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 t[r]
Trang 1F 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
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
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 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 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 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
Trang 2s 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 electrical ch a rac teristics w ere m easured using MIS capacitors w ith evaporated Al electrode 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
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
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 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
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
Gale Voilage (V) Gale Voilage (V) Fig.2 Electrical characteristic of Si nitride films formed in Ar/N, plasma
V
V /
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)]
Trang 3F igures 3 show th e film thickness (d ),
EOT, refractive index an d th e ra tio of the
as a function of Vb, w here th e No m ixing ratio
w as fixed a t 120 m in The d and n w ere
C-V characteristics T he re su lts can be
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
weak Si-N bonds in th e films, respectively,
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
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 depressing leakage c u rre n t density of SiN film by suitable p lasm a etching effect The
th e nitrogen partia l pre ssu re has decisive effect on th e film quality A Si n itrid e film with 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,
Fig.3 Film thickness (d), EOT (a), refractive density at +1V (c) of the SiN film as a function of Vb