3D Optical microscopy

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Interferometry resolution is set to become a mainstream technology for Z-axis metrology in the laboratory. Yet its limitations, which include vibration sensitivity and operating complexity, have kept it away from the production floor and routine research laboratory measurements. A new, non-contact, high accuracy surface mapping technology combining interferome- try and conventional and digital imaging has

now been adapted for optical microscopes, resulting in a tabletop instrument, the 3DScope 2000, [see Figure 1] that can be used for many applications in the compound semiconductor industry. Nanometer- accurate vertical resolution is combined with micrometer-to-miUimeter accurate horizon- tal resolution, in an instrument that has several application and operation related advantages, over other techniques.

N anometer-resolution 3D optical microscopy for I Il-V quality control

Figure 1. Example of one configuration of the 3DScope

2000, the new interferometric instrument.

Insensit ive to vibration, this e q u i p m e n t allows interferometer- l ike t echno logy to opera te in labo- ratory and p roduc t ion f loor envi ronments , w i th minimal requ i rements for v ibrat ion isolation or for o the r ambient-rela ted control .

Its table top capabil i ty offers all the flexibility, ver-

satility and user-friendliness of the white-l ight

microscope .Accessor ies such as various types of

objectives, i l luminat ion modes and visualisation

are available, and 3D measu remen t can be added

wi thou t affecting them.

3D m eas u r em en t is as s imple as any o the r fea-

ture of the microscope, enabl ing a shor t and

u n d e m a n d i n g operat ional learning curve. The

opera tor sees the same field of v iew that is be ing

measured and needs only a shor t set-up t ime for

. . . . . . . . '£71 gr~, ' ~ " ~ ]}:i::~:}

each m e a s u r e m e n t ( involving no scanning) .The

measurement of the whole field of view and the

data computa t ion lasts just a few seconds.What

you see is wha t you get! An additional benefi t is

that the conventional view of a colour CCD cam-

era can be combined wi th the 3D surface data.

For 3D measurement , an optical sensor w i th par-

tially cohe ren t i l luminat ion is incorpora ted on

top of a white-l ight mic roscope and enables 3D

t o p o m e t r y measu remen t s in a highly accurate,

s imple and robus t way. The image ob ta ined by

the mic roscope at the set magnif icat ion is opti-

cally cont ro l led by an electronically cont ro l led

optical manipulator. UrLlike s tandard interferome-

try, no reference b e a m is used, therefore vibra-

tions, tu rbulence , or dust part icles do not change

the optical path, ne i the r do they degrade the

measu remen t accuracy.

This 3D m e a s u r e m e n t capabil i ty results in an

analys is /d iagnos t ics tool tha t is useful b o t h on

the p roduc t ion f loor and in the laboratory, After

manipulat ion, the resul t ing image pa t t e rn of the

ent i re field of v iew of the object is ob ta ined on a

m o n o c h r o m e CCD camera. From these intensity-

only images, the ref lec ted wavef ront f rom the

object inspec ted is fully analysed for phase

and ampl i tude data by propr ie ta ry algorithms,

tha t model the optical system and the optical

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3D Optical microscopy

Figure 2. Resist residue on a GaAs wafer The residue is trans- parent and barely wsible with a white-hght microscope (left)

but clearly seen using interference imaging (middle). The line profile (right) shows the residue thickness at nm resolution.

J

manipula t ion .This analysis gives surface topogra-

phy and reflectivity in the ent i re field of view,

w i thou t scanning in the X,Y, or Z-axes.

Once the 3D m e a s u r e m e n t is complete , a variety

of data analysis and archival funct ions can be

per formed, including 3D viewing, zooming in

and out, cross sect ioning, statistical analysis, data

saving and loading, and repor t preparat ion. The

optical design of the 3D sensor imaging and illu-

mina t ion system enables 3D m e a s u r e m e n t at any

magnif icat ion of the microscope , w i th no

changes in the 3D module, yet mainta in ing the

Z-a~s m e a s u r e m e n t per formance .

Key application -- GaAs MMIC metal lift-off The GaAs IC metal lift-off p rocess plays a very

impor t an t role in IC manufac tur ing due to the

incompat ibi l i ty of the III-V substrate w i th readily

e tchable metals like a luminum. Metals such as

plat inum, t i tanium, and gold are commonly used

for con tac t format ion dur ing the manufac ture of

these ICs and they canno t be easily e t ched with-

out mult iple aggressive e tch ing steps. The lat ter

are of ten difficult to control , t ime consuming,

and almost always attack the c o m p o u n d semi-

conduc to r substrate. Many metal lift-off process-

es have b e e n repor ted , each involving several

steps. The mos t commonly used for gallium

arsenide devices is image reversal, lift-off layer

(LOL), t r imethyl a m m o n i u m hydroxide (TMAH)

photores i s t develop inhibit , and polymethyl

methacry la te (PMMA) bi-layer, bu t some are los-

ing desirability. For those unfamiliar w i th the

process, a typical image reversal p rocedure

involves a pos t -exposure hot-plate bake or

ammonia-gas oven bake before resist develop-

ment , but some also have drawbacks such as a

l imited photores i s t shelf life, due to the use of a

chemical additive for enhanc ing the re-entrant

features.

A LOL process uses non-photosensi t ive polymers

that have a h igher dissolution rate compared to

the resist top layer. It is essentially a double-layer

process involving two coating s teps .The re-

en t ran t characterist ic is control led by the LOL

layer bake t ime and tempera ture .A typical TMAH

inhibi t ing solvent is ch lo robenzene and therefore

this process is losing favour due to skin and eye

irri tat ion and envi ronmenta l concerns . Poly

methyl methacryla te resist lift-off is also a double-

layer process, very robust, bu t t ime consuming.

Wafer t h roughpu t is severely l imited because of

the requ i rement of separate deep ultraviolet

f lood exposure and individual developing.

Thus, in lift-off pa t te rn ing that uses one of these

techniques the photores is t acts as a stencil to

define the location of the metal pat tern. After the

photores is t has b e e n exposed and developed, it is

essential that there is no residual resist remains in

the developed out areas, or the evaporated metal

may not adhere or make good electrical contac t

wi th the semiconductor . Therefore, the de tec t ion

of any photores is t residue before metal evapora-

t ion is important , bu t is almost impossible to

detec t by convent ional optical microscopy. This

is because the th in resist residue film is in mos t

cases colourless and transparent , and can barely

be seen by white-l ight microscopes.

However, w h e n using the 3DScope 2000 instru-

ment , the phase informat ion con ta ined in the

light ref lected from the pa t t e rn allows the identi-

fication of photores i s t residues as seen in Figures

2 and 3. In addition, profile measu remen t s (Figure

2, r ight) or 3D maps (Figure 3, r ight) can accu-

rately measure the th ickness of the residue wi th

n a n o m e t e r resolution.

For c o m p o u n d semiconductors , the value of this

equ ipmen t resides in the ability to rework the

photo l i thographic p roce s s .When detected, mis-

takes or unaccep tab le developing steps can still

III-Vs REVIEW THE ADVANCED SEMICONDUCTOR MAGAZINE VOL 16 - NO 4 - MAY 2003 43

3D Optical microscopy

4 4

Figure 3. Resist residue on silicon dioxide. The residue is transparent and barely visible with a white-l ight microscope

(left) but clearly seen using interference imaging (middle). The 3D view of the sample (right) shows the residue

thickness at nm resolution.

3 D V i e w i . .-i . . . . . i

be cor rec ted at this stage, because metal has no t

yet b e e n depos i ted on the wafer.Thus, any inade-

quately p rocessed wafers de tec ted by the inspec-

t ion can have their photores i s t s t r ipped off and

reworked, avoiding costly wafer scrapping or

tedious failure analysis d o w n the l ine.The non-

destruct ive nature of this analysis, its speed and

the 3D visualisations can directly affect the

device yield f rom a be t te r unders t and ing and

be t te r control of the process.

Resist Profiles In many semiconduc to r processes, the cont ro l of

the dielectr ic and metal edge profiles are critical

parameters .Therefore , to provide dissolut ion of

the resist after the metal layer has b e e n deposit-

ed, lift-off resist s t ruc tures mus t be re-entrant to

pe rmi t access of the resist solvent. These struc-

tures are difficult to image w i th a scanning elec-

t ron mic roscope w i t hou t des t ruct ive cross

sectioning. Moreover, convent iona l 2D imaging

canno t de te rmine w h e t h e r the desired resist lip

overhang exists. Figure 4 illustrates h o w this

ins t rument can also be used to non-destruct ively

measure the profile of a bi-layer overhanging

resist s t ruc ture .Al though the profile does not

echo the precise cross section, it can provide a

reasonable measure of the w i d t h of the overhang

and the he igh t above the substrate surface.

As previously noted, the profile of the resist layer

de te rmines the shape and cross sect ion of the

e t ched pat tern . This resist con tou r (for isotropic

e tching) can be imaged (Figure 5) the inspec-

t ion used to character ise the resist profile, saving

systems downt ime and wafer scrap. O the r appli-

cat ions in c o m p o u n d device fabr icat ion include

m e a s u r e m e n t of profiles of metal bond ing pads

and thickness of overlying oxide passivat ion

layers.The actual th ickness of the passivating

layer as it varies across the device topography

Figure 4. Lift-off process. Characterization of overhang- ing bi-layer photoresist profile

prior to gold deposition.

Photo resist #2 Photo resist #1 GaAs base ~._

Measured line profile

After gold deposition I J

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3D Optical microscopy

Photo resist ~--

GaAs base ~--

Measured profile

After via etch

i i E i

i u i r

15 #m

i i --b I

I

i i

40 ~Lm

can also be accurately measured and rapid feed-

back available f rom this i n s t rumen t can reduce

systems d o w n t i m e and avoid d o w n line process-

ing s teps on defective wafers.

In conclusion, a n e w analytical t echno logy that

combines in te r fe romet ry pr inc ip les w i th imaging

concep t s is n o w available and offers benef i ts to

III-V wafer processing. It allows simple and

s traightforward 3D measu remen t at n a n o m e t e r

level in the Z-axis, br inging h igh-end Z-axis

m e a s u r e m e n t to the factory and laboratory

f loors.Tedious and costly p rocedures can be

replaced w i th more friendly and cost effective

tools, simplifying opera t ions and cut t ing costs.

Figure 5. Non-destructive measurement of a photoresist slope prior to an isotropic via etch.

Contact: David Banitt, CEO, Nano-Or Techno- logies, Lod, Israel Emaih banitt@nanoor .corn

Dr. Alan Mills, contribut- ing editor reporting from Silicon Valley. PO Box 4098, Mountain View, CA 94040, USA. Tel/Fax: +1-65o-968-2383/8416

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