The investigation on the influence of temperature, agitation, size of eich-window, etch time on etch rate and the surface roughness were carried out.. Development o f [r]
(1)V'NU Journal 0Í'Science, Mathematics - Physics 25 (2009) 161-167
Research on optimal silicon etching condition in TMAH solution and application for MEMS structure fabrication
D in h V an D u n g *
Hanoi Pedagogical University No 2, Hanoi, Vietnam
Received 15 May 2009
Abstract, rhe research o f optimal condition for etching silicon in TMAH solution with controlled ctch rate and low surfacc roughness is the purpose of this study The investigation on the influence of temperature, agitation, size of eich-window, etch time on etch rate and the surface roughness were carried out With the TMAH concentration of 20% in weight, the optimal etching conditions were as follows: temperature of about 80 - 90 agitation of 150 - 200 rpm The etch rate is controlled in range o f 0.49 - 0.72 |im/min The etched surface roughness was lower than 70 nm Development o f TMAH application, a useful procedure for fabricating MEMS structures (piezoresistive accelerometer) was sugíỉcstcd
Ktywords: MEMS technology, TMAH, silicon ctching, accelerometer, MEMS fabrication.
I In trodu ction
A nisotropic etching o f silicon is a key technology for fabricating various 3-dim ensional structures for M icro Electro M echanical System s (M E M S) Althouiih deep reactive ion etching (D eep RTF) ha« b cco m c popular for realizing high aspect ratio silicon m icrostructurcs, the advantages o f silicon anisotropic etching te chnology such as low process cost, better surface sm oothness and lower environm ental pollution m a k es them as a com plem entally technology
T h e most popular chem ical etchants arc Potassium H ydroxide K O H and Tetra M ethyl A m m onium H ydroxide (C H 3)4N O H (TM A1Ỉ) K OH was proposed for application very early T he salient good points o f this etchant are its possibility o f m aking structures w ith high aspect ratio because o f the strong anisotropy in etch rate H ow ever, the draw back o f K O H is its incompatibility w ith the I C
technology [1] T o o v erco m e the disadvantages o f K O H , Tetra M ethyl A m m o n iu m H ydroxide ( I'M A ll) was later introduced as a fruitful solution for M E M S fabrication [2]
TMAỈ1 IS a w eak organic h ydroxide without any metal ions, so it is quite com patible w ith the IC te ch n o lo g y T he chem ical p roduct IS nontoxic and easy in handle [3,4] C o m p a red to etching in K OH solution, SÌO2 and SÌ3 N are better m ask materials for etching processes in T M A H Especially, the studies o f Tabala [4-6] sh o w that, the A1 etching in T M A H solution can be decreased to extrem ely low etch rate when som e appropriate am o u n t o f silicon is diluted into the T M A H solution T he presence o f integrated circuits as w ell as A1 interconnections is obvious in M E M S fabrications If the adequate etching condition o f w h ich the solution is assum ed not to etch A1 is found, then the p roblem o f A1
* E-mail: dinhvan_dung@yahoo.com
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connections protection and integrated circuits protection in silicon etching p rocess can seem t o he s olved I 'h u s , the use o f T M A H as the e tc h a n t s e em s to be opliinal in the c a s e w h e n only a o n e -S ld c aligner is available and the thermal SÌO2 is used as the mask material I 'h e point will be e s p ecially useful for M E M S devices fabricated by ctchinu through w hole silicon w afer such as beam structures in accclcromcters, g y r o s c o p e s Some papers reported that, the 'Ĩ M A H solution w ith the concentrations higher than 20 wt % allows fabricating sm ooth surfaces [6 J | T h e silicon clch rate and the sm oothness o f etched surface can be im proved considerably by conlrolhnii the c o n d itio n s Í' etchinu proccss
In this study, the iniluences o f temperature, agitation, etch time, size o f etch-vvindows on the clch rate and etched surfacc roughness were carried out Based on the obtained results, the o p tim a l conditions for etching silicon in T M A H w ere suggested Finally, a useful p rocedure for fabric atm g M E M S structure using b eam s as m echanical elem ent (for exam ple; M E M S p ie zoresistive accelerom eter) with ihe T M A H etchant was presented
2 Experim ents
In order to maintain the concentration and tem perature o f T M A H solution as well as the agitation at controlled speed durm g etching process, a special etching system IK A -W E R K E M O D E L 5000 w as used (Fig 1) The system consists o f a therm ostat w hich allow s heating and maintaining tem perature o f the solution in the rancc o f room tem perature to V and a magnetic stirrer with controlled speed o f - 650 round per m inule (rpm) T he T M A IỈ solution is contained in a vessel o f 5-litcr volume covcrcd by a double-w all hd A cool w ater flow running through the lid can make chcm ica'1 product vapor condensed during the etching proccss in order to maintain the constant conccntralion o f solution
Silicon w afers o f 2-inch, (100) orientation, n-l>pe, and polished double side, phosphorous doped with sensitivities o f - 20 i l c m , 260 ± 25 |.im thickncss w ere used for experiments The protection m aterial w as only SÌO2 layer formed on two sides o f silicon w afer by w et thermal oxidation In the oxidation condition o f 1100° c , wet oxygen 1.5 1/min flow, oxidation
time hours, the thickncss o f the oxide layer w as about 1.1 ụ m T he oxide layer is sufficient to protcct the samples during etching tim e including the case o f etching through w hole w afer thickness Etch- w idow s were form ed by photolithography technique In the invcsligalions on influence o f tem perature, stirring and etch time, the size o f ctch-windovv were choscn at X 1 m m F or the studies on inilucnce o f etch-w indow size, the etch-w indow was chosen with the edge size o f 1, 2, 4, , an d mm
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The average ctch rate w as calculalcd by dividing the ctched groove height by etch time Here, the etch height w as m e a su re d by the instrum ent o f Interferom elric Optical P rofilom etcr L E IC A D M R M with very high accuracy T he profile o f etched surface can also be m easured by using this instrument The process p ro g ram o f M D connected lo com puter allow s calculating average arithm etic roughness Ra and peak ro u g h n e s s Rt (distance from the lowest point to the highest point on the etched surface)
I'hcy are im portant quantities determ ining the ctched surface quality in M E M S fabrications
3 Results and d iscu ssion
The Figs and sh o w the results on the influence o f tem perature on etch rate and etched surface roughness The ex perim ental conditions were chosen as follows: T M A H 20% in w eight, agitation o f
150 rpm clch l ime o f 30 mmul cs. E tc h in g In T M A H 20%
100
y = e * • e *(-6 ,3 x ) R= 9
2
1/T (10^ K ')
3 3.2
Etching in TMAH 20V.
Ra Rt
Tanperaturs (*C)
t'lg z i he i n l l u e n c e 0Í te m p e r a t u r e o n e t c h rate. I'ig The influence u f Iciupcialuic un buifacc
roughness
The etch-rate increases strongly with increasing tem perature T he fit curve has a shape o f an cxnoncntial cur\'c T he small deviation o f experim ental points com pared to the fit curve show s that the (.J ta n e d result co rresp o n d s to the theoretical rule T he activation energy is calculated Ea = 0.548 eV which IS in good a g ree m en t w ith the result o f Tabata [5]
At around 60°c, the etched-surface (100) is the roughest At about 50°c, the average surface roughness is m uch low er than at 60°c At these low tem peratures, the intersection betw een wall side (111 and bottom side (100) is unclear T here are niany pyram ids on the b ottom surface (100) At ^0°C' the etch-ratc IS significantly higher and the intersection betw een (111 and (100) planes is clear, the e.ched surface ( 100) is sm oother H ow ever, there are still some peak s on the surface and the average roughness is still high
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Etching in TMAH 20% Etching in TMAH 20%
A g itation (rpm )
100 150 200
Aqifation (rpm)
250
Pig The influence of agitation on etch rate Fig The influence of agitation on surface roughness
In the Figs o f and 5, the results on the influence o f agitation are shown The experim ents were done in the T M A H solution o f 20% in weight at tem perature o f 80°c T he stirring has a good action on the etched product a n sp o rtatio n from etched-surface into the solution and fresh solution to the etched surface So it m a kes etch process faster But the agitation at very low speed or very high speed can m a k e opposite effects T h e stirring speed o f 150 rpm is optim al for all Ra and Rt roughness T he etch-rat? has largest value w h en the solution w as s tiư e d at about 150 rpm
The i n f ’iences o f the size o f etch-w indow on etch rate and surface roughness are presented in Figs and The etch conditions w ere the following: the agitation o f 150 rpm , 30 m inutes, temperature
80°c, T M A H 20% in weight T here is a light difference in etch-rate at different sizes o f etch-window T he difference w as caused b y m any reasons such as the position o f th e w afer and the holder in the solution, the declination o f surface ( 100) com pared to horizon p la n e T h e surface roughness has also diffcrcncc b u l not riiucVi w hen ihc i>ixc o f clch-w hiduw chaugcb W ith liic clch-wiuduw \aigci UlUii A
1 m m ^ no rule o f the influence w as found in the cases
Etching in TMAH 20V. Etching in TMAH 20%
30
25
Ị 0
s 15
s 10
ú
5
2
The Size etch-vwndow (mm2) The Size of etch-window {rm ứ )
Fig The influence of size o f window on etch-rate
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Finally, the results on the influence o f etch time on etch rate and routihness are reported The cx p e n m c n la l c o n d itio n s w ere chosen as tem perature 80°c, 150 rpm, T M A H 20% in weight With increasing etch time, the etched groove depth increases The transportation process o f etched products iVom the etch -su rfac c into solution and fresh solution to silicon surface b eco m es more difficuli This m akes the average etch-rate to decrease lightly with time W hereas, the am ount and the h d g h t o f pyram ids will increase w hen increasing etch-timc That is the explanations for the results in F ig and lig
Etching in TMAH 20V. Etching in TMAH 20%
Time (min)
Tim e (m m )
Fig rhe dependency of average etch-rate on time Fig The influence of etch lime on surface roughness
A good clchcd surface w as obtained in the etching condition o f T M A H 20 wt.%, temperature 80^C, and 150-rpm agitation as show n by SEM picture m Fig 10 T h e etched surface (100) was ver>' smooth with Ra = 12 nm , Rt = 63 nm
4 T.MAH ap p lication for MF3MS structure fabrication
w IS etched m T M A H solution, but the ctch rate decreases very sharply w hen the suitable amount ui silicon w as d issolved into the solution (Fig 11) [6 ] The etch rate can be low er than 0.001 |am/min if the amount o f 3.5 silicon mol IS dissolved into the T M A H 22 wt % solution
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Fig 10 SEM picture of etched surface in TMAH 20
wt %, 80 “c , ISOrpm.
JO
•Uj
n (
f > i s s o l \ c t l S i l i c o n ( m o l I t
[-’’ig 11 The dependence o f A1 etch rate on diluted silicon amount in the TMAH 22wi.% solution [4|
Silicon wafer; standard cleaning Diffusion
Oxidation
ị Pattern aligner marks
ịhtching tor aligner marks
Pattern ctch win dow from back side
Etching from back side
Pattern resistor
Pattern contact
Al Evaporation
P a U c r n c o n t a c t
Pattern cich w in d o w from iront side
Etch from front side
I r = *
L
Encapsulation
(7)D V Dung / VNU Journal o f Science Mathematics - Physics 25 (2009) Ỉ 61-167 167
Bascíỉ on the low A1 ctchm g in T M A H , a useful suggestion for fabricating M E M S using beam structure as m echanical sensitive elem ent such as in piezoresistive acc elerom eter w as given in Fig 12 In the procedure, we use only the photolithography instrum ent o f one side aligner The aligner marks for locating position p recisely from tw o sides o f the w afer are formed by etching through whole the w afer thickness T h e protection material during etching process is only SÌO m ak in g by simply ihcm ial oxidation T h e first steps are carricd out to make aligner m arks; next the steps o f making m em b ran e and m esa m ass are done T he electrical part is fabricated before releasing the beam structure in order to w o rk easily lithography process Then, the step o f releasing b eam structure by etching through w hole the w afer is done without m aking A1 protection layer in solution
5 C o n c lu sio n s
The obtained results show that, It is quite possible to control etch rate and low er etched surface roughness by controlling tem perature and agitation suitably In the T M A ỈI solution o f 20 % in weight, the optim ized silicon etching condilion in w hich the silicon etch rate is high and the etched surface r o u g h n e s s IS l o w are as follows: temperature o f 80 - ^ c , agitation o f about 150 - 200 tpm W hen the solution tem perature increases from 80 to 90 ^ c, the etch rate increases considerably from 0.49 to 0.72 Ịim /m in At about 150-rpin stirring, the average arithmetic roughness is low er than 70 nm For the M E M S structures with thickness larger than |im , the surface roughness low er than 70 n m IS q u ite a c c e p ta b le
The A1 ctching at low ctch-rate in T M A H makes the protection o f electrical part in etching process to becom e m uch easier For the technology condition m w hich a one-side aligner is available and the thermal SÌO2 is used as the m ask m aterial, aligner m arks for locating precisely position from two sides ot' w afer were form by etching through w hole the w afer thickness, the use o f T M A H as the etchant seems to be technology solution allow s being successful in fabrication o f com plex M E M S structures, especially beam structure such as in accelerom eter, g y ro sco p e T h e suggested solution for a c c c l c i u i n c t c r i a b i i c a l i o n IS q u i l c p o b b i b l c i u c a n y u u l III I h c t c ' c h n o l o g y c o n d i t i o n i n 11 I M S laboratory
A c k n o w le d g e m en t This research w as carricd out in Institute o f E lectronics F undam ental (lEF), University o f South Paris, France and International Training Institute for M aterials Science (ITIMS), Hanoi U niversity o f T ec hnology, V ietnam
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