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1 2012-5-21 © 2011 Keithley Instruments, Inc.© 2011 Keithley Instruments, Inc.

Non-Volatile Memory –Characterization and Measurement Techniques

Alex Pronin

Keithley Instruments, Inc.

2 © 2011 Keithley Instruments, Inc.

Why do more characterization?

• NVM: Floating gate Flash memory– Very successful; lead to explosion in consumer

market (tables, smart phones)– There is concern that standard Flash is “hitting the

wall” and cannot sustain Moore’s law of storage expansion

• Response:– Switch to 3D Flash technologies– New materials and new technologies

• In test and measurement there is increased demand for characterization

– Need for fast voltage and current transient measurements

– Integration of DC and pulse measurement– Simplicity of use


Outline

• Review types of NVM• NVM test requirements and

overview• Common NVM test parameters• Capabilities of Keithley’s Model

4225-PMU Ultra-fast I-V Module for NVM characterization

• Examples of test projects and characterization

– Flash Memory– PRAM/PCM– FeRAM

• Measurements optimization


NVM types

• FG Flash, CTF/SONOS charge trapping Flash, 3D Flash• PCM/PCRAM – Phase change memory• Resistive: ReRam/RRAM, CBRAM• MRAM – Magnetoresistive, STT-MRAM: spin transfer torque MRAM• FeRAM – Ferro electric RAM• CNT RAM – carbon-nanotube RAM


“Ideal Memory” attributes

• Low cost and high density with scalability road map

• Fast read/write (similar to or faster than existing DRAM speeds)

• High endurance (to address DRAM or SSD applications)

• Long retention• Low power and voltage requirements• Compatible with existing logic circuits

and semiconductor processes


New NVM test requirement

• Traditional requirements– Accurate DC measurements (SMU source

measurement units) for Vt, Set/Reset Resistance, etc.– High fidelity pulses– Optimized integration of DC and pulse instruments

• Transient characterization capability for new NVM technologies

– Provides fundamental data on intrinsic material properties– Initial implementations:

• Use load resistors and capacitors in combination with digitizing scopes; “one-off”, customized system, which requires high maintenance

• Limited accuracy, difficult calibration– Development of new pulse measure instrumentation

• Pulse generation functionality combined with measure• Simultaneous measurement of high speed voltage and

pulses• Integration with DC and ease of use


NVM test parameters

• Pulse amplitude• Pulse amplitude fidelity (ringing,

overshoot, and undershoot )• Ability to output multi-pulse waveforms• Pulse timing (rise, fall time, and pulse

width )• Dynamic, simultaneous ultra-fast current

and voltage measurement• Remote pulse amplifier (to decrease

parasitic effects of interconnects)• Current compliance or current control • Switching and integration between pulse

and DC instruments• Channel synchronization


Models 4225-PMU and 4225-RPM

• The Model 4225-PMU Ultra-Fast I-V Module is a single-slot instrument card for the Model 4200-SCS

• Two channels of voltage pulse sourcing with integrated, simultaneous real-time current and voltage measure for each channel

• Each channel can independently source ±10V or ±40V into high impedance, 80V amplitude in differential mode

• Two A/D converters per channel that sample the voltage and current simultaneously, with 200M Sample rate

• Timing features from 20ns to 40s, automatically synchronized to within ±2ns across channels

• Arbitrary segment waveform (Segment ARB®) capability for multi-level or multi-pulse waveforms


Multi-level waveforms with Segment ARB® capability

• Multi-level pulse waveform– 4 pulses, 3 levels

• Created by 16 linear voltage segments (grey numbers) and 4 measurements (red boxes)

• Flash memory program and erase waveform

– 2 pulses, 3 levels


Model 4225-RPM (Remote Pulse Unit)

• The Model 4225-RPM Remote Amplifier/Switch is an optional addition to the Model 4225-PMU

• Located near the device-under-test• Allows for low current pulse

measurement ranges (from 10mA to 100nA)

• Decreases parasitic effects of interconnects

• Provides switching for – Source measurement units (SMUs)– CVU signals– Pulses from PMU– No re-cabling to change instruments


Connection setups for NVM testing

2-terminal test device 4-terminal test device


Flash testing

• Program and erase conditions for Fowler-Nordheim tunneling• Test hardware

– One Model 4225-PMU with two Model 4225-RPMs– Two SMUs


Connection to four-terminal floating-gate Flash device


Flash data: program and erase Vt

Program

Erase


Flash program pulse

Program Voltage and Current Waveforms

Gat

e V

olta

ge (V

) Gate C

urrent (A)

Time (s)

Simultaneous gate voltage (blue) and gate current (red) measurement

FN current

parasitic current


Flash endurance


What is PRAM?

• PCRAM – Phase Change RAM• Based on chalcogenide alloys• Amorphous phase – after “RESET”

– High resistivity > 1MOhm– Melting of material– Fast cooling (RESET)– Fall time for PRAM materials is about 30ns or

even faster• Crystalline phase – after “SET” operation

– Low resistivity <~1kOhm– Lower energy state– Raising temperature above crystallization

temperature– Re-crystallization by slower cooling (SET)– Re-crystallization rates dropped from

hundreds of ns to 30ns


PRAM material testing

R-load technique Connection diagram for PRAM tests


PRAM I-V characterization

• Blue line: voltage pulse vs. time• Red line: current response vs. time• Rise time: 10µs, can be as short

as 20ns• Note: switching for high resistive state

to low resistive state at 0.7 V

• I-V curve measured by the PMU with RPMs. This test uses the same data shown to the left.


Definition of PRAM R-I curve

Diagram explains the R-I curve measurement sources from the RESET-measure-SET-measure waveform.

The amplitude of SET curve is swept, while measuring RESET resistance (red M), SET current (green M), and SET resistance (blue M)


PRAM RI curve, data

• Voltage (blue) and current (red) waveform• RESET pulse puts DUT into a high resistance state • SET pulse sets the low resistance state• The second and last pulse measures the resistance

of the RESET/SET state• R-I curve shows the variation of the SET resistance

as the SET pulse amplitude is swept


PRAM endurance

• SET resistance is plotted as a function of total number of cycles

• SET resistance increases as applied stresses increases


Ferro-electric memory

• Ferro-electric memory is based on polarization switch due in the presence of critical electrical field

• It is NOT ferro-magnetic memory• Read-write processes are destructive

intrinsically and require refreshing

• Characterization tests• Hysteresis curve of polarization

(charge) vs. field• PUND (positive-up-negative-down)

test

References:http://www.ramtron.com/about-us/what-is-f-ram.aspx


FeRam test setups

Traditional: Sawyer-Tower circuit• C0 (load cap) >> C• Vin ~ V• Vo << Vin, measures

charge/polarization• Requires ability to measure

small voltage

Measurement with Pulse Measure Unit• Measures current and charge directly• No need for load cap• Does not require measurement of

small voltages


FeRAM hysteresis waveform measurement

• Voltage Up/Down/Up sweep waveform (blue line on the left graph)

• Measure response current from low side of FeRamcap (red line)

• Integrate current to charge and plot charge vs. voltage to get Hysteresis curve (black curve on the right graph)

Voltage

Current


FeRAM PUND (positive-up-negative-down) test

Procedure:•Force PUND pulse sequence using Segment ARB functionality of PMU/RPM (blue curve)•Measure response current (red line)•Integrate current to obtain (area under curve)

• P, Pa• U, Ua• N, Na• D, Da• Psq, and Qsw

•Advantage: low noise measurement


FeRAM endurance

Test•Psw (P-U)•Qsw (1/2(P-U+N-D))•Total endurance is about 1e8 cycles

•Test time is in minutes


ReRAM and CBRAM testing

• In many cases, ReRAM and CBRAM testing is done with SMU (DC sweep)

• DC current compliance is used –– But SMU compliance circuitry

is not instantaneous! It takes hundreds of microseconds to milliseconds

• We provide Segment Waveform, dynamic Voltage sweep to characterize I-V curve for any sweep test

• DC test time is always >0.1ms• Pulse test time with PMUs can be

as short as 40ns

Diagram for one of the multi-pulse tests included in the NVM library


Capacitive charging effects during pulse transitions


Avoiding capacitive charging effects

• Measuring the current on the LOW side of the DUT, away from the applied pulse, is the key to avoiding the charging effect

• The side of the device connected to Channel 2 has a dV/dt that is essentially 0

• This approach is sometimes called pulsing on the high side and measuring the current on the low side

• There is no capacitive charging current seen during the pulse transitions. Only the current flowing through the resistor DUT is seen.

High

Low

dV/dt ≈0

No charging effects, just current flowing through the test device


NVM Testing with the Model 4200-SCS

• Hardware minimum requirements– Model 4200-SCS with KTEI V8.2 or higher– Two SMUs, either medium power (Model 4200-SMU)

or high power (Model 4210-SMU). – One Model 4225-PMU with two Model 4225-RPMs– Compiler to modify the NVM test modules– Software components– KTEI V8.2 with standard suite of characterization tools– NVM library and NVM project, with example test setups

and data for PRAM, FLASH NAND, and FeRAM• For additional information, see application note

– #3141 Non-volatile Memory Pulse I-V Characterization Techniques at http://www.keithley.com/data?asset=56338


Summary

• Most of the NVM technologies have similar electrical characterization requirements

– Multi-level pulse waveforms– Simultaneous current and voltage sampling during the test– Wide range of voltage sourcing and current measurements

• The Model 4200-SCS with Models 4225-PMU and 4225-RPM simplifies complex NVM test requirements

– Hardware that does not require load resistors or load capacitors– Integrated, simultaneous fast current and voltage transient measurements– Segment ARB® capability for multi-level, multi-pulse waveform creation

• Included project contains sample tests and data for multiple NVMtechnologies


Contact Keithley for further information

Worldwide HeadquartersWithin the USA: 1-888-KEITHLEYOutside the USA: +1-440-248-0400

Email: [email protected]

Additional offices: www.keithley.com

Europe:Germany: (+49) 89 849 307 40Great Britain: (+44) 118 929 7500

Asia:China: (+86) 10-8447- 5556Japan: (+81) 3-5733-7555Korea: (+82) 2-574-7778Taiwan: (+886) 3-572-9077

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