface roughness


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Orange peel coupling

Smooth(5x5

m)

Rough(1x

1m

Hotspots / Orange peel coupling

FRONT END BUILD-UP


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No porous dielectrics

No large-grain-boundary metals

Extra smoothing polish on dielectricor metal layer before mag dep

Beware of hillocks and voids in Cu

Beware of dishing Self-aligned Cu cap (etch back +

backfill + smoothing CMP)

Beware of residual slurry particles In situ megasonic cleaning or use of

abrasive-free slurries

FRONT END BUILD-UP

Cu CMP issues


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edge right side

center edge

0,17 0,24 nm rms

FRONT END BUILD-UP

ILD surface roughness


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MAGNETIC STACK

DEPOSITION / ANNEAL

M2M2

oxide

SiNx

V3 Strap / etch stop

Reference layer

Storage layer

Hard mask

13

M4 Bit-Line

VM2

MM1

MM2 - Strap

Pads

V2

Process Magnetic PVD / Anneal

Target RA / TMR uniformity < few %

Key issues MgO integrity (pinholes, hot spots, ), layers roughness, proper magnetics

Materials contamination

Impact Cells resistance distribution, switching current distribution


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Base electrode

Seed layer

Pinning layer

Tunnel barrier

Capping layer

Spin injectionlayer

Thermal barrier

Etch stop layer

Contact to select transistor + diffusion barrier

Ta(5) or NiFeCr(10) : Promotes texture of crit ical layers

PtMn(20) : AF layer sets direct ion of reference layer

CoFeB(2) / Ru(0.8)/CoFe(2) : SAF, immune to external fields

MgO (1.1) : Defines cell R & TMR

Reference layer

NiFe(3) / CoFe(2) : Stores data (2 stable states)

Storage layer

Protects MTJ during process

Top Electrode

MAGNETIC STACK

DEPOSITION / ANNEAL


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Magnetic film stack grown entirely without breaking vacuum asinterface properties are of high importance in magnetics.

Conventional PVD tools poorly suited to MTJ film deposition, as they

lack the necessary control of film thickness, film uniformity, and

surface roughness. Additional chamber required for finely-controlled oxidation of tunnel

barriers

Specialized deposition tools derived from HDD industry

TMR read

head

MAGNETIC STACK DEPOSITION


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10 targets, rotating drum

Deposition chamber

Oxidation module :Plasma oxidation

Natural oxidation

RF reactive deposition



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inMRAMinMRAM2013MAGNETIC STACK DEPOSITION


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Sputter Ta in chamber (used as H2O getter) during MgO deposition.With Ta getter

w/o Ta getter


Influence of water partial pressure


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Anneal (Magnetic)

Set pinning layer(s) with desired magnetization direction.

T~250-300C (depends upon AF material)

In-plane MTJ requires magnetic field, H~1T, low skew

Perpendicular MTJ may not require magnetic field anneal

Anneal (Structural)

MgO / CoFeB crystallization

Perpendicular layers crystallization

Beware, magnetic films can change stoichiometry dramatically

MAGNETIC STACK ANNEAL


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Amorphous CoFeB

Polycrystalline (001)

textured MgO

Amorphous CoFeB

Ru spacer

Cap layer (Ta or Ru)

As-deposited Annealing

bcc CoFe

crystalline (001)

textured MgO

Ru spacer

Cap layer (Ta or Ru)

After annealing

Improvement of MgO

crystallization

Crystallization of bcc

CoFe from the MgO

interface and expulsion

of the B out-of the CoFeB

alloy

Crystallization of bcc

CoFe from the MgO

interface and expulsionof the B out-of the CoFeB

alloy

B rich CoFeB

bcc CoFe

B rich CoFeB

Important to attract B away from the tunnel barrier during the crystallization process

Insert B getters nearby free and reference layer (Ta, Ru, Ti, Nb, Zr, Hf)

MAGNETIC STACK ANNEAL

Growth of Tunnel Barrier


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Ru-based SAF reference layer important for anneal at T>300C

Mn diffusion l imits anneal T at ~400C

Ultrafast (flash) anneals also allow to get good recrystallization while preventing interdiffusion

Higher Tanneal better (MgO) but issues with interdiffusion in the metallic layers

MAGNETIC STACK ANNEALInfluence of annealing T


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inMRAMinMRAM2013MTJ ETCH

M2M2

oxide

SiNx

V3

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Process Hard mask Cl-RIE Ash / Clean + MTJ etch (IBE or RIE) / Clean

Target >85-deg sidewall (e.g. CD gain), limited O/E

Key issues Prevent sidewalls redeposit ion, magnetic layers damage/corrosion, CD cont rol

Insert proper etch stop layers / post etch clean

Impact Cell Resistance distribution, R/W performances, reliability

M4 Bit-LineVM2

MM1

MM2 - Strap

Pads

V2


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CD variation

Parasitic resistances

parallel (shunts)

serial (contact R)

Edge tapering / defects

Influences switching process

Materials damage

Influences witching process

Decreased TMR (read margin)

Corrosion (reliability)

Parallel R shunt

CD variation

Contact R

Etch remains the major process challenge

MTJ ETCH ISSUES


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MTJ ETCHDistribution Considerations (Read)


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MTJ ETCHDistribution Considerations (STT Write)


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Low reactivity with etched films

Control of incident angle

(sidewalls clean)

Similar etch rates across a wide range

of different materials

HM HM

MTJ MTJ

Low-density MRAM Cell High-density MRAM Cell

Etch

Clean

Non volatile species

(conductive sidewall redeposits)

Poor selectivity (wrt. mask)

Shadowing (limits density / AR)

Low throughput

No 300mm history (uniformity ?)

ION BEAM ETCHING (IBE)


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Taper angle (~ 10) for physical removal of

sidewall redeposits.

Tradeoff between optimal cleanliness, damage

of upper layers and CD control

Option 1 : Single angle etch (e.g. 70-80)

Good CD control

Important redepositions

Option 2 : Dual angle etch

High angle etch for CD control Low angle sidewalls clean

Can etch down to bottom layer

MgOTa

TaPtMn

Metallic

Redeps

ION BEAM ETCHING (IBE)


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Applied MaterialsHot cathode DPS+ Etcher

2MHz

Tc=150-250C

High throughputs

Good CD control / Vertical sidewalls

Volatile etch by-productLimited sidewalls redepositions

No shadowing effectsPossibility to process dense (low pitch)devices

REACTIVE ION ETCHING (RIE)

Process of choice in

semiconductor industry

Poor volatility of magnetic materials by-

products at moderate T

Process is very materials (stack)dependent


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Chlorine, Bromine RIE Chlorine attack of magnetic films (corrosion, undercut)

Poor reactant volatility (sidewall redeposits)

Degraded magnetic properties (halogen exposure)

Carbonyl-based RIE (methanol or CO/NH3) more volatile byproducts

higher selectivity with metallic masks (Ti,Ta)

low propensity towards corrosion in these chemistries

Option 1 : Stop on MgO Prevents impact of sidewals redeps

Poor selectivity for MgO stop (footing / residues)

Additional masking/etch step required (cost, cell size)

Option 2 : Full MTJ etch High selectivity to underlayer possible

healthy overetch to scrub MTJ sidewalls

Sidewalls redeps critical Moderate taper angle



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1- Hard mask etch : TiN, Ta, TN

Cl2 / F based chemistries in DPS+

T>150CStop on Ru etch stop (capping) layer

3- Magnetic Tunneling Junction etching :

One step process in DPS+ reactorCO/NH3, NH3 or N2H2 chemistries

Tcat.=150-250C

Formation of volatile carbonyl based products

expected at elevated T

Three-step process in RIE process

2- Post Etch Treatment :

Importance ofin situ clean before MTJ etch

Avoid Chlorine diffusion within the magnetic stack NH3 clean to remove Cl and F from Ru surface

SiO2 HM

TA-MRAM Stack

Ta

Ta

Ru

FeMn

NiFe

RuCoFe

PtMn

MgO

CoFeB

CoFe

Hard mask

Storage

Reference




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Chlorine effect on MTJ stack :

NH3 (100 sccm) - 5mTorr - 800Ws / 0Wb 2min

To avoid metallic salt formation and corrosion,chlorine must be removed.

Dechlorination using NH3 plasma :

Evidence of chlorine contamination at the surface

Metallic Salts formation on Ru after air exposure

Efficiency of NH3 Clean

step to remove Cl and Ffrom Ru surface.

200 nm

REACTIVE ION ETCHING (RIE)Post etch treatment

( )


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From IBE .

to Chlorine

to carbonyls

(methanol, CONH3)

to hydrogen

(NH3, N2H2)

to IBE !

Important physical damage to MTJ

Volatile by-products but

decomposed by plasma !

Nice morphology

Degraded magnetics

Mixed RIE/IBE process

(IBE for critical layers

and/or sidewalls clean)

IBE-

like

IBE-

like



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inMRAMinMRAM2013LINER DEPOSITION

M2M2

oxide

SiNxV3

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Process Nitride PECVD

Target stress: -50 to -150 MPa; mean thickness: 300 30A, < 2% 1s unif,, > 70% conformality

Key issues Conformal coverage, Queue time (corrosion)

Impact Reliability

M4 Bit-LineVM2

MM1

MM2 - Strap

Pads

V2

i MRAMi MRAMLINER DEPOSITION


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In-situ MTJ encapsulation after etch better but notmandatory

Silicon oxide (TEOS precursor) void-free / conformal films at T


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STRAP ETCH

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M4 Bit-LineVM2

MM1

MM2 - Strap

Pads

V2

M2M2

oxide

SiNxV3

RIE (mostly Cl-based) of metal strap (usually Ta)

Option 1 : After cell etch Photo on topology

Option 2 : Before cell etch Photo alignment (inflated cell size)

Option 3 : Before MTJ deposition Front-end build up (roughness) must be adjusted

i MRAMinMRAM

TOP CONTACT TO MTJ


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M4 Bit-LineVM2

MM1

MM2 - Strap

Pads

V2

M2M2

oxide

SiNxV3

M4

Two strategies

M2M2

Oxide or SoG

SiNxV3

Direct contact (CMP-open)

Contact via (damascene process)

TOP CONTACT TO MTJ

inMRAMinMRAMTOP CONTACT TO MTJ


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Fewest processing steps (cost)

Need good control of CMP

Shadowing during MTJ etch.

Standard process

Overlay implies large cell size

Beware of punch-through (contamination)

CMP open

Use (conducting) hard mask

as self-aligned contact

Contact via

Additional via in

damascene process

TOP CONTACT TO MTJ


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inMRAMinMRAMTOOLING


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ContactCell

CMP open / Damascene fill Process

Cu CMP

Mag deposition

Mag anneal

Cell Photo

Cell Etch / Clean

Encapsulation

Strap Etch

ILD

Oxide CMP

Trench Photo

Trench Etch/Clean

Cu seed

Backside clean

Cu fill

Cu CMP

Backside clean

Strap Photo

Backside clean

Etch stop layer Dep

Via Photo

Via Etch/Clean

Surface polish

/

/

inMRAMinMRAMTOOLING


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inMRAMinMRAM2013TOOLING

Specific tools

Standard / Dedicated tools

Standard / Generic tools

FURNACE DEPOSITION PHOTO ETCH / STRIP WET / Clean CMP

MAGNETIC SOLUTIONS

MRT2000

SINGULUS TIMARIS

(for MTJ + stap + HM dep)

NIKON /ASML DUV (193 nm)

+ track TEL ACT 8

AMAT Centu ra DPS+ RIE

(for TMR +HM + Metal)

LAM SEZ

(for back side cleaning)

EBARA FREX -200

orAMAT mirra mesa (for

Cu CMP)

AMAT CENTURA or

NOVELLUS C2 SEQUEL

(for low-T oxyde / nitride)

NIKON /ASML UV (248 nm

or i-line) + MUV track

VEECO IBE TOOL FOR TMR

STACK

SEZ orSEMITOOL RAIDER

(for wet)

EBARA FREX -200

orAMAT mirra mesa (for

Oxide CMP)

AMAT ENDURA orLAM EXL

(for seed & Pads deposition)

APPLIED CENTURA MxP+or TEL UNITY

(for oxide/nitride etch)

SEMITOOL EPA

(for Cu fill)

TEL SCCM or LAM EXL

(for slug etch back)

inMRAMinMRAMMAGNETIC METROLOGY EQUIPMENT


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Need for rapid in-line (magnetic) process monitoring

Measurement of process yield Standard serpentine-comb and via chain structures

Probes MTJ cell / contacts / lines resistances

Measurements of the MRAM-specific layers

Requires magnetic fields sweep (slow) Must be non destructive

Probes magnetics and magneto-transport properties

(Hc, Hexch, RA, TMR, )

On-chip testing of arrays Standard e-test without magnetic fields

Probes cell functionality (read/write, reliability, )

MAGNETIC METROLOGY EQUIPMENT

inMRAMinMRAMM t O ti l K Eff t (MOKE)


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Non Contact, fast Small Signal sensitivity

No sample size limitation

Spot measurement on wafer (~m)

In line wafer mapping Penetration depth limited

No absolute magnetization

Principle : Rotation of light polarizationafter reflection from magnetic surface

Magneto-Optical Kerr Effect (MOKE)

inMRAMinMRAM


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inMRAMinMRAMCurrent-In-Plane tunneling (CiPTec)


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Problem : How to measure

RA, TMR in blankets films ?

Current In Plane tunneling (CiPTec)

BARRIER

TOP LAYER

[FREE]

BOTTOM LAYER

[PINNED]

I+

V+

V-

I-

x xx

Rt x

Rb x

RA

x

RA

x

BARRIER

TOP LAYER

[FREE]

BOTTOM LAYER[PINNED]

I+ V+ V- I-

x xx

Rt x

Small probe pitch measures Rt

Intermediate probe pitch

measures RA and MR

RT // RB

RT

Probe pitch

Rsq

Large probe pitch measures RT // RB

( ) +

= 2ln2||

00

xK

xK

R

RRR

I

VR

B

TBT

inMRAMinMRAMCurrent-In-Plane tunneling (CiPTec)


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No need to pattern

Local measurement (~100m)

Non destructive if done in scribe line

Mapping possible

Need special capping layer (probe-to-stack contact)

May require low conductivity underlayer

Tips wear / cost

Current In Plane tunneling (CiPTec)

inMRAMinMRAMMagnetic QSW


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2013Magnetic QSW

Principle : Standard Electroglass wafer

prober platform with built-inquadrupole

magnet for rotating field generation


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THANK YOU !