HP AN4156 8_Evaluation of Oxide Reliability Using v Ramp J Ramp Tests

8/14/2019 HP AN4156 8_Evaluation of Oxide Reliability Using v Ramp J Ramp Tests

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Evaluation of Oxide Reliability

Using V-Ramp / J-Ramp Test

Application Note 4156-8

HP 4155B/4156B

Semiconductor Parameter

Analyzer


2/8

Introduction

With the size of gate area and the

thickness of the oxide shrinking

along with device geometries, cre-ating a reliable thin oxide has be-

come an important issue. The

integrity of the thin oxide in a MOS

device is a dominant factor deter-

mining overall reliability of a

micro-circuit.

This application note describes V-

Ramp and J-Ramp tests using the

HP 4155B/4156B semiconductor

parameter analyzer, based on the

JEDEC Standard No. 35.

They can promptly give useful

feedback to the manufacturing

process about oxide reliability.

What are V-Ramp and

J-Ramp?

Both Voltage Ramp (V-Ramp) and

Current Ramp (J-Ramp) are one of

so called Wefer Level Reliablity

(WLR) tests. In V-Ramp test, volt-

age across an oxide is increased at

a constant linear rate until failure

occurs. On the other hand, J-Ramp

test increases current through an

oxide logarithmically in linear time.

They both record total charge

(Qbd) and breakdown voltage

(Vbd). These parameters are meas-

ured on a large number of test sam-

ples and usually plotted as

cumulative breakdown distribution

versus breakdown charge density

or breakdown electrical field on

probability chart. The manufactur-

ing process should be maintainedso that this distribution becomes

closer to the ideal shape. V-Ramp is

more sensitive at detecting break-

down at lower voltages (i.e. logic,

micrprocessors). J-Ramp which is

more sensitive at higher voltages

(i.e. EPROM, flash). V-Ramp test is

more commonly applied to test

structures which have relatively

large oxide areas. On the other

hand, J-Ramp test is more often ap-

plied to small scribe-line test

structures.

V-Ramp Procedure

First, the test structure is checked

for integrity by measuring the leak-

age current after applying a con-

stant voltage (Vuse).

Second, voltage is applied to the

test structure in steps until the ox-

ide ruptures, compliance occurs, or

voltage exceeds specified limit This

ramped test should be performed

in accumulation mode.

Finally, a post stress analysis is

performed to record the failure

2

Structure Breakdown?

T

F

F

T

I leak > 1A

Current Compliance

F

Force Vuse

Measure I

Force Vstress

Measure I

Qbd = Qbd + I*T

T Initial Failure

Stop Test

Determine Failure

Type

Post Stress Test

START

END

Initial Failure

Stop Test

Determine Failure

Type

Post Stress Test

Qbd = 50C/cm^2

or Compliance?

T

F

F

T

Vmeas < Vuse

Vmeas < .85*Vprev

F

Force IstressMeasure Vmeas

T

Force I0

Measure V

Qbd = Qbd + Istress*T

Istress = Istress * FEND

START

Figure 1. Flowchart of V-Ramp

Figure 2. Flowchart of J-Ramp


3/8

category. Figure 1 shows the flow

of the V-Ramp Test.

J-Ramp Procedure

First, an initial test is made to de-

termine the quality of the oxide ca-

pacitor by monitoring the voltage

after applying a current (typically

1A). If the voltage does not reach

the use condition voltage within

specified time, it fails the initial

procedure.

Then, current is applied to the test

structure in steps until oxide rup-

tures, or total charge exceeds thespecified value. The current is in-

creased by multiplying a certain

factor (F) by the previous stress

current. JEDEC No.35 specifies

that F should be less than square

root of 10. Breakdown is defined by

a 15% drop in voltage. This ramped

test should be performed in accu-

mulation mode.

Finally, post stress test is per-

formed to determine the failure

category. Figure 2 shows the flowof the J-Ramp Test.

Problems with V-Ramp and

J-Ramp

Since oxides can rupture suddenly,

changing the step, monitoring the

voltage, and judgement of the exit

condition must be done very

quickly to insure reliable measure-

ment results. Automation using a

computer is mandatory for both of

these tests. Several JEDEC require-

ments make creating the test pro-

gram difficult.

It is desirable to keep the step time

as uniform as possible to measure

Qbd precisely and consistently.

During the ramped test, maintain-

ing a constant step time may be dif-

ficult. Using the clock in the

external computer, with its resolu-

tion and accuracy limitations, you

cannot set small and consistent

step durations. Even if you use the

sweep function of the instrument

and set the delay time, any range

changing that occurs during sweepwill add unpredictable step varia-

tion as shown in Figure 3.

HP 4155B/4156B Solution

The HP 4155B/4156B has built-in

HP Instrument BASIC. The whole

V-Ramp or J-Ramp tests can be

automated using HP Instrument

BASIC which is suitable for instru-

ment control and scientific calcula-

tion. Thus, an external computer is

no longer necessary.

Initial stress measurement, ramped

test, and post stress test are done

by loading the pre-saved measure-

ment setup files. These setup files

can easily be created by fill-in-the-

blank manner from the front panel.

This eliminates one of the most dif-

ficult parts of a program, instru-

ment control.

Of course, each setting parameter

can be changed from the program

later. So, any setting parameter

which is determined by compli-

cated calculation or by measured

results can be updated later.

You can make use of the HP

4155B/4156B's analysis function to

extract the resultant parameters.

Extraction of Vbd and calculation

of Qbd is possible by setting the

USER FUNCTION and moving the

marker to the breakdown point.

Complicated programming to ana-

lyze the data is eliminated.

The requirement of maintaining a

precise and constant time step

throughout the ramp test, is accom-

plished by using a fixed range, and

by using the output trigger function

(see Figure 4). The output trigger

function gets its name from the

ability of the HP 4156B to synchro-

nize with external instruments.

3

Measure Time

Range change

Delay Time

Figure 3. Duration Varies Due to Range Change and Inconsistent Measurement

Output Trigger

Step Delay Time

Measure Time

Delay Time

Overhead

Time

Figure 4. Constant Duration by Using Output Trigger Function


4/8

Step delay time for external instru-

ments is required to provide consis-

tent settling time for reliable

measurements. You can make use

of this function to keep the step du-ration constant during the ramped

test. Precise Qbd values can be ob-

tained by using this function by set-

ting time steps to 100s resolution.

V-Ramp with the HP4155B

/ 4156B

V-Ramp test

The entire procedure described be-

low is fully automated by the HP

Instrument BASIC.

First, an initial test checks leakage

current when Vuse is applied to the

test structure. To make a single

spot test, the HP 4155B/4156B is

set to sampling mode and the num-

ber of steps is set to "1". The post

stress test, which is made at the

end of entire test, is identical to the

initial test. Both measurements are

performed by loading presaved

measurement setup files. The setup

file can easily be created or modi-

fied from the front panel.

Then the ramped test is performed

using the sweep measurement

function. Figure 5 and 6 show the

SWEEP SETUP and MEASURE

SETUP pages.

Constant duration through out the

ramped test is desirable. The fol-

lowing setup procedure maintains

a constant step time interval during

the ramped test.

1 Enable trigger output function

in OUTPUT SEQUENCE page

(Figure 7).

2 Set DELAY TIME in SWEEP

SETUP page.

3 Set STEP DELAY time in OUT-

PUT SEQUENCE page.

4 Set measurement range to

FIXED range in MEASURE

SETUP page.

4

ME A S U R E : S WE E P S E T U P 9 4 J U N 2 5 0 6 : 3 1 A M

V o l t a g e R a mp S w e e p M e a s u r e me n t

* V A R I A B L E V A R 1 V A R 2

U N I T S MU 1 : MP

N A ME V g

S WE E P M O D E S I N G L E

L I N/ L O G L I NE A R

S T A R T - 5 . 0 0 0 V

S T O P - 4 0 . 0 0 0 V

S T E P - 5 0 . 0 mV

N O O F S T E P 7 0 1

C O M P L I A N C E 5 0 . 0 0 m A

P O WE R C O M P O F F

* T I M I N G

H OL D T I ME 0 . 0 0 0 0 s

D E L A Y T I ME 4 0 . 0 ms * S WE E P S T OP A T C OMP L I A N C E S t a t u s

* C O N S T A N T

U N I T S MU 4 : MP V S U 1 V S U 2

N A ME V s u b V S U 1 V S U 2

MO D E V V V

S O U RC E 0 . 0 0 0 0 V 0 . 0 0 0 0 V 0 . 0 0 0 0 V - - - - - - - - -

C O MP L I A N C E 1 0 0 . 0 0 mA - - - - - - - - - - - - - - - - - - - - - - - - - - -

ME A S U R E : ME A S U R E S E T U P 9 4 J U N 3 0 0 9 : 1 0 A M


* M E A S U R E M E N T R A N G E

U N I T N A ME R A N G E Z E R O C A N C E L OF F

S MU 1 : MP I g F I X E D 1 0 0 mA O F F [ 1 n A ]

S MU 4 : MP I s u b F I X E D 1 0 0 mA O F F [ 1 n A ]

V MU 1 V MU 1 A U T O - - - - - O F F

V MU 2 V MU 2 A U T O - - - - - O F F

( * : Ol d d a t a i s u s e d . )

* I N T E G T I M E

T I ME N P L C

S H O R T @ 6 4 0 u s 0 . 0 3 2

ME D 2 0 . 0 ms 1L ON G 3 2 0 . ms 1 6

* WA I T T I M E

0 * ( D E F A U L T WA I T T I ME )

ME A S U R E : O U T P U T S E Q U E N C E 9 4 J U N 3 0 0 9 : 1 3 A M

V o l t a g e R a mp S w e e p M e a s u r e m e n t

* O U T P U T S E Q U E N C E * T R I G G E R S E T U P

U NI T N A ME MO D E E N A B L E / D I S A B L E E N A B L E

1 S MU 4 : M P V s u b V F U N C T I O N T R I G O U T

2 S MU 1 : M P V g V S T E P D E L A Y 2 1 . 4 ms

3 S MU 2 : M P P O L A R I T Y P O S I T I V E

4 S MU 3 : M P

5 V S U 1 V S U 1 V

6 V S U 2 V S U 2 V

7 S MU 5 : M P

8 S MU 6 : M P

9 P G U 1

1 0 P G U 2

Figure 5. SWEEP SETUP Page

Figure 6. MEASURE SETUP Page

Figure 7. OUTPUT SEQUENCE Page


5/8

5 Set WAIT TIME to zero in

MEASURE SETUP page.

JEDEC No.35 specifies that the

ramp rate should be from 0.1 to 1.0MV / cmsec. If you define step volt-

age (Vstep), the duration of each

step is calculated as follows.

Duration=Vstep/(Ramp rate Tox)[sec]

Tox: Oxide thickness [cm]

The delay time and step delay time

should be chosen so that they sat-

isfy the following equations.

In case of V-Ramp:

Duration=1.2 ms + (DELAY TIME)

+(STEP DELAY - 100 s)

In case of J-Ramp:

Duration=1.5 ms + (DELAY TIME)

+ (STEP DELAY - 100 s)

where: 1.2 ms and 1.5 ms are meas-

urement overhead.

DELAY TIME should be set large

and according to the above formula

so that the measurement is per-

formed close to the end of eachstep. STEP DELAY time must be

larger than sum of setting INTEG

TIME and measurement overhead.

To keep the step duration constant,

don't use USER FUNCTIONs while

voltage is swept. Analysis with

USER FUNCTIONs can be done af-

ter the ramped test procedure is

completed.

Select SWEEP STOP AT COMPLI-

ANCE Status in the SWEEP SETUP

page so that sweep aborts immedi-ately if compliance occurs.

The calculation of Qbd in equation

below, can be performed by a HP

4156B USER FUNCTION. The set

up of USER FUNCTION should be

made after the ramped measure-

ment to keep the duration con-

stant. Time is defined as

multiplication of measurement data

(@INDEX) and step duration. When

the marker is moved to the break-

down point, Qbd appears on the

GRAPHIC page.

Qbd =i(t) dt

The search for breakdown voltage

is made in the HP Instrument BA-

SIC program. JEDEC defines

breakdwon voltage as the voltage

when current becomes ten times

larger than the expected current or

the slope (log Ig /Vg) changesby between 1.5 to 3. Expected cur-

rent is decided by either a pre-

measured curve or theoretical

curve. Theoretical current is called

the Fowler-Nordheim current,

which is given as follows.

J = A E2 e(-B / E)

where

A , B : Constants in terms of effec-

tive mass and barrier

height.

E : Electric field

Or, since current starts flowing

right after oxide rupture, it will be

OK to detect the breakdown volt-

age one step before the Ig compli-

ance as a simplified alternative.

Another V-ramp test for

studying ultra thin oxide

leakages

The main purpose of WLR V-ramp

tests is to monitor the qualitative

tendency of the process. Parame-

ters such as Qbd give prompt proc-

ess control feedback by quickly

stepping the stress voltage to

5

C H A N N E L S : U S E R F U N CT I ON D E F I N I T I ON 9 4 J U N 1 8 1 0 : 5 7 A M


* U S E R F U N C T I O N

N A M E U N I T D E F I N I T I O N

T i me s e c @I N D E X * . 0 6 2 5

V b d V @MY 2

Q b d C I N T E G ( I g , T i me )

G R A P H / L I S T : G R A P H I C S ME D I U M 9 4 J U N2 2 0 3 : 4 8 A M

o

*

V o l t a g e R a m p S w e e p M e a s u r e m e n t

M A R K E R ( 2 8 . 0 6 2 5 0 0 0 s e c - 5 . 2 4 2 5 m A - 2 7 . 4 0 0 V )

V b d - 2 7 . 4 0 0 0 0 0 0 0 0 V

( A ) Q b d - 1 1 . 3 3 3 5 0 9 3 7 5 m C ( V )

- 1 . 0 0 - 4 0 . 0

I g V g

d e c a d e - - - - - -

/ d i v / d i v

- 1 . 0 0 p - 5 . 0 0

0 . 0 0 T i m e ( s e c ) 5 . 0 0 / d i v 4 2 . 0

Figure 8. USER FUNC SETUP Page

Figure 9. V-Ramp Example Measurement Result


6/8

breakdown, in uniform time

increments.

For thin oxides (less than 100 ang-

stroms thick), low voltage "directtunneling" leakage must be accu-

rately studied. The HP 4156B is ide-

ally suited for this purpose due to

its ability to measure currents

down to fA levels at high speeds.

Use auto ranging in the MEASURE

SETUP page to increase accuracy.

Add a hold time only if you sweep

from a non-zero start point. Add de-

lay time only for large voltage step

size or if probes are not fully

guarded. Leakage sweeps, such asshown in Figure 10 can be made in

less than one minute using the HP

4156B with a well guarded probe

station such as the Cascade Sum-

mit series prober.

J-Ramp with the HP

4155B/4156B

The entire procedure described be-

low is fully automated by the HP

Instrument BASIC.

The initial test is performed usingthe sampling mode function. Speci-

fied current is forced and voltage is

repeatedly measured with constant

time interval. The measurement

stops when the voltage reaches the

specified threshold value, in this

case 2V. Figure 11 and Figure 12

show CHANNEL DEFINITION and

SAMPLING SETUP page

respectively.

After the initial test, the ramped

test is performed using the sweepmeasurement function. Figure 13

and Figure 14 show SWEEP SETUP

and MEASURE SETUP page

respectively.

Also with the J-Ramp test, constant

step time duration throughout the

ramped test is desirable. The dura-

tion is fixed constant by setting the

output trigger function, delay time,

6

G R A P H / L I S T : G R A P H I C S S H O R T 9 4 J U N 3 0 0 8 : 4 5 A M

o

. 0 0 1 C M 2 C A P T O X = 1 6 0 A n g s t r o m s

M A R K E R ( - 2 2 . 7 5 0 V - 2 7 . 5 9 0 m A )

( A )

- 1 0 0 . m

I g

d e c a d e

/ d i v

- 1 0 f

- 5 . 0 0 V g ( V ) 2 . 0 0 / d i v - 2 5 . 0

C H A N N E L S : C H A N N E L D E F I N I T I O N 9 4 J U N 1 9 0 0 : 1 0 P M

* M E A S U R E ME N T M O D E

S A MP L I NG

* C H A N N E L S

ME A S U R E S T B Y S E R I E S

U N I T V N A ME I N A ME MO D E F C T N R E S I S T A N C E

S MU 1 : MP V g I g I C O N S T 0 o h m

S MU 2 : MP 0 o h m

S M U 3 : M P

S MU 4 : MP V s b I s b V C O N S T

S M U 5 : M P

S M U 6 : M P

V S U 1 V S U 1 - - - - - - - V C O N S T

V S U 2 V S U 2 - - - - - - - V C O N S T

V MU 1 - - - - - - - - - - - - - - - -

V MU 2 - - - - - - - - - - - - - - - -

P GU 1 - - - - - - -

P GU 2 - - - - - - -

G N D U - - - - - - - - - - -

ME A S U R E : S A MP L I N G S E T U P 9 4 J U N 1 9 0 0 : 1 2 P M

* S A MP L I N G P A R A ME T E R * S T O P C ON DI T I O N

MO D E L I N E A R E N A B L E / D I S A B L E E N A B L E

I N I T I A L I N T E R V A L 1 0 . 0 0 ms E N A B L E D E L A Y 0 . 0 0 0 0 0 0 0 s

N O . O F S A MP L E S 1 0 0 N A ME V g

T O T A L S A MP . T I ME 1 . 0 0 s T H R E S H O L D 2 . 0 0 0 0 0 0 0 V

E V E N T | V a l | > | T h |

H O L D T I M E 0 . 0 0 0 0 0 0 s E V E N T N O . 1

F I L T E R O N

* C O N S T A N T

U N I T S MU 1 : MP S MU 4 : MP V S U 1 V S U 2

N A ME I g V s b V S U 1 V S U 2

MO D E I V V V

S O U R C E 0 . 0 0 0 0 A 0 . 0 0 0 0 V 0 . 0 0 0 0 V 0 . 0 0 0 0 V

C O MP L I A N C E 1 0 0 . 0 0 V 1 0 0 . 0 0 mA - - - - - - - - - - - - - - - - - -

Figure 10. Low Level Ig-Vg Characteristics Using Auto Ranging

Figure 11. CHANNEL DEFINITION Page (Initial Test)

Figure 12. SAMPLING SETUP Page (Initial Test)


7/8

step delay time, and zero wait time

as explained in the V-Ramp test.

The current is increased as IFn (n

is integer). To increase the currentwhile satisfying this equation, use

the LOG sweep function of the HP

4155B/4156B. Three kinds of F val-

ues can be chosen, which are

10(1/10), 10(1/25), and 10(1/50). The stop

current is calculated by solving the

following equation.

where

MaxQ : Maximum charge density

given by user (50 C/cm2 in

JEDEC No.35)Area : Oxide area [cm2]

In case of the HP 4155B/4156B, the

sweep measurement won't stop

right after the breakdown occurs.

After the specified sweep is over, it

can quickly search for the break-

down point. Figure 15 is an exam-

ple of J-Ramp ramped test.

Conclusion

The HP 4155B/4156B provides you

with features that greatly simplify

V-Ramp / J-Ramp tests. Pre-saved

setup files and auto analysis drasti-

cally reduce the programming ef-

fort. The built in HP Instrument

BASIC controller eliminates the

need for an external computer. Pre-

cise Qbd value is acquired by main-

taining a constant step time

duration through out the ramped

test.

7

ME A S U R E : S WE E P S E T U P 9 4 J U N 1 8 1 1 : 0 8 A M

* V A R I A B L E V A R 1 V A R 2

U N I T S MU 1 : M P

N A ME I g

S WE E P M O D E S I N G L E

L I N / L O G L O G 1 0

S T A R T - 1 . 0 0 0 u A

S T O P - 5 0 . 0 0 mA

S T E P - - - - - - - - -

N O O F S T E P 4 7

C O MP L I A N C E 4 0 . 0 0 0 V

P O WE R C O M P O F F

* T I M I N G

H OL D T I ME 0 . 0 0 0 0 s

D E L A Y T I ME 2 5 . 0 ms * S WE E P S T O P A T C OMP L I A N C E S t a t u s

* C O N S T A N T

U N I T S MU 4 : M P V S U 1 V S U 2

N A ME V s b V S U 1 V S U 2

MO D E V V V

S O U RC E 0 . 0 0 0 0 V 0 . 0 0 0 0 V 0 . 0 0 0 0 V - - - - - - - - -

C O MP L I A N C E 1 0 0 . 0 0 m A - - - - - - - - - - - - - - - - - - - - - - - - - - -

ME A S U R E : ME A S U R E S E T U P 9 4 J U N 3 0 0 9 : 0 7 A M

* M E A S U R E M E N T R A N G E

U N I T N A ME R A N G E Z E R O C A N C E L OF F

S MU 1 : MP V g F I X E D 4 0 V O F F

S MU 4 : MP I s b F I X E D 1 0 0 mA O F F [ 1 n A ]

( * : O l d d a t a i s u s e d . )

* I N T E G T I M E

T I ME N P L C

S H O R T @ 6 4 0 u s 0 . 0 3 2

ME D 2 0 . 0 ms 1

L O N G 3 2 0 . ms 1 6

* WA I T T I M E

0 * ( D E F A U L T WA I T T I ME )

G R A P H / L I S T : G R A P H I C S S H O R T 9 4 J U L 0 5 0 8 : 5 1 A M

o

*

C U R S O R ( - 2 . 3 5 0 0 0 0 0 0 m s e c 1 6 . 6 6 6 6 8 8 6 6 7 G A - 1 0 0 . 8 7 0 8 3 1 7 5 V )

M A R K E R ( 2 . 1 5 0 0 0 0 0 0 s e c - 1 9 . 5 1 m A - 2 7 . 1 1 2 4 V )

Q b d - 4 . 1 6 6 7 9 4 7 5 0 0 m C

( A ) V b d - 2 7 . 1 1 2 4 0 0 0 0 0 V ( V )

- 1 . 0 0 - 4 1 . 0

I g V g

d e c a d e - - - - - -

/ d i v / d i v

- 1 0 0 . n 0 . 0 0

0 . 0 0 T i m e ( s e c ) 2 0 0 . m / d i v 2 . 3 5

Figure 13. SWEEP SETUP Page

Figure 14. MEASURE SETUP Page

Figure 15. J-Ramp Example Measurement Result


8/8

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HP AN4156 8_Evaluation of Oxide Reliability Using v Ramp J Ramp Tests

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Transcript of HP AN4156 8_Evaluation of Oxide Reliability Using v Ramp J Ramp Tests