Performance improvement of a large range metrological

AFM through parasitic interference feedback artifacts

removing by using laser multimode modulation method

Qi Li*, Sitian Gao, Wei Li, Mingzhen Lu, Yushu Shi

National Institute of Metrology (NIM), 100013 Beijing, P.R China

ABSTRACT

A large range multi-functional metrological atomic force microscope based on optical beam deflection

method has been set up at NIM one year ago. Being designed intended to make a traceable

measurement of standard samples, the machine uses three axes stacked piezoceramic actuators, each

axis with a pair of push-pull piezo operated at opposite phases to make orthogonal scanning with

maximized dimensional up to 50×50×2mm3. The stage displacement is measured by homodyne

interferometer framework in x,y,z direction, from which beams are aligned to intersect at cantilever tip

to avoid Abbe error, an eight times optical path multiplier interferometer mirror is researched to

enhance fringe resolution. There is also a new compact AFM head integrated with LD, quadrant PD,

cantilever, optical path and microscope, the head uses special track lens group to guarantee laser spot

focused and static on the back of the cantilever, no matter whether or not the cantilever have lateral

movements; similarly, reflect beam also focused and static in the center of quadrant detector through

convergence lens group, assumed no cantilever bending on vertical direction. Attribute to above design,

the AFM have a resolution up to 0.5nm. In the paper, further improvement is described to reduce the

influence of parasitic interference caused by reflection from sample surface using laser multimode

modulation, the results shows metrological AFM have a better performance in measuring step, lateral

pitch, line width, nanoroughness and other nanoscale structures.

Keywords: single longitudinal mode, optical feedback, laser hopping, parasitic interference, high

frequency modulation

1. INTRODUCTION

In nanometer and micrometer scale measurements, Atomic Force Microscope (AFM) is an

essential tool in the field for visualizing surface morphologies, measuring structure dimensional and

investigating sample’s characteristics. Currently, there are a large number of commercially available

AFMs used in semiconductor and MEMS manufacturing, solar panel and display panel inspections, as

well as biological physiological researching areas[1-2]

. Because these commercially available AFMs

need to be calibrated and trace back to laser wave length, the NIM has researched a long range

metrological AFM to provide measurements traceable to SI unit standard, the resolution is up to 2nm

with standard uncertainty 20nm.

*E-mail:liqi@nim.ac.cn

Scanning Microscopies 2013: Advanced Microscopy Technologies for Defense, Homeland Security, Forensic, Life, Environmental, and Industrial Sciences, edited by Michael T. Postek, Dale E. Newbury, S. Frank Platek, Tim K. Maugel,

Proc. of SPIE Vol. 8729, 872906 · © 2013 SPIE · CCC code: 0277-786X/13/$18 · doi: 10.1117/12.2015663

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Figure 1, the NIM’s mAFM apparatus construction

NIM’s metrological AFM can calibrate transfer standards (step height, 1D, 2D pitch and 3D

pyramid) which are traced to laser wave by interferometers in X, Y, Z directions. In order to attain on

line height direction profile calibration, NIM’s AFM operates at constant force mode that uses optical

beam deflection (OBD) method to transfer cantilever distortion into deflection signals. These signals

then generate feedback to control PZT platform’s height position to maintain constant force. At last,

through z direction interferometer mounted under platform, height information is traced to laser waves.

Figure2. cantilever is fixed with double curvatures

cylindrical lens, whose focus is on the cantilever, so no

matter how the head moves laterally, collimated laser

always focuses on the same position on back of

cantilever, the only difference is which part on lens

surface the laser transmits.

Figure3. the solid line is coincidence with condenser’s

optical axis, the dotted line is similar to paraxial light,

and the equations below are set up: the bending angle

of cantilever is α=3Δz/(2z), object distance is L=a+b+c,

object aperture angle is 2α, magnified displacement

ΔS=XtanU’=3X L Δz/(z L’)

Base

xy coarse stage

Sample holder & cubic mirror

AFM head & probe

Metrological frame

Interferometer

z coarse stage

6-DOF nanostage

Cubic mirror

Interferometer

Interferometer

3-DOF nanostage

Cover

AFM head support

(a) (b)

Quadrant

detector

Condenser

Mirror

Tip

Mirror

Source

`Condenser lens Fiber

Nanometer platform Tip

Tracing lens

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Figure 2 is OBD element mounted on our AFM, differentiating from other OBD design, the

element uses separated X-Y plane and Z direction piezoelectric ceramics scanners instead of three

dimensional PZT tube, which has small bending in horizontal movements; moreover, separated PZT

scanners can adapt high resonance scanner in Z direction to accelerate measuring speed in the mean

time adapt long range plane scanner to enlarge the scanning size. Another novel design of our element

is it divides element into fixed and moving parts, because usually the back width of cantilever is no

more than 50μm and laser spot’s dimension normally is between 5-15μm, it is easy the spot partly or

wholly exceeds the back width of cantilever, for this reason, our design makes laser spot follow the tip

movement in horizontal directions in the mean time keep deflection angle reflected by cantilever

unchanged, which is very important in OBD method.

The structure and optical path of our OBD element can be seen in Figure 3, 4, it consists fiber, two

pieces 45o plane mirror, two tracking lens, corner cube reflector, condenser lens and quadrant photo

detector, Among above components, tracking lenses belong to moving parts, the other components are

stationary and belong to fixed parts. Laser emitted from fiber is first reflected by 45o mirror to change

propagation direction vertically; then two tracking lens with certain focal length and intervals make

sure laser spot is focused on the back of cantilever, besides this, tracking lenses and cantilever are both

mounted on plane PZT scanner with fixed relative position, making deflection angle only sensitive to

fluctuations on z direction; next, beam is sequentially reflected by the second 45o mirror, corner cube

reflector and goes through condenser lenses at last projects onto photo detector, the final displacement

magnification ratio is 1970 in our OBD method.

2. OBD NOISES SOURCES AFFECTING mAFM

Generally speaking, there are two implementation methods in measuring z direction nano scale

height. One is OBD method which through feedback of tip bending to keep the deflection beams

stationary on PSD (indicated as zero position) and measuring loop vertical displacements to get

traceable height information. The other is interferometer deflection measuring method which

measuring interference distance between laser interferometer and back of cantilever to get traceable

height information.

Compared with interferometer deflection measuring method, OBD method has simple

experimental setup as well as optical alignment, and theoretical studies demonstrate that these two

methods have nearly the same sensitivities[3]

, however some facts seriously limit the precision of OBD

method which causes tens times performance losses. In this study, the paper analyzes the factors limits

OBD performance and studies a laser multimode modulation method to improve it.

2.1 Mode hope and parasitic interference noises in OBD method

In apparatus research process, there are two kinds of distinct noises to affect OBD zero position

precision. One noise phenomenon is laser power fluctuations observed from PSD’s addition and

subtraction signals, the other noise phenomenon is periodic laser fringes pattern displayed on CCD and

oscilloscope. Through investigation it can be proved that the first noise is from optical feedback

reflected by optical elements and object surfaces[4-6]

, this can be explained as follows: in apparatus,

OBD method usually adapts single longitudinal mode semiconductor laser as a source because its noise

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lever is much lower than multimode laser’s; but when optical feedback occurs, the light back to laser

cavity makes single mode laser transfer to multi mode lasers with most power concentrating on one

mode[7-8]

. This mode with the biggest power is not invariant, mode competition occurs between

different modes, and this induces mode hoping[9-10]

. Besides, optical feedback also causes external

interference between cavity’s facet and reflecting surfaces, which causes power fluctuations with half

wavelength of optical path. The second noise is from parasitic interferences scattered by both tip and

object surface, this is also a distance dependent noises which attributes to coherence of single mode

laser.

In order to explain mode hope and parasitic interference noises, first an OBD device without noise

reduction is set up. In this device, the source uses Opnext HL6319G AlGalnP laser diode, the

wavelength is 635nm, maximum output 10mW, and integrated PD as feedback control. The driver

circuit uses iCHaus iC-Wk chip operates ranging from 2.4V-6V voltage output and 10μA to2.5mA

current output. The quadrant detector uses HAMAMATSU S5981. The schematic diagram is seen in

figure 4. In the circuit, power is supplied to laser diode, and also PD signal is feedback to chip’s

monitor pin, which protects laser diode from destroying by sudden voltage and current jump. Except

this, there are no extra efforts to reduce noises and keep laser stable, at last, in the experiments we

observed phenomena in detector (see in figure 5)

Figure 4 experiment layout without current modulation

from above are quadrant detector sum voltages depict laser intensity variances. In this situation, the

light is reflected by cantilever and no object is loaded. From oscilloscope we find the DC voltage is

600mV; after subtract DC component, the AC components with minimum amplitude fluctuations is

approximately 23mV and maximum amplitude exceeding 80mV, which are 3.8% and 13% of DC

respectively. According to the principle of semiconductor laser[11-12]

, the minimum noises can be

considered as light instantaneous noise, but except for this noise lever, the AC noise wave amplitudes

can increase to 80mV, and the changes are periodical with approximately 2 minutes period intervals.

Further more, these noises varies on both amplitudes and central position of averaged voltages.

Because the voltages are proportional to laser radiation intensities, the jumps of averaged voltage levels

also mean the changes of laser power.

iC-WK

Iout

Monitor

Laser diode

Tip

Cylindrical

lens,

Condenser

Quadrant

detector

Current driver chip

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Figure 5,one cycle intensity fluctuation caused by optical feedback, this is time variance noises only occurs when

partially light is reflected, the signals displayed on oscillscope is AC voltags which has bypassed the DC voltage.

The max noise is 80mV while the min noise is 20mV and believed as instantaneous noise

The experiment is done in clean room and there are no external parameter changes, so it is

reasonable to believe that it is the state of semiconductor laser alters; moreover, in experiment adjust

process, we found when optical elements are not strictly perpendicular to axis, it is observed clearly the

reflected laser spot from the element surface, so it can be further deduced the optical feedback is the

main reason causing these phenomena.

Next, we add a mirror under the cantilever tip to research parasitic interference. Because our AFM

mainly calibrates the silicon mask linewidth with very smooth surface, this experiment set up is

Max:20mV,10mV

DC voltage 600mV

is bypassed

exceeds 60mV,10mV/div

about 45mV, 10mV/div about 20mV, 10mV/div

about 50mV,10mV/div exceeds 60mV,10mV/div

elapse 27s elapse 61s

elapse 90s elapse 125s

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reasonable. Unlike optical feedback noise which is time related, it is thought parasitic interference is

distance related. To testify this phenomenon experiment uses a Npoint N-XYZ100B nano PZT platform

and a plane mirror fixed on it. The platform is driven by periodical linear variant triangular waves to

move forward and backward, the step and time intervals of which are set to 100nm and 200ms

respectively. At last the laser intensities are displayed in figure 6, the green waveform is from Npoint

monitor output, the step shaped triangular wave is because the PZT only operates in step moving

method; the yellow waveform is from detector, through it can clearly observe the phenomena of

parasitic interference superimposed by optical feedback noises, it shows that starting at some certain

time within an optical feedback period, the average laser power fluctuates greatly (approximately

45mV in figure) and noise amplitudes also increase very large (approximately 35mV).

It also shows light intensities display stepped changes in accordance with PZT steps, because PZT

moves one step very fast (but intervals between adjacent steps are very long) and oscilloscope sweeps

at 200ms/div a relatively low speed, the signal steps are actually the light intensities when PZT pauses;

Besides this, it can be also observed that the light signals obey some repeatability, these are interference

phenomena.

The phenomena can be discussed as follows: it is known the PZT step distance is 100nm, and

semiconductor laser wavelength is 650nm, because one roundtrip includes forward and backward

optical paths, the coherence length will be 325nm if exists interferences. In figure 6, although integral

multiple of PZT step length are not exactly coincidence with half wavelength, and every pause PZT

may not be able to stay exactly at the peak or zero cross node positions, we can still find the tendencies

that every three steps the waveform will repeat the period which is 300nm approximately a half

wavelength.

Figure 6, optical feedback noises superimpose parasitic noises, the green waveform is PZT driver signals observed

from monitor output, the yellow signals is laser intensity signals observed from quadrant detector. The burr parts is

because optical feedback effects.

Because OBD method relies on addition and subtract signals as “zero position” indicator, the

existence of intensity jumps seriously affect the position of “zero position”, besides this, higher noise

level also reduces resolution of OBD method, so in next section, a high frequency current modulation

method to reduce mode hoping and laser coherence is researched.

PZT 50mV/div, 0.1μm/step

optical feedback

base intensity

parasitic interference

10mV/div, 200ms/div optical feedback 10mV/div, 200ms/div

50mV/div, 0.1μm/step

half wavelength

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As discussed above, in order to improve the precision of OBD method, the optical feedback and

parasitic interference noises must be reduced. In our OBD’s device, a high frequency current

modulation method is adapted. The principle of the high frequency current modulation is in first several

nanoseconds after lit the single-mode laser is in transient state and oscillating in multimode

condition[13-15]

, if at this movement the laser power supply is superimposed by a high frequency

modulated current, then the laser oscillation will maintains on multimode state. The modulation forces

laser mode to transfer between adjacent modes, which prevents laser from instable mode competing.

Because in this situation mode transition is regular and periodical, the laser power fluctuation is also

period. If now the modulation frequency is adjust to exceed the quadrant detector response frequency,

signals appeared on oscilloscope will be stable and optical feedback noises can be greatly reduced. For

parasitic interference noises, the high frequency current modulation causes high frequency mode

transition, and this reduces optical coherence of laser light, so it is harder to generate interferences

between cantilevers and object surfaces.

In device we uses TI LMH6525 driver chip to generate a sine wave current that both amplitude

and frequency can be adjust. The chip accepts 4.5V≤Vs≤5.5V voltage and maximum 20μA currents,

and through changing external resistors, the modulated frequency is from 200MHz to 600MHz; and

modulated amplitude is from peak to peak 100mA. The schematic diagram in figure7 shows the

experimental setup, and laser source is the same one used in previous experiment. In schematic, the

current input is connected to the PD output as feedback channel, and its output connects to two level

op-amps; the first level is operated as a voltage follower, and the second level works as a voltage

comparator with its positive input to an adjustable offset voltage and the negative input to feedback

channel; after that, the op-amp output is connected to LMH6525 chip and produces modulated

amplitude and frequency; at last, the modulated current is offered to semiconductor laser. This circuit

has mainly two functions, the part before chip offers APC control to prevent laser damage; the part

about the chip offers current modulation function.

Figure 7, laser source with high frequency and amplitude current modulation, the first op-amp operates as a

voltage follower, the second op-amp operates as APC elements, two variable external resisters are used for

controlling the frequency and amplitude.

TI

LMH6525

Laser diode

Tip

Cylindrical lens,

Condenser

Quadrant

detector

Feedback

Iout

A

B

C

D A+B+C+D

3 HEIGHT FRENQUENCY LASER CURRENT MODULATION AND OPTIMIZATION

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Power pulsed train

Current

Power

P0

threshold

Frequency modulation

Figure 8, current modulation experiment, including

modulation circuit, quadrant detector, amplification

circuit, AFM head, mirror and PZT platform.

Figure 9, the relationship between sine current and

power pulsed train, when stationary offset point is

fixed, through adjusting modulation depth, sine

waveform or discontinuous pulses can be obtained.

Figure 10, noise contrast with and without current

modulation, when modulation is on, the noises level are

stable on the instantaneous noise value 20mV; when

modulation is off, the noise is about 70mV

Figure 11, after applying current modulation, the

optical feedback and parasitic interference noise are

greatly reduced, the noise lever of laser intensities is

about 20mV

After the circuit is build up, next work is to research optimal amplitude and frequency

configuration values. In current modulation, there are three factors affecting final modulation effects:

the offset point, modulation depth and modulation frequency. Figure8 shows the relationship between

laser power and current parameters. In semiconductor laser there is threshold above which the gain of

light will exceed the loss and then laser is emitted. So in the optimization process, the first thing is to

determine threshold and offset point. According to Opnext HL6319G parameters, the threshold is

50mA, max operation voltage is 2.7V and max operation current is 95mA, so the current offset point P0

is set to its typical value 70mA. After the offset point is fixed, the next thing is to consider the

modulation depth. Modulation depth is related to current amplitude, if the current amplitude is adjusted

20mV

Modulation

On|Off 70mV

20mV/div bypass DC

20mV/div, 200ms/div

50mV/div, 200ms/div

current modulation circuit

laser detector amplification circuit

laser spot

PZT platform mirror

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to above the threshold, then the output power is a sine waveform; otherwise if the current adjustment

crosses over the threshold, then the output power is a high pulsed train. In amplitude optimization

process, we identified some characteristics through radiation power meter and oscilloscope: 1, the

power varies proportionally to current changes with approximately 0.5mW/mA; 2, in the case

modulation current crosses over threshold the averaged output power is larger than power without

modulation; 3, the modulation method reduces noises only in situation the pulse train generates, and

this can be explained by that on high frequency pulse train output, at the beginning of tens of

nanoseconds each time the current exceeds threshold, the single mode laser is initially in the transient

state that existing multi longitudinal modes, and there are no mode contentions during these intervals;

moreover, because the modulation frequency is about hundreds of MHz, the current is beneath the

threshold before the mode contentions occur. In experiment, it is found that when modulation depth is

set to 25mA, the laser has output around 5.7mW compared with no modulation output 5.1mW, and the

noise keeps approximately 20mV. The third factor should be adjusted is modulation frequency,

according to specifications, LMH6525 can adjust current frequency from 200MHz to 600MHz, and

S5981 detector’s cut off frequency is 20MHz, which guarantees detector do not respond laser

modulation intensity variances, but high frequency can induces electronic noises, so in our circuit some

filter components are mounted. The experiment shows when works on 500MHz, our circuit has the

smallest residual noises. The final results show in figure 9 and 10, when modulation switch is off, the

noises change dramatically with max value approximately 70mV; when the switch is on, the noises

level then reduces and keeps stable on 20mV. The final noise ratio after modulation is 3% which is

nearly a quarter of before.

4. CONCLUSION

In the AFM apparatus research process, it is found the laser power in OBD component fluctuates

with time and displacement variances. Because the apparatus is on clean room with fine temperature

controlling, this influence can be excluded. Through experiments it is shows that when distance

between surface and laser facet is fixed, the laser intensity noises appears as period time variances;

when distance is changed by PZT driver, the laser intensity noises appears as period space variances;

when tip and object are removed, the noises lever is about 20mV and keeps stable, which can be

considered as photon noises. From above results it can be induced the time dependent periodical noise

is caused by reflected light impinging into laser cavity which generates optical feedback and mode

hopping, and the space dependent periodical noise is caused by optical path variances which generates

unwanted interferences. Next, in order to reduce these noises, a high frequency current modulation

method is applied, through which the sine wave current oscillates above and beneath laser gain

threshold rapidly, making the single mode laser stay on multimode transient state at initial tens of

nanoseconds every time it exceeds threshold point, so the mode hopping is avoided. The experiment

results show that after high frequency current modulation noises is reduced from 70mV to 20mV, and

because laser is on multimode state which reduces optical coherences, it also make OBD apparatus

insensitive to parasitic interferences.

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