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Circuit failure identification using focused ion beam and transmission electron microscopy characterisation techniques
ELSEVIER
Microelectronic Engineering 49 (1999) 181-189 www.elsevier.nl / locate/mee
Circuit failure identification using electron microscopy characterisation
focused ion techniques.
beam
and
transmission
R. Pantel, G. Mascarin and G. Auvert France Telecom CNET, Chemin d u vieux chene BP 98 F-38243 Meylan France * In this p a p e r we present the development and improvement of techniques used in CNET Meylan during the last few years for localisation, cross sectioning and observation o f circuit failure using focused ion beam, scanning and t r a n s m i s s i o n electron m i c r o s c o p y techniques. Failure analysis examples are given to illustrate the methods used. Defect localisation is carried o u t using potential contrast and electrical testing. The imaging a n d ion milling capability of the focused ion beam technique is used for cross sectioning the failure area with an accurate control of the Iocalisation. The physical observation o f defects on the cross section is carried out either in situ using scanning ion microscopy or ex situ using electron beams. For higher resolution the defective area is ion milled using the focused ion b e a m technique to produce a thin lamella containing the defect which can be then observed with s u b - n a n o m e t e r spatial resolution using t r a n s m i s s i o n e l e c t r o n microscopy. Chemical analysis can also be carried out in the t r a n s m i s s i o n e l e c t r o n microscope using electron energy loss spectroscopy and electron energy filtering to complete the failure identification. The high resolution compositional maps give a clear identification of the materials in the defect area and allows us to give some h y p o t h e s i s a b o u t the origin of the circuit failure.
I. INTRODUCTION.
With the growing complexity o f CMOS integrated circuits tiC) failure analysis is becoming more and m o r e challenging. A typical logic CMOS 0.25 ~m circuit contains several million o f transistors, six or more i n t e r c o n n e c t i o n levels. Finding a defect in s u c h a circuit is s o m e t i m e s very difficult. Strategies have been developed to include, in the wafer, modules w h i c h allows defect localisation using electrical testing. On RAM m e m o r y electrical test allows coordinates identification of failed m e m o r y cells. However the size o f the defect could be very small and a v e r y precise Iocalisation is needed. High resolution observation in situ in scanning electron microscope (SEM)
or focused ion beam (FIB) s y s t e m s combined with b e a m induced c o n t r a s t potential can also be very useful f o r precise localisation of failure area. After electrical localisation the defect should be further analysed. F o r that purpose the FIB technique h a s already proven to be a powerful cross sectioning and imaging method for defect analysis [1]. Combined with SEM observation, fast defect identification is now routinely possible. However SEM imaging gives medium spatial r e s o l u t i o n observation and poor contrast on fiat or smooth FIB cross sections. For higher resolution o b s e r v a t i o n and m i c r o a n a l y s i s the t r a n s m i s s i o n electron microscopy (TEM) t e c h n i q u e should be used on specimen p r e v i o u s l y thinned. Few years ago FIB s p e c i m e n preparation for physical observation in
* T h i s work w a s p e r f o r m e d within a cooperation b e t w e e n CNET a n d ST Microelectronics. t
0167-9317/99/$ - see front matter PII: S 0 1 6 7 - 9 3 1 7 ( 9 9 ) 0 0 4 3 8 - 4
© 1999 Elsevier Science B.V. All rights reserved.
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Figure 1: Classical FIB cross s e c t i o n carried out in a wafer or a circuit f o r observation a t 30 ° incidence angle.
Figure 2: Open FIB cross section c a r r i e d o u t n e a r a cleaved face for o b s e r v a t i o n a t 0 ° incidence angle.
the I E M has been used for subn a n o m e t e r r e s o l u t i o n defect i m a g i n g [2,3] a n d m o r e recently FIB-TEM e l e c t r o n energy loss spectroscopy (EELS) f o r c h e m i c a l a n a l y s i s [4]. In this paper we present a n d discus a m e t h o d developed a n d used d u r i n g t h e last few y e a r s in CNET M e y l a n for defect Iocalisation, cross sectioning, high resolution physical observation of circuit s t r u c t u r e s a n d defects using FIB, SEM a n d TEM t e c h n i q u e s . The m o r e recent powerful h i g h resolution chemical analysis techniques in the TEM: electron energy l o s s spectroscopy (EELS) a n d energy f i l t e r i n g TEM (EFTEM) a s s o c i a t e d w i t h FIB p r e p a r a t i o n for defect i d e n t i f i c a t i o n a r e also presented. The r e s u l t s of a p p l i c a t i o n on failure a n a l y s i s are discussed a n d interpreted.
3. FOCUSED SECTIONING
2. E X P E R I M E N T A L D E T A I L S .
The following e q u i p m e n t were used: a FIB Seiko 8300, a FIB Micrion 9500EX, a SEM H i t a c h i 4500, a TEM P h i l i p s CM200 FEG equipped w i t h a n i m a g i n g filter G a t a n GIF 200.
ION
BEAM
CROSS
The FIB t e c h n i q u e t a k e s a d v a n t a g e of the properties of a finely focused i o n gallium b e a m (30 to 50 keV energy). When s c a n n e d on the specimen, l o w c u r r e n t a n d small b e a m d i a m e t e r a l l o w s ion m i c r o s c o p y (SIM) i m a g i n g b y collecting the s e c o n d a r y electrons. A t higher current and larger beam diameter, the s p u t t e r i n g properties o f the b e a m is used for cross s e c t i o n i n g [1]. The i m a g i n g capability allows a precise control of t h e cross section w h i c h c a n be a c c u r a t e l y located in a s m a l l s t r u c t u r e or a defect. Two cross section types are possible: a classical cross section as described i n figure 1 a n d a cross section n e a r a cleaved plane (open cross section) as described in figure 2. Each cross section type h a s s o m e advantages. The classical cross section c a n be carried out at a n y place of a wafer or a circuit w i t h o u t special p r e p a r a t i o n . T h e cross section is observed easily in s i t u using SIM or ex situ at 30 ° i n c i d e n c e angle using SEM.
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Figure 3: SIM a n d SEM o b s e r v a t i o n of the FIB cross section of a n AI-W s t a c k e d v i a structure: a) 30 keV SIM observation, b) 20 keV SEM observation, c) 20 keV S E M o b s e r v a t i o n after c h e m i c a l delineation.
The cross section n e a r a cleaved face is advantageous for high quality observation in SEM a t 0 ° incidence angle. This cross section type is also c h o s e n i n case of Auger or EDX microanalysis. In the n e x t section SIM a n d SEM i m a g e s of c o n t a c t s in a n open cross section are presented. 4. SCANNING ION AND ~ O N OBSERVATION OF FIB CROSS SECTION The f a s t e s t w a y to observe a FIB cross section is in situ SIM i m a g i n g w h i c h c a n be easily done by s p e c i m e n tilting at 60 °. Figure 3a p r e s e n t s a SIM image of a n aluminium tungsten stacked via structure. The SIM image is c o n t r a s t e d due to specific materials electron e m i s s i o n c o n t r a s t a n d c h a r g i n g effects. The oxides a p p e a r s d a r k due to p o s i t i v e charges deposited by the gallium ions o f 30 keV in energy. The limits of SIM imaging are: a p o o r resolution (30 n m in the case of the i m a g e of figure 3a) a n d a destructive action f o r repeated o b s e r v a t i o n s . SEM is k n o w n as a n observation technique allowing a b e t t e r r e s o l u t i o n (1 to 2 nm). Therefore ex
situ SEM imaging is v a l u a b l e to c o m p l e t e t h e in situ information. Figure 3b shows a SEM image of t h e same FIB cross section t h a n in figure 3a. The c o n t r a s t is poor due to a fiat a n d s m o o t h surface t o p o g r a p h y of the FIB cross section a n d to the electron e m i s s i o n coefficient of Al, Si a n d SiO2 w h i c h a r e r o u g h l y s i m i l a r at 20 keV. However f i n e details can be evidenced such as a s m a l l hole (see arrow). The SEM c o n t r a s t can be i m p r o v e d using c h e m i c a l delineation. As it is s h o w n in Figure 3c, the i n t e r f a c e s are better evidenced a n d the hole of fig 3b is enlarged. Ion beam induced electrons e m i s s i o n is a complex process a n d t h e SIM images, w h i c h are c o n t r a s t e d b u t have a m e d i u m resolution, are n o t e a s y to interpret. Therefore c o m p a r i s o n w i t h SEM w h i c h give i m p r o v e d r e s o l u t i o n is valuable for SIM contrast i n t e r p r e t a t i o n . For example the h o l e observed in figures 3b a n d 3c was v i s i b l e on the SIM image, in figure 3a, as a bright point (see arrow). In the next section we p r e s e n t l o c a l i s a t i o n a n d o b s e r v a t i o n of defect using FIB.
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FIB c r o s s section a t arrow place
Figure 4: SIM potential c o n t r a s t a n d FIB cross section of a resistive c o n t a c t chain: a) SIM image, b) SIM observation of the FIB cross section carried out at the a r r o w place showing a n o n o p e n e d c o n t a c t in the middle.
5. FIB POTENTIAL CONTRAST DEFECT LOCALISATION AND CROSS SECTIONING
FOR FIB
Potential c o n t r a s t c o m b i n e d w i t h e t c h i n g in the FIB can be used f o r l o c a l i s a t i o n a n d cross sectioning o f electrical defects. Figure 4a shows a SIM image of a h i g h l y resistive c o n t a c t c h a i n observed in situ in the FIB. Some parts of t h e c h a i n are positively c h a r g e d a n d does n o t emit electrons. A FIB cross section w h i c h can easily be m a d e in a c h a r g i n g p a r t (arrow on the SIM image) is s h o w n i n figure 4a. The c h a r g i n g effect is explained by the presence of a n oxide layer at t h e b o t t o m of the middle c o n t a c t as it is observed on the SIM image of the FIB cross section in figure 4b. This fast a n d efficient FIB p o t e n t i a l contrast technique is accurate f o r I o c a l i s a t i o n of h i g h resistive or opened i n t e r c o n n e c t i o n s since low c u r r e n t is used (100 pA) a n d c h a r g i n g is sensitive to few volts. The capability of r e c o n f i g u r i n g interconnections, using ion i n d u c e d m e t a l d e p o s i t i o n w h i c h is very used i n FIB e q u i p m e n t s for circuit m o d i f i c a t i o n , c a n also be u s e d for more a d v a n c e d defect localisation.
6, FIB SPECIMEN PREPARATION TEM OBSERVATION
FOR
As s h o w n previously SIM a n d S E M observation t e c h n i q u e s have l i m i t a t i o n s in t e r m of resolution a n d c o n t r a s t . T h e s e limitations c a n be o v e r p a s s e d u s i n g TEM w h i c h is k n o w n to give high r e s o l u t i o n a n d c o n t r a s t e d images. As the TEM is limited to s m a l l specimens of 3 m m in length, we use a p r e - p r e p a r a t i o n t e c h n i q u e . The s m a l l s p e c i m e n is extracted from the circuit o r wafer (figure 5) using a d i a m o n d w h e e l dicing saw equipped w i t h a n o p t i c a l imaging system which allows the localisation of t h e failure a r e a [3]. TEM i m a g i n g n e c e s s i t a t e s very t h i n l a m e l l a s w h i c h can be prepared u s i n g FIB ion milling. For t h a t purpose t h e small s p e c i m e n T - s h a p e d b a r is stuck o n a t r u n c a t e d ring a n d m o u n t e d on the X-Y table of the FIB m a c h i n e as s h o w n i n figure 6. The area of i n t e r e s t w h i c h is c o n t a i n e d n e a r the top of the wall is t h i n n e d using our specific g a l l i u m i o n etching process a l r e a d y described [4]. This e t c h i n g process leads to very t h i n m e m b r a n e s (less t h a n 100 nm) w h i c h a r e compatible w i t h high r e s o l u t i o n T E M observation a n d m i c r o a n a l y s i s .
R. Pantel et al. I Microelectronic Engineering 49 (1999) 181-189
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CCD camera
Figure 5: TEM specimen e x t r a c t i o n u s i n g m e c h a n i c a l s a w i n g of a wafer.
Figure 6: T shape s p e c i m e n s t u c k on t h e TEM ring a n d FIB t h i n n i n g for T E M observation.
In the next section we s h o w a n application of very precise defect l o c a l i s a t i o n a n d FIB p r e p a r a t i o n f o r TEM o b s e r v a t i o n .
Therefore a FIB-TEM preparation was carried out to verify this h y p o t h e s i s a n d identify the defect. B o t h the sawing a n d FIB t h i n n i n g have b e e n very carefully positioned to c r o s s sectioned a group of s u s p e c t e d contacts. The precise point chosen for o b s e r v a t i o n is indicated in figure 7 a (circle on the upper left of the bit m a p test). Figure 7b s h o w s the SIM image o f the corresponding specimen after localised sawing a n d FIB thinning. T h e cell c o o r d i n a t e s are m a r k e d in o r d e r to indicate the c o r r e s p o n d e n c e with the b i t m a p test of fig ure7a. Figure 8 s h o w s the TEM o b s e r v a t i o n of t h e FIB t h i n n e d specimen. The five c o n t a c t s are o b s e r v e d i n figure 8a. A higher m a g n i f i c a t i o n s h o w s , in figure 8b, bright layers u n d e r two o f the three middle contacts. T h e s e s l a y e r s of 30 to 50 n m t h i c k have a low d e n s i t y and are located only on the n+ d o p i n g region. This suggests a corrosion p h e n o m e n a occurring d u r i n g the c o n t a c t processing on this n + / p + / n + j u n c t i o n . The h y p o t h e s i s of resistive c o n t a c t s to explain t h e failure of t h e m e m o r y cells is t h u s confirmed. This e x a m p l e s h o w s the p o t e n t i a l i t y of coupling electrical testing a n d very
7. ~RICAL LOCALISATION OBSERVATION
AND
TESTING FIB-TEM
Electrical test is one of the m o r e sensitive a n d reliable w a y of defect l o c a l i s a t i o n in p a r t i c u l a r in p e r i o d i c s t r u c t u r e s such as m e m o r y . An e x a m p l e is s h o w n in figure 7a w h e r e a partial v i e w o f a b i t m a p test of a RAM m e m o r y is presented. Two 16 X 16 bits m e m o r y b l o c k s a r e s h o w n a n d the c o o r d i n a t e s of the ceils are indicated. The failed cells are m a r k e d b y "F". All the failed cells are aligned o n vertical c o l u m n s a n d placed p e r i o d i c a l l y every four cells in the h o r i z o n t a l direction (1st, 5th, 9 t h .... cells). T h i s p a r t i c u l a r d i s t r i b u t i o n leads to suspect a group of five aligned c o n t a c t s on n + / p + / n + j u n c t i o n s w h i c h are located a t s i m i l a r place a n d w i t h the s a m e s p a t i a l s e q u e n c e in the m e m o r y design (every four cells).
R. Pantel et al. / Microelectronic Engineering 49 (1999) 181-189
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Figure 7: Electrical t e s t a n d localised FIB-TEM p r e p a r a t i o n ot a I
Figure 8: a) TEM observation of the FIB prepared thin wall s h o w n in figure 7 c o n t a i n i n g five contacts, b) Higher m a g n i f i c a t i o n showing two c o n t a c t s on n+ silicon p r e s e n t i n g a s u s p e c t e d layer of low d e n s i t y material at the t u n g s t e n silicon interface.
R. Pantel et al. / Microelectronic Engineering 49 (1999) 181-189
a
b
TEM p i c t u r e
187
EELS s p e c t r a
~ l e c t r o n l~,nergy tOSS i e v l Figure 9: TEM o b s e r v a t i o n a n d EELS analysis of a FIB t h i n n e d resistive c o n t a c t chain, a) TEM bright field picture, b) EELS spectra obtained on points a a n d b of the TEM picture.
precise 1ocalisation (100 nm) using FIB for high r e s o l u t i o n TEM o b s e r v a t i o n o f defects. 8. ELECTRON ENERGY SPECTROSCOPY ANALYSIS DEFECTIVE CONTACTS
LOSS OF
In all the examples shown previously, we have p r e s e n t e d p h y s i c a l o b s e r v a t i o n s of defective circuits. A complete identification of defect necessitates sometimes a chemical a n a l y s i s a n d next sections d e s c r i b e EELS a n d EFTEM c h e m i c a l a n a l y s i s o f FIB p r e p a r e d failed s t r u c t u r e s . Electron e n e r g y loss s p e c t r o s c o p y is an analytical technique currently e n c o u n t e r e d in t r a n s m i s s i o n e l e c t r o n microscopy. The EELS technique consists in an energy dispersive a n a l y s i s of the t r a n s m i t t e d e l e c t r o n s using a m a g n e t i c field deflection. T h e electron energy loss spectrum gives
information on the s p e c i m e n and specially the i n n e r shell ionisation edges allow for a t o m identification [5]. This technique is limited to s p e c i m e n s with t h i c k n e s s of a b o u t 100 n m w h i c h can be p r e p a r e d using FIB etching. S o m e results of EELS a n a l y s i s o n defective i n t e r c o n n e c t i o n s s t r u c t u r e s a r e n o w presented. Figure 9a s h o w s the TEM picture of a highly resistive c o n t a c t chain. The l o w e r metal is t u n g s t e n a n d the c o n t a c t is filled with CVD tungsten. Between t h e s e m a t e r i a l s a diffusion b a r r i e r is visible i n bright c o n t r a s t . Figure 9b s h o w s an EELS s p e c t r u m o b t a i n e d after focusing an electron p r o b e (of a few n a n o m e t e r in spot size) t h r o u g h the diffusion barrier. The s p e c t r u m a f t e r electron b a c k g r o u n d e x t r a c t i o n s h o w s two fine p e a k s identified as Ti-L a n d O-K i o n i s a t i o n edges. The oxidation of titanium during barrier deposition
188
a
R. Pantel et al. / Microelectronic Engineering 49 (1999) 181-189
Zero Loss image
b
Oxygen map
C
Fluorine mrtp
Figure I0: TEM a n d energy filtering TEM observations of a c o n t a c t extracted from a f a i l e d circuit a n d FIB thinned, a) TEM bright field picture, b) a n d c) EFTEM compositional m a p s of respectively oxygen a n d fluorine.
e x p l a i n s the high resistivity of t h e contacts. The EELS technique allows compound identification using fine s t r u c t u r e o b s e r v a t i o n of s p e c t r a n e a r t h e i o n i s a t i o n edges. For e x a m p l e in figure 9b the O-K fine structure s p e c t r a is v e r y s i m i l a r to the one of a reference s p e c t r a of TiO2. In the next s e c t i o n we p r e s e n t a n extension of EELS for fast compositional mapping. 9. ENERGY FILTERING OBSERVATION OF CONTACTS DEFECTIVE CIRCUIT
TEM IN A
The energy filtering TEM t e c h n i q u e w h i c h h a s been developed a few y e a r s ago, extends the p o t e n t i a l of EELS (i.e. p u n c t u a l analysis) to fast c h e m i c a l mapping. Using the energy filter equipment, chromatic electron t r a n s m i t t e d images c a n b e a c q u i r e d on a CCD c a m e r a [6]. C o m b i n i n g and processing images acquired near the energy edges of core level i o n i s a t i o n allows a t o m i c c o n c e n t r a t i o n m a p p i n g . Figure 10 s h o w s EFTEM o b s e r v a t i o n of a c o n t a c t t h i n n e d using FIB in a defective circuit.
Figure 10a s h o w s the TEM i m a g e w h e r e a deep isotropic etching o f a l u m i n i u m is observed. Figure 10b a n d 10c show respectively the c o m p o s i t i o n a l m a p s of oxygen and fluorine. T h e s e s observations clearly evidence a c o r r o s i o n p r o b l e m and an A I F x - A I O y c o m p o u n d f o r m a t i o n at the b o t t o m o f c o n t a c t s w h i c h can explain a c i r c u i t defectivity. Chemical a n a l y s i s is m o r e a n d m o r e useful for circuit failure analysis particularly on c o n t a c t s and thin diffusion barriers. The energy f i l t e r i n g TEM is an operative s o l u t i o n for f a s t compositional mapping. I0.
CONCLUSIONS
Techniques developed in CNET Meylan u s i n g FIB, SEM a n d FIB-TEM f o r localisation, cross sectioning a n d h i g h resolution physical observation of circuit defects are presented. C h e m i c a l analysis of m a t e r i a l s and failed structures using EELS a n d EFTEM a r e also presented. The c o m b i n a t i o n of t h i s t e c h n i q u e s allow identification, fine localisation and roots origin interpretation of complex circuit failures.
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R Pantel, G. Mascarin, J.P. G o n c h o n d and D. Lafond, Q u a l i t y and Reliability Engineering Intemational, No 10 (1994) 3 1 9
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R. Hull, D. B a h n c k , F.A. Stevie, L.A. Koszi a n d S.N.G. Chu, Appl.Phys. Lett. Vol 62, No 26 (1993) 3408
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R. Pantel, G. Auvert, G. M a s c a r i n a n d J.P. Gonchond, : Proceedings of 13th I n t e r n a t i o n a l Congress on E l e c t r o n Microscopy ICEM'13, Paris, F r a n c e J u l y (1994) 1007
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R. Pantel, G. Auvert a n d C~ M a s c a r i n , Microelectronlc Engineering, 37 {1997) 49
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Microelectronic Engineering 49 (1999) 181-189 www.elsevier.nl / locate/mee
Circuit failure identification using electron microscopy characterisation
focused ion techniques.
beam
and
transmission
R. Pantel, G. Mascarin and G. Auvert France Telecom CNET, Chemin d u vieux chene BP 98 F-38243 Meylan France * In this p a p e r we present the development and improvement of techniques used in CNET Meylan during the last few years for localisation, cross sectioning and observation o f circuit failure using focused ion beam, scanning and t r a n s m i s s i o n electron m i c r o s c o p y techniques. Failure analysis examples are given to illustrate the methods used. Defect localisation is carried o u t using potential contrast and electrical testing. The imaging a n d ion milling capability of the focused ion beam technique is used for cross sectioning the failure area with an accurate control of the Iocalisation. The physical observation o f defects on the cross section is carried out either in situ using scanning ion microscopy or ex situ using electron beams. For higher resolution the defective area is ion milled using the focused ion b e a m technique to produce a thin lamella containing the defect which can be then observed with s u b - n a n o m e t e r spatial resolution using t r a n s m i s s i o n e l e c t r o n microscopy. Chemical analysis can also be carried out in the t r a n s m i s s i o n e l e c t r o n microscope using electron energy loss spectroscopy and electron energy filtering to complete the failure identification. The high resolution compositional maps give a clear identification of the materials in the defect area and allows us to give some h y p o t h e s i s a b o u t the origin of the circuit failure.
I. INTRODUCTION.
With the growing complexity o f CMOS integrated circuits tiC) failure analysis is becoming more and m o r e challenging. A typical logic CMOS 0.25 ~m circuit contains several million o f transistors, six or more i n t e r c o n n e c t i o n levels. Finding a defect in s u c h a circuit is s o m e t i m e s very difficult. Strategies have been developed to include, in the wafer, modules w h i c h allows defect localisation using electrical testing. On RAM m e m o r y electrical test allows coordinates identification of failed m e m o r y cells. However the size o f the defect could be very small and a v e r y precise Iocalisation is needed. High resolution observation in situ in scanning electron microscope (SEM)
or focused ion beam (FIB) s y s t e m s combined with b e a m induced c o n t r a s t potential can also be very useful f o r precise localisation of failure area. After electrical localisation the defect should be further analysed. F o r that purpose the FIB technique h a s already proven to be a powerful cross sectioning and imaging method for defect analysis [1]. Combined with SEM observation, fast defect identification is now routinely possible. However SEM imaging gives medium spatial r e s o l u t i o n observation and poor contrast on fiat or smooth FIB cross sections. For higher resolution o b s e r v a t i o n and m i c r o a n a l y s i s the t r a n s m i s s i o n electron microscopy (TEM) t e c h n i q u e should be used on specimen p r e v i o u s l y thinned. Few years ago FIB s p e c i m e n preparation for physical observation in
* T h i s work w a s p e r f o r m e d within a cooperation b e t w e e n CNET a n d ST Microelectronics. t
0167-9317/99/$ - see front matter PII: S 0 1 6 7 - 9 3 1 7 ( 9 9 ) 0 0 4 3 8 - 4
© 1999 Elsevier Science B.V. All rights reserved.
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Figure 1: Classical FIB cross s e c t i o n carried out in a wafer or a circuit f o r observation a t 30 ° incidence angle.
Figure 2: Open FIB cross section c a r r i e d o u t n e a r a cleaved face for o b s e r v a t i o n a t 0 ° incidence angle.
the I E M has been used for subn a n o m e t e r r e s o l u t i o n defect i m a g i n g [2,3] a n d m o r e recently FIB-TEM e l e c t r o n energy loss spectroscopy (EELS) f o r c h e m i c a l a n a l y s i s [4]. In this paper we present a n d discus a m e t h o d developed a n d used d u r i n g t h e last few y e a r s in CNET M e y l a n for defect Iocalisation, cross sectioning, high resolution physical observation of circuit s t r u c t u r e s a n d defects using FIB, SEM a n d TEM t e c h n i q u e s . The m o r e recent powerful h i g h resolution chemical analysis techniques in the TEM: electron energy l o s s spectroscopy (EELS) a n d energy f i l t e r i n g TEM (EFTEM) a s s o c i a t e d w i t h FIB p r e p a r a t i o n for defect i d e n t i f i c a t i o n a r e also presented. The r e s u l t s of a p p l i c a t i o n on failure a n a l y s i s are discussed a n d interpreted.
3. FOCUSED SECTIONING
2. E X P E R I M E N T A L D E T A I L S .
The following e q u i p m e n t were used: a FIB Seiko 8300, a FIB Micrion 9500EX, a SEM H i t a c h i 4500, a TEM P h i l i p s CM200 FEG equipped w i t h a n i m a g i n g filter G a t a n GIF 200.
ION
BEAM
CROSS
The FIB t e c h n i q u e t a k e s a d v a n t a g e of the properties of a finely focused i o n gallium b e a m (30 to 50 keV energy). When s c a n n e d on the specimen, l o w c u r r e n t a n d small b e a m d i a m e t e r a l l o w s ion m i c r o s c o p y (SIM) i m a g i n g b y collecting the s e c o n d a r y electrons. A t higher current and larger beam diameter, the s p u t t e r i n g properties o f the b e a m is used for cross s e c t i o n i n g [1]. The i m a g i n g capability allows a precise control of t h e cross section w h i c h c a n be a c c u r a t e l y located in a s m a l l s t r u c t u r e or a defect. Two cross section types are possible: a classical cross section as described i n figure 1 a n d a cross section n e a r a cleaved plane (open cross section) as described in figure 2. Each cross section type h a s s o m e advantages. The classical cross section c a n be carried out at a n y place of a wafer or a circuit w i t h o u t special p r e p a r a t i o n . T h e cross section is observed easily in s i t u using SIM or ex situ at 30 ° i n c i d e n c e angle using SEM.
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Figure 3: SIM a n d SEM o b s e r v a t i o n of the FIB cross section of a n AI-W s t a c k e d v i a structure: a) 30 keV SIM observation, b) 20 keV SEM observation, c) 20 keV S E M o b s e r v a t i o n after c h e m i c a l delineation.
The cross section n e a r a cleaved face is advantageous for high quality observation in SEM a t 0 ° incidence angle. This cross section type is also c h o s e n i n case of Auger or EDX microanalysis. In the n e x t section SIM a n d SEM i m a g e s of c o n t a c t s in a n open cross section are presented. 4. SCANNING ION AND ~ O N OBSERVATION OF FIB CROSS SECTION The f a s t e s t w a y to observe a FIB cross section is in situ SIM i m a g i n g w h i c h c a n be easily done by s p e c i m e n tilting at 60 °. Figure 3a p r e s e n t s a SIM image of a n aluminium tungsten stacked via structure. The SIM image is c o n t r a s t e d due to specific materials electron e m i s s i o n c o n t r a s t a n d c h a r g i n g effects. The oxides a p p e a r s d a r k due to p o s i t i v e charges deposited by the gallium ions o f 30 keV in energy. The limits of SIM imaging are: a p o o r resolution (30 n m in the case of the i m a g e of figure 3a) a n d a destructive action f o r repeated o b s e r v a t i o n s . SEM is k n o w n as a n observation technique allowing a b e t t e r r e s o l u t i o n (1 to 2 nm). Therefore ex
situ SEM imaging is v a l u a b l e to c o m p l e t e t h e in situ information. Figure 3b shows a SEM image of t h e same FIB cross section t h a n in figure 3a. The c o n t r a s t is poor due to a fiat a n d s m o o t h surface t o p o g r a p h y of the FIB cross section a n d to the electron e m i s s i o n coefficient of Al, Si a n d SiO2 w h i c h a r e r o u g h l y s i m i l a r at 20 keV. However f i n e details can be evidenced such as a s m a l l hole (see arrow). The SEM c o n t r a s t can be i m p r o v e d using c h e m i c a l delineation. As it is s h o w n in Figure 3c, the i n t e r f a c e s are better evidenced a n d the hole of fig 3b is enlarged. Ion beam induced electrons e m i s s i o n is a complex process a n d t h e SIM images, w h i c h are c o n t r a s t e d b u t have a m e d i u m resolution, are n o t e a s y to interpret. Therefore c o m p a r i s o n w i t h SEM w h i c h give i m p r o v e d r e s o l u t i o n is valuable for SIM contrast i n t e r p r e t a t i o n . For example the h o l e observed in figures 3b a n d 3c was v i s i b l e on the SIM image, in figure 3a, as a bright point (see arrow). In the next section we p r e s e n t l o c a l i s a t i o n a n d o b s e r v a t i o n of defect using FIB.
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FIB c r o s s section a t arrow place
Figure 4: SIM potential c o n t r a s t a n d FIB cross section of a resistive c o n t a c t chain: a) SIM image, b) SIM observation of the FIB cross section carried out at the a r r o w place showing a n o n o p e n e d c o n t a c t in the middle.
5. FIB POTENTIAL CONTRAST DEFECT LOCALISATION AND CROSS SECTIONING
FOR FIB
Potential c o n t r a s t c o m b i n e d w i t h e t c h i n g in the FIB can be used f o r l o c a l i s a t i o n a n d cross sectioning o f electrical defects. Figure 4a shows a SIM image of a h i g h l y resistive c o n t a c t c h a i n observed in situ in the FIB. Some parts of t h e c h a i n are positively c h a r g e d a n d does n o t emit electrons. A FIB cross section w h i c h can easily be m a d e in a c h a r g i n g p a r t (arrow on the SIM image) is s h o w n i n figure 4a. The c h a r g i n g effect is explained by the presence of a n oxide layer at t h e b o t t o m of the middle c o n t a c t as it is observed on the SIM image of the FIB cross section in figure 4b. This fast a n d efficient FIB p o t e n t i a l contrast technique is accurate f o r I o c a l i s a t i o n of h i g h resistive or opened i n t e r c o n n e c t i o n s since low c u r r e n t is used (100 pA) a n d c h a r g i n g is sensitive to few volts. The capability of r e c o n f i g u r i n g interconnections, using ion i n d u c e d m e t a l d e p o s i t i o n w h i c h is very used i n FIB e q u i p m e n t s for circuit m o d i f i c a t i o n , c a n also be u s e d for more a d v a n c e d defect localisation.
6, FIB SPECIMEN PREPARATION TEM OBSERVATION
FOR
As s h o w n previously SIM a n d S E M observation t e c h n i q u e s have l i m i t a t i o n s in t e r m of resolution a n d c o n t r a s t . T h e s e limitations c a n be o v e r p a s s e d u s i n g TEM w h i c h is k n o w n to give high r e s o l u t i o n a n d c o n t r a s t e d images. As the TEM is limited to s m a l l specimens of 3 m m in length, we use a p r e - p r e p a r a t i o n t e c h n i q u e . The s m a l l s p e c i m e n is extracted from the circuit o r wafer (figure 5) using a d i a m o n d w h e e l dicing saw equipped w i t h a n o p t i c a l imaging system which allows the localisation of t h e failure a r e a [3]. TEM i m a g i n g n e c e s s i t a t e s very t h i n l a m e l l a s w h i c h can be prepared u s i n g FIB ion milling. For t h a t purpose t h e small s p e c i m e n T - s h a p e d b a r is stuck o n a t r u n c a t e d ring a n d m o u n t e d on the X-Y table of the FIB m a c h i n e as s h o w n i n figure 6. The area of i n t e r e s t w h i c h is c o n t a i n e d n e a r the top of the wall is t h i n n e d using our specific g a l l i u m i o n etching process a l r e a d y described [4]. This e t c h i n g process leads to very t h i n m e m b r a n e s (less t h a n 100 nm) w h i c h a r e compatible w i t h high r e s o l u t i o n T E M observation a n d m i c r o a n a l y s i s .
R. Pantel et al. I Microelectronic Engineering 49 (1999) 181-189
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CCD camera
Figure 5: TEM specimen e x t r a c t i o n u s i n g m e c h a n i c a l s a w i n g of a wafer.
Figure 6: T shape s p e c i m e n s t u c k on t h e TEM ring a n d FIB t h i n n i n g for T E M observation.
In the next section we s h o w a n application of very precise defect l o c a l i s a t i o n a n d FIB p r e p a r a t i o n f o r TEM o b s e r v a t i o n .
Therefore a FIB-TEM preparation was carried out to verify this h y p o t h e s i s a n d identify the defect. B o t h the sawing a n d FIB t h i n n i n g have b e e n very carefully positioned to c r o s s sectioned a group of s u s p e c t e d contacts. The precise point chosen for o b s e r v a t i o n is indicated in figure 7 a (circle on the upper left of the bit m a p test). Figure 7b s h o w s the SIM image o f the corresponding specimen after localised sawing a n d FIB thinning. T h e cell c o o r d i n a t e s are m a r k e d in o r d e r to indicate the c o r r e s p o n d e n c e with the b i t m a p test of fig ure7a. Figure 8 s h o w s the TEM o b s e r v a t i o n of t h e FIB t h i n n e d specimen. The five c o n t a c t s are o b s e r v e d i n figure 8a. A higher m a g n i f i c a t i o n s h o w s , in figure 8b, bright layers u n d e r two o f the three middle contacts. T h e s e s l a y e r s of 30 to 50 n m t h i c k have a low d e n s i t y and are located only on the n+ d o p i n g region. This suggests a corrosion p h e n o m e n a occurring d u r i n g the c o n t a c t processing on this n + / p + / n + j u n c t i o n . The h y p o t h e s i s of resistive c o n t a c t s to explain t h e failure of t h e m e m o r y cells is t h u s confirmed. This e x a m p l e s h o w s the p o t e n t i a l i t y of coupling electrical testing a n d very
7. ~RICAL LOCALISATION OBSERVATION
AND
TESTING FIB-TEM
Electrical test is one of the m o r e sensitive a n d reliable w a y of defect l o c a l i s a t i o n in p a r t i c u l a r in p e r i o d i c s t r u c t u r e s such as m e m o r y . An e x a m p l e is s h o w n in figure 7a w h e r e a partial v i e w o f a b i t m a p test of a RAM m e m o r y is presented. Two 16 X 16 bits m e m o r y b l o c k s a r e s h o w n a n d the c o o r d i n a t e s of the ceils are indicated. The failed cells are m a r k e d b y "F". All the failed cells are aligned o n vertical c o l u m n s a n d placed p e r i o d i c a l l y every four cells in the h o r i z o n t a l direction (1st, 5th, 9 t h .... cells). T h i s p a r t i c u l a r d i s t r i b u t i o n leads to suspect a group of five aligned c o n t a c t s on n + / p + / n + j u n c t i o n s w h i c h are located a t s i m i l a r place a n d w i t h the s a m e s p a t i a l s e q u e n c e in the m e m o r y design (every four cells).
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Figure 7: Electrical t e s t a n d localised FIB-TEM p r e p a r a t i o n ot a I
Figure 8: a) TEM observation of the FIB prepared thin wall s h o w n in figure 7 c o n t a i n i n g five contacts, b) Higher m a g n i f i c a t i o n showing two c o n t a c t s on n+ silicon p r e s e n t i n g a s u s p e c t e d layer of low d e n s i t y material at the t u n g s t e n silicon interface.
R. Pantel et al. / Microelectronic Engineering 49 (1999) 181-189
a
b
TEM p i c t u r e
187
EELS s p e c t r a
~ l e c t r o n l~,nergy tOSS i e v l Figure 9: TEM o b s e r v a t i o n a n d EELS analysis of a FIB t h i n n e d resistive c o n t a c t chain, a) TEM bright field picture, b) EELS spectra obtained on points a a n d b of the TEM picture.
precise 1ocalisation (100 nm) using FIB for high r e s o l u t i o n TEM o b s e r v a t i o n o f defects. 8. ELECTRON ENERGY SPECTROSCOPY ANALYSIS DEFECTIVE CONTACTS
LOSS OF
In all the examples shown previously, we have p r e s e n t e d p h y s i c a l o b s e r v a t i o n s of defective circuits. A complete identification of defect necessitates sometimes a chemical a n a l y s i s a n d next sections d e s c r i b e EELS a n d EFTEM c h e m i c a l a n a l y s i s o f FIB p r e p a r e d failed s t r u c t u r e s . Electron e n e r g y loss s p e c t r o s c o p y is an analytical technique currently e n c o u n t e r e d in t r a n s m i s s i o n e l e c t r o n microscopy. The EELS technique consists in an energy dispersive a n a l y s i s of the t r a n s m i t t e d e l e c t r o n s using a m a g n e t i c field deflection. T h e electron energy loss spectrum gives
information on the s p e c i m e n and specially the i n n e r shell ionisation edges allow for a t o m identification [5]. This technique is limited to s p e c i m e n s with t h i c k n e s s of a b o u t 100 n m w h i c h can be p r e p a r e d using FIB etching. S o m e results of EELS a n a l y s i s o n defective i n t e r c o n n e c t i o n s s t r u c t u r e s a r e n o w presented. Figure 9a s h o w s the TEM picture of a highly resistive c o n t a c t chain. The l o w e r metal is t u n g s t e n a n d the c o n t a c t is filled with CVD tungsten. Between t h e s e m a t e r i a l s a diffusion b a r r i e r is visible i n bright c o n t r a s t . Figure 9b s h o w s an EELS s p e c t r u m o b t a i n e d after focusing an electron p r o b e (of a few n a n o m e t e r in spot size) t h r o u g h the diffusion barrier. The s p e c t r u m a f t e r electron b a c k g r o u n d e x t r a c t i o n s h o w s two fine p e a k s identified as Ti-L a n d O-K i o n i s a t i o n edges. The oxidation of titanium during barrier deposition
188
a
R. Pantel et al. / Microelectronic Engineering 49 (1999) 181-189
Zero Loss image
b
Oxygen map
C
Fluorine mrtp
Figure I0: TEM a n d energy filtering TEM observations of a c o n t a c t extracted from a f a i l e d circuit a n d FIB thinned, a) TEM bright field picture, b) a n d c) EFTEM compositional m a p s of respectively oxygen a n d fluorine.
e x p l a i n s the high resistivity of t h e contacts. The EELS technique allows compound identification using fine s t r u c t u r e o b s e r v a t i o n of s p e c t r a n e a r t h e i o n i s a t i o n edges. For e x a m p l e in figure 9b the O-K fine structure s p e c t r a is v e r y s i m i l a r to the one of a reference s p e c t r a of TiO2. In the next s e c t i o n we p r e s e n t a n extension of EELS for fast compositional mapping. 9. ENERGY FILTERING OBSERVATION OF CONTACTS DEFECTIVE CIRCUIT
TEM IN A
The energy filtering TEM t e c h n i q u e w h i c h h a s been developed a few y e a r s ago, extends the p o t e n t i a l of EELS (i.e. p u n c t u a l analysis) to fast c h e m i c a l mapping. Using the energy filter equipment, chromatic electron t r a n s m i t t e d images c a n b e a c q u i r e d on a CCD c a m e r a [6]. C o m b i n i n g and processing images acquired near the energy edges of core level i o n i s a t i o n allows a t o m i c c o n c e n t r a t i o n m a p p i n g . Figure 10 s h o w s EFTEM o b s e r v a t i o n of a c o n t a c t t h i n n e d using FIB in a defective circuit.
Figure 10a s h o w s the TEM i m a g e w h e r e a deep isotropic etching o f a l u m i n i u m is observed. Figure 10b a n d 10c show respectively the c o m p o s i t i o n a l m a p s of oxygen and fluorine. T h e s e s observations clearly evidence a c o r r o s i o n p r o b l e m and an A I F x - A I O y c o m p o u n d f o r m a t i o n at the b o t t o m o f c o n t a c t s w h i c h can explain a c i r c u i t defectivity. Chemical a n a l y s i s is m o r e a n d m o r e useful for circuit failure analysis particularly on c o n t a c t s and thin diffusion barriers. The energy f i l t e r i n g TEM is an operative s o l u t i o n for f a s t compositional mapping. I0.
CONCLUSIONS
Techniques developed in CNET Meylan u s i n g FIB, SEM a n d FIB-TEM f o r localisation, cross sectioning a n d h i g h resolution physical observation of circuit defects are presented. C h e m i c a l analysis of m a t e r i a l s and failed structures using EELS a n d EFTEM a r e also presented. The c o m b i n a t i o n of t h i s t e c h n i q u e s allow identification, fine localisation and roots origin interpretation of complex circuit failures.
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