VNU. JOURNAL OF SCIENCE. Mathematics - Physics, T.xx. N03AP. 2004
F A B R IC A T IO N O F U LTR A T H IN S IN F IL M A T LOW T E M P E R A T U R E BY U SIN G E C R PLA SM A
L u u H o a i Nam® b, L u u T u a n T a i bc
a. Interdisciplinary G raduate School o f E ngineering Sciences, K yu sh u U niversity I . International T raining Institute for M aterials Science c. D epartm ent o f physics, N a tu re Science University, H anoi S ta te University
A bstract. SiN thin film has been fabricated using ECR plasma irradiation. Ar/Nj m ixe a p la s m a irra d ia tio n a t a lo w te m p erature o f 4 0 0 ° c w a s ca rrie d o u t fo r the g ro w th o f th e film . It is fo u n d th a t t h e nitro ge n pa rtial p re s s u re h a s d e c is iv e e ffect o n th e film qu a lity. A S i nitrid e film ha ving a stru ctu re n e a re s t to s to ic h io m etric c o n s tru c tio n is ob ta in e d b y p re cise control o f N j m ixin g ra tio at 60 % . U n d e r th e o p tim u m co n d itio n , th e as-g ro w n S iN film s h o w s a le a k a g e c u rre n t m o re than th re e o rd e rs o f m a g n itu d e lo w e r th an th a t o f th e rm a l g ro w n S i0 2 h a v in g th e s a m e e q u iv a le n t o xid e th ickn e ss. H igh -reso lutio n tra n s m is s io n ele ctron m ic ro sco p e m ic ro g ra p h s h o w s an atom ically fla t in te rfa ce o f Si a n d S iN film .
1. I n t r o d u c t io n
S i0 2 film s a re widely used in th e fabrication of in teg ra te d circuits. SiO-., is nativ e to Si and with it forms a low defect density interface. F u rth e r ad v a ntages of SiO , include the high receptivity, excellent dielect ic strength, a large band gap, an d a high m elting point However, with th e scaling of th e M OSFET for high perform ance (speed), low static power and low cost devices, S i0 2 g ate dielectrics thickness of several atom ic lay e rs is required. As the physical th ick n ers of S i0 2 gate oxide approaches less th a n 2 nm, th e leakage cu rren t density increases because of direct tunneling effect. Thus, S i0 2 ca n n o t be used for low power ULSI in th e future. To solve th is problem an d extend th e lim it of g ate dielectric, m any kinds of m etal oxides having a dielectric co n stan t la rg e r th a n S i0 2 a r e studied as a replacem ent of trad itional Si dioxide2. SiN is one of im p o rtan t g ate in su la to r, which shows a higher perm ittivity an d b e ttu r b a rrie r for oxygen and boron diffusion relative to S i0 2 having the sam e equivalent oxide thickness (EOT). Thus. Si n itrid e film is a reasonable replacem ent of SiO j film a s gate dielectric.
In this paper, SiN th in film has been fabricated a t a low te m p e ra tu re of 400°c. The grow th of th e Si n itrid e film w as carried out w ith Ar/N2 m ixing plasm a irradiation in an electron cyclotron resonance (ECR) plasm a system . The effect of the N2 p a rtia l pressure on the film quality w as studied in d eta il an d it w as found th a t dilution o f N2 plasm a w ith Ar gas having su ita ble m ixing ratio im proved the nitrogen radical concentration of th e plasm a.
The bias effects, electrical and stru ctu ra l properties of th e Si n itrid e films w ere also investigated.
2. E x p e r im e n t
Si n itrid e film grow th w as perform ed in an ECR plasm a system . An n-type (100) Si su b stra te w ith a resistivity of 8-12 Qcm was se t on a sam ple holder a t th e end of deposition cham ber. Before direct n itrid atio n , Si su b stra te w as tre a te d by RCA cleaning process. The
1 0 0
F a b r ic a tio n o f u l t r a t h i n S i N f i lm a t low.. 101
s u b stra te tem p e ra tu re w as well controlled as low as 400uc d u rin g th e experim ent by ad ju stin g th e holder hea tin g power. Microwave power w as kep t as co n stan t a t 500W. The electrical ch a rac teristics w ere m easured using MIS capacitors w ith evaporated Al electrode having an a re a of 2.25Ì10 1 cm ' by lithography. To investigate th e s tru c tu re of th e Si nitride film, refractive index an d th ickness of th e Si nitride film w ere m easured using an ex-situ spectroscopic ellipsom etry (SE). To observe the flatness of the Si/SiN interface, high- resolution transm ission electron microscope (HRTEM) m easurem ent w as carried out. In addition. X-ray photoelectron spectroscopy (XPS) re su lts elucidated the bonding character of th e as-grown film.
3. R e s u lts a n d D is c u s s io n
T he effect of N.> p a rtia l pre ssu re on film quality w as observed in XPS re su lts3. The chem ical shifted peak position and the full w idth of half-m axim um (FWHM) of N l s signal a s a function of N., m ixing ratio are shown in Fig. 1. In th is figure, dash lines also show the peak position and the FWHM of N l s signal from stoichiom etric Si-jN, film fabricated by therm al CVD as reference. Since it is well known th at th e position and FWHM of XPS peak are very sensitive to th e film bonding stru ctu re , it can be seen th a t a Si n itrid e film fabricated a t th e N, m ixing ratio of aro u n d 60% is n e a re s t to
stoichiom etric stru ctu re . W hen th e N2 mixing ratio was higher th a n 60%, th e film quality becam e worse. T his phenom enon w as maybe caused by N , m olecular incorporation in the film. According to the above discussion, the optimized N2 m ixing ratio for Si nitride growth can be concluded a s 60%.
The norm alized high frequency (100 kHz) C-V ch a racteristic for th e Si nitride film is shown in Fig. 2 (a). T he film w as fabricated a t N2 m ixing ra tio of 60% for 120 m in, where th e EOT w as 2.46 nm. As com parison, a
calculated ideal C-V curve is also shown by a solid line. The interface s ta te density a t the flat band w as estim ated a s 6.4Ì10" eV 'em 2 by using th e high frequency capacitance m ethod. T he C-V ch a rac teristics indicate a Si oxide in terlay e r is needed betw een Si s u b s tra te and Si n itrid e film to im prove the interface quality. T he J-V curve o f th e sam e Si n itrid e film was also show n in Fig. 2 (b).
The leakage c u rre n t den sity w as 4.2110’°
A/cm2 a t +1 V, which w ere m ore th a n two orders of m agnitude low er th a n th a t of therm ally grown Si dioxide film w ith a given
EOT. From th e figure, it also can be seen th a t the breakdow n happened a t 4.8 V, which was corresponding to a breakdow n field of 20 MV/cm.
Gale Voilage (V) Gale Voilage (V) Fig.2. Electrical characteristic of Si nitride
films formed in Ar/N, plasma
o ' ”
\
“V V /
Si.N, 8..N,
1 10 mixing ratio <%) N, mixing ratio (%) F ig.l. N ls XPS chemical shifted (a) peak position and (b) FWHM as a function of N2
mixing ratio (N2/(N2+Ar)]
102 L u u H o a i N a m , L u u T u a n T a i
F igures 3 show th e film thickness (d), EOT, refractive index an d th e ra tio of the leakage c u rre n t density a t + 1V of th e SiN film as a function of Vb, w here th e No m ixing ratio was fixed a t 60% an d th e film grow th tim e w as fixed a t 120 m in. The d and n w ere m easured by SE, a n d EOT w as obtained by C-V characteristics. T he re su lts can be classified into th ree types. It shows th a t the plasm a etching caused by the su b stra te bias application also affected th e SiN film quality.
The strong negative a n d positive biases caused the existence of m uch defects and weak Si-N bonds in th e films, respectively, consequently, th e leakage c u rre n t density increased largely. An optim ized su b stra te bias contributed to depressing leakage cu rren t density of SiN film by su itable plasm a etching effect.
HRTEM m easurem ent indicates th a t th e sam ple has an atom ically flat interface between Si and SiN film. F urtherm ore, this im age indicates th a t th e physical thickness of th e film is about 3.9 nm w ith accuracy.
C onsidering th e EOT value of th is film, the p erm ittivity of th e Si n itrid e film was calculated to be 6.2, which w as 1.6 larger tha th a t of SiO-,
4. C o n c lu s io n
An am orphous Si n itrid e th in film h as been fabricated usin g A r/N , mixed plasm a irradiation in an ECR system a t a low tem p eratu re of 400°c. A fter 120 m in irradiation u n d er th e optim um condition, a Si n itrid e film with EOT of 2.46 nm w as form ed. The as- grown SiN film shows a leakage c u rre n t more th a n two orders of m agnitude lower than th a t of th erm a l grown S i0 2 with a given EOT. An optim ized s u b s tra te bias contributed to depressing leakage c u rre n t density of SiN film by suitable p lasm a etching effect The HRTEM m icrograph show s an atom ically fla t interface of Si a n d SiN film. It 18 found th a t th e nitrogen partia l pre ssu re has decisive effect on th e film quality. A Si n itrid e film with m inim um EOT is obtained by precise control of N2 m ixing ratio a t 60%. It im plies th a t the existence of Ar w ith su itable p a rtia l pressure would increase nitrogen radical concentration in th e Ar/Na mixed plasm a. The XPS re su lts also confirm ed this point.
R e fe re n c e s
1. Applied physic reviews. 90, 5(2001) 2057.
2. G. D. Wilk, R. M. Wallace, and J . M. Anthony, J. Appl. Phys. 89 (2001) 5243.
3. L.Zhao. N.H.Luu, D.Wang, Y.Sugimoto, K.Ikeda, H .Nakashim a and H .Nakashim a, JJAP, 43 (2004) L47-L49.
Fig.3. Film thickness (d), EOT (a), refractive index (b) and the ratio of the leakage current density at +1V (c) of the SiN film as a
function of Vb
