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Focused ion beam lithography
Nuclear Instruments and Methods in Physics Research B80/81 (1993) 1271-1280 North-Holland
Beam Interactions with Mat.Nils &At...
'Focused , ion beam lithography John Melngailis Massachusetts Institnte of Technoloty, Cambridge, MA, USA
Lithography for microelectronics, that is, the exposure and development of resist, is already being carried out in research laboratories at dimensions well below 0.1 h.m . In production the minimum dimensions are likely to approach 0.1 l :m before the end of the decade. This review will examine the use of focused ion beams for ultrafine lithography . Minimum dimensions down to 0 .015 l..m have been reported as well as exposure of 0.25 ltm thick resist with 0.05 pro linewidth for the making of X-ray lithography masks. At this time there are only two techniques for writing original patterns (as opposed to replicating them) at 0.1 Wm and below : electron beams and ion beams. Electron beams are at a mmure state of development and have aavantages in absence oi shot noise and in fast deflection capability. Ion beams on the other hand have demonstrated absence of proximity effect and high resist sensitivity, i .e . potentially faster writing speed . The development of the gas field ion source promises hundredfold inrrense in current density of light ions (H ', He . . . ) in the beam focal spot. In addition, these light ion beams at high energy can be deflected at the speeds needed for lithography . Thus focused ion beam lithography is a serious candidate for future fine pattern writing . 1. Inttroduction Lithography is universally used in the fabrication of microelectronic devices, and in that sense it is the foundation of our information age . In the microelectronics context lithography consists of the exposure of a radiation sensitive polymer film on the surface of a semiconductor wafer and the subsequent development to produce a fine pattern in the polymer (e.g . the exposed (or unexposed) part is dissolved away). The fabrication steps which follow, such as material removal, implantation, or material addition, then alter the part of the substrate that is not covered by resist . The lithography is repeated with different patterns to build, for example, an integrated circuit. Up to now almost all lithography on integrated circuits is carried out by projecting a demagnified image of a mask onto the substrate using ultraviolet radiation . The mask consists of an opaque film such as chrome on a glass or a quartz substrate . The original pattern on the most advanced of these masks is written by exposure of resist by electron beam lithography . The smallest dii,ensions exposed on the wafer have been shrinking steadily, and now 0.8 p m is typical in advanced production . However, as the minimum dimensions shrink further and approach 0.i Win, alternatives to ultraviolet radiation as the exposure vehicle are being considered . A tool is nrAed to write the original pattern on the mask and another t-)i to replicate the pattern on the semiconductor wafer. The candidates most actively pursued today arc proximity X-ray lithography for the replication and electron beam lithography for the pat0î6h-583X/93/$06.00C~ 1993 - Elsevier
tern writing . Ions have also been considered as an alternative both for pattern writing and for replication over the past decade and a half . But the level of research activity has been much lower . Recent developments, however, which may impact both ion pattern writing and ion pattern, replication, motivate us to review ion lithography. There are three types of ion lithography, i .e. three methods for producing alternate irradiated and unirradiated areas on a surface wiih minimum features of 0 .1 p.m and below . They arc: 1) Focused ion beams. A small intense spot with a current density of 6 .1 to 80 /`./cm 2 is deflected over the surface. This is very similar to electron beam lithography and is the main topic of this review. 2) Ion beam proximity printing (also called masked ion beam lithography). Here a stencil mask with open areas is held in close proximity to a resist covered sample and exposed to light ions such as H+, HZ or He' . The ions can come from a standard ion implanter, ; nd this technique of shadow printing can be very fast. For a review, see ref. [1]. The difficulties of making a mask at the final dimension which is either ion transparent or open have limited the applicability of this technique . Lines and sp, ces down to 80 nm have been demonstrated [2j as well as special applica'i-tns whicc the limitations dfe less important [3]. :1) Ion projection lithography . In this technique a stwicil mask is uniformly illuminated by a beam of light ions, and Men by means of electrostatic optical elements the image of the stencil mask is projected onto a sample surface [4], The demagnification permits the mask spccitications to be relaxed, and the removal of
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Va. NOVEL TECHNIQUES (a)
!. Melngailis / F,x,tured `oelxant lnhography
1272
the mask from the proximity of the sample may simplify step and repeat exposures. Recent progress in controlling the field distortion [5], the image stability [61, and in achieving fine lines [7] make ion projection a promising lithography technique . Regardless of wheiher ions or X-rays are used for future ultrafine a technique is needed for writing the original pattern . We will review developments which indicate that ions may prove to be the preferred technique . But first we will look at the ion-resist interaction in general.
260keV Be FIB
20keV EB
INovolak Based Negative Resist PMMA '100 z Y U >_
PMMA xx-xx y
a'
w L' 50
x
a î zz
2. lua-it,Ast Eateraction
0
The sensitivity of resist to radiation is determined by the number of molecules of resist transformed by the incident radiation . In case of positive resists, like PMMA, the radiation breaks up long molecules, and one speaks of a "chain scission efficiency" . The competing process of cross-linking also takes place . In positive resists the scission efficiency is larger than the cross linking, and in negative (single component) resists the opposite is true [8]. Particularly in the case of ions, many resists can act both positive and negative depending on the dose [9,10]. Light ions, such as H, He, or Be in the 100-200 kcV range, lose all of their energy in traversing about a gm of resist, while electrons penetrate much deeper . Thus the sensitivity of r,si,t in terms of ions/cm2 needed is typically two orders of magnitude higher thin the sensitivity to electrons/cmz . An ion is often visualized a, exposing a cylinder of resist of about 10 nm diameter surrounding the point of entry. This picture has not been directly verified, except by the observation that evesi if the ion beam is focused to diameters of 8 rim, the minimum linewidth obtained is still 12 am [i i].
oil
1012
lo is
Ici .
1 0"
DOSE ( tons, electrons /cme)
Fig . 1 . From ref . [121. Ey!iosure characteristics of PMMA and negative novolak resist . The vertical axis shows the %r thickness remaining after development. Note that PMMA is about two orders of magnitude r,orc .,easitir:c to ions #tat, electrons . By sensitivity we generally mean the minimuin dose needed to expose the resist. Some typical va!ucs of sensitivity to ions arc given in table 1 . The resist sensitivity is in most cases obtained from plots of resist thickness remaining vs dose as shown in fig. 1 . The resist sensitivity depends, in part, on baking, on ,uickness, and on development conditions . The dependence on development time, for example, is seen in fig . 2. 2.2. Shot noise
One of the consequences of the high sensitivity of resist to ions is that statisticL! Guctuatioii iii the number of ions in a given minimun, dimension featoie clement (shot noise) must be considered. This feature element, often called a pixel, can be taken as a square at the minimum dimensions. Ttkus if N ions are on the
Table 1 Examples of organic resist exposure with ion beams Ion H'
Energy (kcV) 120
H' He'
1(M)
Be 2 '
1.
Bc 2 ' Ga' Ga'
280 IOJ 30
lit 14)
Resist PMMA (pos.) PMMA PMMA PMMA Novolak (Shipley SAL 601) (neg .) PMMA PMMA SAL 601 silylated
Min . dose (ions/cm z ) 3x10' 2
Min. line width ( )
Remuiks
Ref.
Thru stencil mask
[40)
2x 10" 3x 10" 1 .8x 10'2
0 .08
Thru stencil mask
[421 [411
2x10"
0.1 (0.6 I
m thick)
Focused ion beam
[121
2x 10" 6x10 11
(t 05
thick) 0.06 (0 .12 Wm thick resist) 0.08
Focused ion beans Focused ion beam Focused ion beam
(391 [431 [351
8x 1Ol'
(0.3
tLm
J. Mehigailis / Focused ion heron lithography ê m c E w N
cY u t tIon Dose Thickness of PMMA remaining vs dose after various development times as indicated in the inset . The highest contrast (steepest drop-off) is for the 30-60s develop ment. The initial thickness of the PMMA is less than the range of Be`' is ns .
Fig. 2. From rcf. [101.
average incident on a pixel, the range over which N can vary statistically is of order FN . For a sensitivity of 10'= ions/cm = each 0.1 Wm x 0.1 I .tm pixel would have 100 ;-- 10 ions incident . (There is, of'_ot :rsv, a nonncgligiblc pre :)ability of having a number of ions outside of this range of t 10.) Experimentally, and somewhat contrary to this statistical argument, well-formed, continuous lines ha-c been demonstrated with the number of ion, per l ., .:I in the range of 25 to 35 [13,141 . Below this number. lines at minimum dimensions become discontinuous . 2.3. Range Ideally the range of the ions should match exactly cite resist thickness so that the desired pattern is devclopcd all the way down to the substrate . The range it ;eases with increasing energy and decreasing ion mass. Some typical values are shown in table 2. If the lithography is to be carried out directly over Table 2 Ion range in PMMA resist (or depth to which exposed resist is developed) ion"H' He' Be Si
Ga
Energy (keV)
Range (wm)
Reference
40 1io 240 40 120 240
0.52 1 .12 1 .85 0.44 0.96 1 .54 0.45 0.72 1 .2 0.65 0.074
[401
IIAi
2tN1 20(1
100
1401 1441 [9,101 (9,101 [131
1273
active devices, for example by exposing transistor gates, then the ion penetration into the substrate may lead to damage or unwanted doping. In some cases matching the resist thickness to the ion energy may be a solution [151 . Thus after the eons pass through the resist and reach the substrate, they have vary little energy left . In general some penetration into the substrate will occur . For light ions, protons or He, the effects may be minimal, while for heavier ions such as Be or Si the damage and the substrate implantation are harder to avoid . NMOS transistors have been fabricated using protons for all levels of lithography. The devices were annealed at 450°C . No statis: :ealiy siguifieant effects of the irradiation were found [; 61 . GaAs MESFETs have also been fabricated where I rotons were used only for the gate lithography which i:. expected to be the most critical . The unwanted effet.ts of proton irradiation could be avoided by any one of four techniques : a sacrificial SiO, layer, the correct resist thickness, a 450°C, 5 s anneal, or removal of 50 nm of the surface by wet etching [171 3. Focused ion beams Focused ion beam systems capable of lithography have been devcioped mainly in the US and in Japan. For reviews of focused ion beams and üppiications, see refs . [18-231. The same systems can also be used for direct maskless implantation of dopants and many unique devices have beer. demonstrated. This has been reviewed elsewhere [231 . The essential system elements arc the focused ion beam column, the precision x-y stage, and the beam and stage control circuitry. 3.1 . lon co/untn An example of an ion column is shown schematically in fig . 3a. A point source of ions is followed by electrostatic lenses which tocus the ions back into a point on the sample . Precise beam deflection is accomplished by applying voltages to electrodes adjacent to the beam path, and beam blanking is done by deflecting the beam out of an aperture and onto the surface of a plate. A crossed electric and magnetic field velocity filter acts to separate masses so that only the desired mass species is propagated down the column. 3.2. Sources - The liquid metal ion source is used in almost all of the focused ion beam systems today . It consists of a needle surrounded by a liquid metal reservoir. See fig, 3b . (Heat may be needed,) The liquid slides dowr the needle and is pulled into a cusp by the electric field Va . NOVEL TECHNIQUES (a)
J. Melngaifis / Focused ion beam lithography
1274
FOCUSED ION SYSTEM
BEAM
(8)
SOURCE
produced by the bias voltage on an extraction electrode. The ions are emitted from the point of this cusp. (For a de=tailed discussion of liquid metal ion source, see ref. [20] .) The most commonly used source metal is Ga . However, Au/St, and Au,/Si/Be alloys have also
been used for lithography because of the lighter mass ions available in these sources . For Ga ions no mass separator is needed and the column can be considerably simpler titan shown in fig. 3a. The remarkable fact is that these ion sources particularly Ga, and Au/Si ope=rate stably for many hours, certainly long enough to do resist exposures. Provided the lenses arc properly designed and precisely rn~whined, and electrical noise
and vibration arc controlled, beam diameters down to 0.05 am can be readily achieved . The two main fundamental performance limitations arc the virtual source size and the energy spread of the ions emitted from the sourrre . The virtual source size (typically 50-100 nm) requires that demagnification I+c used to achieve smaller focal spots on the sample . Focal spot, down to 8 nm have been achieved using
ACCEL LENS
DEFLECTION PLATES ELECTRON DETECTOR
SAMPLE,X " Y STAGE
-- 10 x demagnification [11]. The energy spread of the ions (AE - 5 to 10 eV) leads to chromatic aberration, that is ions of different energies arc focused at diffcrc^t depths . This requires that the cone angle of the ions be reduced if small diameters are to be achieved. Thus the energy spread has the effect of limiting the current density in the focal spot . - The gas field ion source is m-.ire complex but promises higher performance . It has been studied [2429] since the mid 70s, but has not proved practical because the emission was not stable and the structure was cumbcrsumc . Its potential advantages for lithography were, however, recognized [30] . Therefore, the
recent advances in the technology which have resulted in stable operation of 170 it [31] and in the operation of the source in an ion column [32] are exciting developments. A schematic of the gas field ion source is shown in fig . 4. It consists of a cryogenically cooled single, crystal tip, usually tungsten, with gas He, H, or Nc for example, surrounding the tip at a pressure of about ?0 -° Torr. As in the liquid metal ion source, a biased annular extraction electrode faces the tip. The gas
preferentially ionizes on a specially grown surface protrusion (a few nm size) and the ions are emitted in a narrow ronc - 1°, which is well matched to the operation of the ion column . The energy spread of the ions DE is typically - 1 eV anti the virtual source size is estimated to be of order 1 nm . Because of this the angular brightness is estimated to be 1O t° A/cm`sr which is three to four orders of magnitude higher than IONS
Fig. 3. (a) Schematic of the 150 kV column in use at MIT. (b) Schematic of liquid metal ion source with enlarged view of tip showing liqu; d pt,lled into a cusp by the elec=tric field.
for the liquid metal ion sourer .. Accordingly the current density in the focal spot has been calculated from existing data to be 80 A/cm' for F1 2' and 87 A/cm' for He' [33].
J. Melngnihs / Focused ion beam lithography
IONS Fig . 4 . Schematic of the gas field ion source of the type used in refs. [31-33]. The structure holding the emission tip has to be in a gimbal mount so that the tip can be tilted "- 15° and the emission can bu made to go down the axis of the system perpendicular to the extraction electrode. This superior performance is based on two de~elonments : the use of ultraclean conditions, i .e. 10- "' Torr vacuum and purified gases, and the controlled, in situ growth of the reliable small emission tips . The implications of this development for lithography will be easier to appreciate after we review the focused ion beam exposure results obtained so far . But first we will briefly look at the components of the beam writing process . 3.3. Stage and beam control For focused ion beam lithography we must have the ability to deliver a given dose in desired patterns on the surface . To do this, the system must be able to deflect the beam, to turn it on and off (beam blanking), and to align to existing features . In addition, since most patterning is likely to be over entire wafers, or at least over a chip, and since the maximum practical deflection of th,. ion beam is only a few hundred micrometers, :he stage holding the sample must be moved, and the . stage motion and beam deflection must be accurately aligned to one another. - Beam deflection is accomplished by a transverse electric field generated by suitable electrodes . Simple parallel plates or octupoles are typically used, see fig . 3 . Although deflection appears to be straightforward, it is responsible for some of the system limitations such as writing speed and field size . The deflection speed is limited by the ability of the electronics to generate the usually up to 60 V pulses with high precision, low noise, and very fast rise and fall times, and by the transit time of the ions between the deflection electrodes . Some values of the time for °,n ion tc, traverç t: t cot ,^.re ^oiven in t lhln 2 ,. - Beam blanking is usually done further up in the column, where the beam has lower energy. Together
1275
with deflection On th e simple, it is an essential part of any writing. Blanking is done by deflecting the beam above an aperture preferably where the beam has a crossover . For the system depicted in fig. 3 the blanking plates are between the E x B mass separator and the mass separating aperture, and they deflect the beam pcipundicular to the deflection direction of the F_ x B mass separator . Because the blanker has a finite length and displares the beam laterally before sweeping it off th,: apes lure, the beam has "tails" both is time and in space . The blanking pulse needs to rise fw ;t from zero and return cleanly and quickly to zero, but can have a noisy maximum . The transit time between the blanking plates, see table 3, also limits the maximum blanking rate . - Alignment except for the writing of an original mask pattern, alignment to existing features is essential . Alignment is achieved by using, the focused ion beam itself to image . In this mode the machine is used much like on a scanning alPctron microscope (SERO The beam is raster scanned over the sample and secondary electrons emitted from the point of ion incidence are collected by a channel electron multipiier located above the sample (see fig. 3). Since the alignment marks are covered by resist, the image formed is of the resist topography. For 0 .1 wm thick PMMA covering the alignment masks, alignment accuracy of t0 .1 wm has been reported [9] . However, if thicker resist needs to be used, the alignment marks may need to be uncovered, for example . by laser ablating the resist-or by ion milling [9] . - Pattern writing is accomplished by combining the deflection, blanking stage motion, and alignment (if needed). The sample is located on a precimon x-y stage whose position is measured to t 10 nm by a laser interferometer system. The stage position is fed to the computer that controls the beam deflection and blanking . Patterns are usually written in the vector mode, where a blanked (virtual) beam is deflected to the position of a desired feature where it is unblanked and deflected to fill in the feature . The position and size of the features to be lithographically cxpo~ed are in a computer file. This data is Table 3 Time to travel 1 cm [ns] Species Electron Proton H, He
Be Ga
Energy [keV] 30 0.097 4.17 5.90 8.32
12.5
34.7
100
300
0.053 2.28 3.23 4.56 6.84 19.0
0.031 1 .32 1 .86 2.63 3.95 11 .0
Va . NOVEL TECIINIQUES (a)
1276
J. Mchigaihs / Focaacd inn N-arrr hrlugrapha,
converted to beam deflection voltages and appropriate stage commands by the on-board computers.
IonBeam
4. Lithography results The concept of exposing resist with focused ion heenis ,predates the liquid metal ion soué,e [34]. Since the early eighties, when columns began to be developed with good optics and with liquid metal sources, numerous demonstrations of focused ion bean? lithography have been reported (see refs. [9,22,23] and references .herein). In negative resist (SAL 601-ER7) lines down to 0.05 pm width have been demonstrated in thin 0 .12 pm resist and good quality 0.1 Wm lines have been exposed in 11.6 wm thick resist . See fig . 5. The dose needed for exposure is m the range of 1 to 2 x 10 12 ions/cm= . In this resist the exposed portion is not developed away . The minimum features demonstrated arc certainly adequate for future microfabrication needs . The resolution capabilities of the positive resist PMMA are even better. Fig. 6 shows 0.05 p.m gold features which were plated into 0 .3 Wm thick resists for the purpose of making an X-ray mask . These leaturcs were exposed with 280 keV B,: = ' ions. The finest features have been achieved in a specially configured column using Ga' ions and a high degree of dlemagnification. The minimum width lines exposed in PMMA arc in the 12-15 nm range [11,14,36] . This minimum beam diameter in a Ga' beam at 50 kV has also been demonstrated by exposure of InP and subsequent chemical etch [36] . See fig. 7. Another important feature of ion lithography is the absence of proximity effect [12,35,36] . This is illus-
F---f 0 .5 Nm Fig. 5 . From ref . 19] (on left). Perspective view of negative resist lines (SAL Gilt-12117) exposed wilb 260 keV Bc2 ' ions at a dose of 2x 11112 ions/cm= linewidth is 0.1 Wm and resist thickness is 0.6 wm . (On the right) 0.05 wm features in the same resist of thickness 11 .12 wrn "i-ed with 260 keV Be -` ions at I x 10 12 ions /cm2 .
Cold
D,, ,1,, p&Mal,
J
--n
Dissolve PMMA Fig . 6. From ref. [39[. Focused ion beam exposure of PMMA and electroplating of features used to fabricate X-ray lithography masks . First PMMA is exposed with 280 keV Be 2 ' ions at I x 111 1 ' ions/cm` and d nA(1ped then gold « nlawd up from a plating babe (a thin. film of gold under the PMMA). Finally, the PMMA is dissolved resulting in the features sho-n in the photo. 150 riles we ; c exposed with pairs of lines ~) Pro long. trated in fig. 8 where the width of a narrow line is unchanged by the presence or absence of close proximity large area features . This is not true in the case of electrons which scatter widely as they penetrate into a solid and leads to the need for proximity effect correction, i.e . the dose has to be adjusted to account for the variable background produced by the electron scattering. The proximity effect can be -educed by exposing with a high energy 100 keV eicctroi, beam [45], particularly if it is done over a thin membrane as in the fabrication of X-ray lithography masks . But even with 100 keV electrons over a membrane the writing of patterns ~s still proximity corrected for forward scattering [45]. An indirect measure of the proximity effect is the fact that the minimum dose needed to expose a large area in PMMA at 100 keV is 600 WC/cm= when it is over a membrane (the valu . , ussd in table 4) but it is 4(M la,C/cm= when it is over a solid substrate [46]. This indicates that backscattered electrons produce a large background. Nevertheless, impressive 0.075 lam features including lines, spaces, and arrays, have been exposed by the I(Hl keV electrons in 0.75 wm thick resist and gold plated to a thickness of 0.6 Wm [45] . So far we have been discussing exposure of single layer positive and negative resist by ions or elections . An alternate scheme is to use surlace imaged resists combined with directional, reactive ion etching [35,37] .
1277
J, hlr/ngulhs / Focused ion brarrt litltngrnphy 4.1 . Focused ion beasts us electron beasts
Of these two techniques capable of writing original patterns below 0.1 pro which is preferable? The main advantages of ion beams over e-beams arc: - Resist sensitivity (fast exposure) This will be discussed below. - Absence of proximity effect. As shown above in fig . 8 ions deposit their energy where needed and do not produce unwanted scattering . - improved stability. This is less well documented . However, ions are heavier and move more slowly so they are less susceptible to magnetic field disturbances. Also unwanted charging effects may be ab-
Fig. 7 . From ref . [361. A 15 nm line exposed in InP using 50 keV Ga' ions (dose in line - 2 .5 x 10 13 ions/cm 2, dose in pad - 4 x If) 14 ions/cm 2 etched after exposure in ammonium hydroxide and water, 1 : 15, for 1(I to 30 s). 'Phis illustrates some of the finest features exposed by ion beams . (See also refs . [11,14J .)
The process is illustrated schematically in fig. 9. Incidentally this process is also being developed for exposure of resist with deep ultraviolet radiation. 193 nm wavOcagth, and below where the resist is no longer transparent . The silylation process has the advantage that Ga' ions can be used which are available at current densities about ten times higher than Bc2 ' but which fia~e a limited range in the resist . An example of feature,; exposed by this process is shown in fig . 10. Proximity effects have also been found to be absent in the ion exposure of silylated resist (fig . 8) [35,371. Many inorganic films can also act as "resists" . The ion irradiation can be used to modify the solubility of films in wet or plasma etches. For a tabul-ttion of recent results, see ref. [23], see also fig. 7, ref, [361. However, in most cases the dose needed for exposure is about two orders of magnitude higher than it is for organic resists. One important reason for interest in inorganic resists is that they are potentially usable in in situ processing, i .e . deposition, exposure, development, and subsequent processing, all done in vacuum .
tym 0.3 Nm
Fig. 8. (upper) From ref. [351. SAL 601 resist exposed to 240 keV Bee' ions, 4x 10 13 ions/cm 2 , and developed using the silylation process (fig. 9) . No proximity effect is observed, i .e . the width of the exposed line in the center is unchanged by the presence of large exposed areas immediately adjacent. (lower) From ref. [361, Absence of proximity effect illustrated by exposing 60 un thick PMMA with 50 keV Ga - ions and ,ifting off 3:, nm of AuGe . Linewidth is 24 nm and the gap between the pads is 130 nm . Va NOVEL TECHNIQUES (c :
1278
J. Meingailis / Focused ion beam lithography
FOCUSED 10-4 PEAM EXPOSURE
i 11
1411
4111
111
iii
CROSSLINKED
SILTLATION nMSDMA, ti0-t00^c
Fig. l'.). From ref . [351 . Features produced in SAL 601 resist by tht! silylation process (fig 9), exposed -46 'In well Gs" ions. On the left the minimum width lire is seen in a top view to be 0 .08 am. On the right we see vertical side walls produced by "development" in the OZ reactive ion etching,
SILdLATED
Fig .
9.
A schematic of the citylation prtea :., rei. (351 or [37) .
sent because ions "clean off" surfaces that they strike and are less likely to build up charged surface films than electrons . Conversely the main advantage: of electron beams OVCT ion 1 ,eams arc : - f0sence of shot noise effects There are many more (typically a huadred times) electrons than ions needed pe . unit area to produce exposure . Thus statistical fluctuation is less '.ikuly to play a role . - Transit time effects arc unimportant . The deflection and blanking speed of ion systems is ultimai, .y li.a-
ited by the transit time through the column . ElccIrcnc at the same energy travei 43 times faster even than protons. Sce table 3. - Electrons produ,~e no implantation effects. Ttiis is important only fer direct writing on a wafer. - Mature state of development. Historically e-beam development for lithography started about ten years before ion beam development . Therefore, to overcome this head start ion beam lithography would have needed to show very dramatic advantages. Among the listed disadvantages both sides can point to ways of overcoming them, and both camps can show laboratory demonstrations of microfabrication well below 0.1 i,m.
Table 4 Electron beam vs focused it .n beam writing time fo; ultrafine features . Note - in the case of ion beams ti-e minimum linewidth is written with a single pass of the beam . This has been shown (39,35,361 to yield well formed lines. The linewidth can be adjusted by varying the dose . In (he case of electrons the 75 run minimum linewidth features are written with five passes of a 15 nm diameter beam (ref. [381) or three passes of a 28 run diameter beam (refs. [45,461) in order to have good control of the linewidth . If multiple passes are used with ion beams the writing time will be increased, e.g . using tw. passes will increase the writing time by about o factor e,
Source Features written Beam current Resist sensitivity
Writing time
e-beam (ref. [381)
e-beam (ref. [4'5,461)
Ion beam (ref. [391)
Field emission e-gun 25 kV 75 nm lines 1 .17 nA 150 WC/Cm 2 (9x 10 1° e/cm 2 ) (PMMA 0.1 wm thick)
Field emission 100 kV 75 mn lines 5 nA 600 WC/cm2 (PMMA 0.73 Wln thick over thin Si membrane) 12 pCArr,2 (n'ga-c. rhemic3lly amplified 1200 s/r,m 2 PMM. k
Bee' at 280 keV
1300 s/mm`
24 s/mm 2 for negativ_ resist
50 nm lines 20 pA 1 x 10 11 ions/cm2 (PMMA 0 .3 gin thick)
1700 s/mm 2 or 710 s/mm2 if scaled to 75 nrl features
Ion beam 66-wû, ref. [351) Ga * 30 keV
1(n, beam (projected Ref. [32,331)
75 nm lines 176 pA 8X1011 (positive SAL 601 silylation)
75 not lines 3.6 nA 1 .5 x 10 11/ul.' (positve resist fMMA)
7 .6 s/mm 2
H, 100 keV
1 .5x 10 12 (negative resist SAL 601) . .9 ,/m,,2 for PMMA 0 .i9 s/mm2 for negative resist
J. Melngaills / Focused ion beam lithography
One of the fundamentt.l issues which directly affects the utility of a lithographic technique is the exposure time as determined by the resist sensitivity. In table 4 we compare electron beams to various kirni~ of ions. For electron beams we take the brightest reported field emission type sources which will yield the fastest writing time . The results reported so far show that the writing time per unit area (large area sensitivity: ions or electrons per unit area divided by the particle current) is comparable or slightly shorter for 13eZ * ions than for electrons. However, the Be e+ ions come from an alloy source and are only a fraction of the total beam, and, becau-e of the energy spread, the acceptance angle r:as to he limited as discussed above . The writing current is, therefore, only 20 pA . Fer the gas field ion sources being developed the projected current is 3 .6 nA, i.e . 360 times more ions/s in the beam (20 pA Be` + is equivalent to 10 pA singly charged ions) . The writing time with H z ions for example is projected to be 174 times faster than with electrons frt positive resist and 35 times faster for negative. resist . The transit time and shot noise effects also will not limit the lithography with these light ions. A 0.1 x 0.1 h, .m area even for the most sensitive resist (1 .5 x 10" ions/cm') would have 150 ions which is at least three times higher than the measured shot noise limit. As for the transit time effects: at a current of 3 .6 nA, 150 ions are delivered in 6 .9 ns . The 106 keV H, ions traverse 1 cm in 3 .2 ns . The blanking and deflection electrodes can be less than 1 cm in length, or they can be segmented, as in some oscilloscopes, with pulses applied to them in synchronism wkh the passing ion . Thus the transit time will not post a limit to the writing speed fir the light ions. In the case of the surface imaged silylated resist, Ga* ions. can be used. Ga + ion beams are now readily available at low energies (25-50 keV) for repair processes If we were to use these for the surface imaged lithography (see above and table 4), the time to deliver 80 ions needed to expose a 0.1 x 0.1 gm area would be 7() its . A 50 keV Ga* ion traverses 1 cm in 27 ns . Thus again the transit time should not pose a limit, i .e. the current available in the beam is limited by other factors ;anyway.
5 . Conclusions Many examples of successful resist exposure with ions at dimensions well bciutt 0.1 pin leave no doubt that the technique works . In addition, focused ion beam lithography does not puffer from proximity effects and can have many times shorter exposure times than electrons, particularly if gas field ion sources are used . Thus focused ton beam lithography is worthy of
1279
development for the writing of original ultrafine patterns
Acknowledgements The author wishes to thank R.L. Kubena and S. Mastui f^r oroviding copies of figures, and M.A . McCord for pr )viding a preprint and additional enlightenment on e-beam lithography. Although. the writing of this re-I :ew paper is not directly sponsored . the author's work at MIT on foe-ased ion beam lithography is supported by DARPA/ARO and by Sematech/SRC
References [i] J .L . Bartelt, Solid State Technol . 29 (1986) 215. [21 J .N. Randall, J. Vac . Sci . Technol . A4 (1986) 777 . [31 D .P . Stumbo, S. Sen, G.A . Damm, F .O. Fong, D.W. Engler, K.F. Fong and J.C . Wolfe, J . Vac . Sci . Technol. B9 (1991) 2879. [41 G. Stengl, H . Löychner, E . Hammel, E.D . Wolf and J .J . Muray, Emerging Technologies for In Situ Processing, eds . D.J . Ehrlich and V .T. Nguyen (Nijhoff, 1989) p . 113 . [51 G. Stengt et al ., 36th Int . Symp . on Electron Ion and Photon Beams, Orlando Florida, 1992, Paper C38. J. Vac. Sci. Technol. B (Nov/Dec 1992) 2838. [61 W.H. Briinger, M . Torkler and L.M. Buchmann, ibid., p. 2829. [73 G . Stengl, et al ., ibid., p. 2824 . [81 C.G . Willson, in: Introduction to Microlithography, eds. L.F. Thompson, C.G. Willson and M.J . Bo,den, ACS Symp. Series 2"19 (1983) p. 92-98. [9) S . Matsui, Y. Kojima, Y . Ochiai and T. Honda . J . Vac. Sci . Technol . B9 (1991) 2622 . [101 J.S . Huh, M.I . Shepard and J . Melngailis, ibid ., p . 173 . [111 R.L. Kubena, J .W. Ward, F.P. Stratton, R.J . Joyce and G.M . Atkinson, ibid ., p . 3079. 1171 S Matsui, Y. Kojima and Y. Ochiai. Appl. Phys . i ett . 53 (1988) 868. [131 S. Matsui, K. Mori, K. Saign, T. Shiokawa, K. Toyoda and S . Namba, J. Vac. Sci. Technol. B4 (1986) 845 . [141 R .L. Kubena, F.P. Stratton, J.W . Ward, G.14 . Atkinson and R.J . Joyce, J . Vac. Sci. Technol. B7 (1989) 1798. [151 P . David and L. Karapiperis, Microelectron. Eng. 3 (1985) 173. [161 J .L . Bartelf, C.W. Slayman, J.E. Wood, J .Y . Chen, C.M . McKenna, C.P . Minning, J .F. Coakley, R.E. Holman and C .M . Perrygo, J. Vac . Sci . Technol . 19 (1981) 1166 . [171 S .W. Pang, T.M. Lyszc7arz, C.L. Chen, J .P. Donnelly and J.N . Randall, J . Vac. Sci . Technol . B 5 (1987) 215. 1181 L.R . Harriott, Appl . Surf. Sci . 36 (1989) 432. [191 S . Namba, Nud . Instr. and Meth . 1339 (1989) 504. [201 P.D . Prewett and G .L .R. Mair. Focused Ion P-i~ :r^^ Liquid Metal Ion Sources (Wily, 1991). [211 J. Orloti, Sci . Am. .^.65 (N'J1) Sti. [221 d. Melngaihs, J . Vac. Sci. Technol. B5 (19s l) 469 . Va. NOVEL T'3CHNIQUES (a)
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J. Mefngnihs / F-,.,d ton beam hthography
[231 J. Melngailis, in: Handbook of VLSI Microlithography, eds. W.B . Glendinning and J.N. Helbert (Noyce, 1991) p. 554. 124) R Levi-Setti, Proc. 7th Ann. . .canning Electron Microscope Symp. (IIT Research Institute . Chicago, 1974) Part 1, p. 125. [251 J.H . Orloff and L.W . Swanson, J. Vac. Sci. Technol . 1 2 (1975) 12119. [261 K. Horiuchi, T. Itakura and H. Ishikawa, J. Vac. Sci. Technol. B6(1988) 241. ,271 J. Orloff and L.W . Swanson . J. Appl. Phys. 5 0 (1979) 6026 . (281 G.N . Lewis, 11. Paik, J. Mioduszewski and B.M . Siegel, J. Vac. Sci . Technol . B4 (1986) 116. [291 P.R . Schwoebel and G.R. Hanson, J . Vac. Sci . Technol. B3 (1985) 214. [301 B.M . Siegel, 1 Vac. Sci Teehon1 136 (19xx) 350 1311 R. Biirret, K. Jousten, K. Bährioger and S. Kalbnzer, J. Phys . D21 (1988) 1835 . 1321 Ch . Wilbertz, Th. Maisch, D. F16ttner, K. Böhringer, K. Jousten. and S. Kalbitzer, Nucl . Instr . and Meah . B63 (1992) 120. 1331 S. Kalbimr, Mar Planck Inst . für Kernphysik, private communication . [341 R.L. Seliger and W.P. Fleming, J. Vac. Sci . Technol. 1 0 ("773) 1127 : and J. Appl . Phys. 45 (1974) 1416. (3° ; M.A . Hartney, D.C . Shaver, M.I. Shepard, J. Velngailis,
V. Medvedev and W.P. Robinson . J. Vac. Sci . Technol. B9 (1991) 3432 . [361 G.M . Atkinson . F.P. Stratton, R.L. Kubena ald S.C . Wolfe (ElPB 199:.) to be published J. Vac. Sci . Technol . R(Nov/Dec 1992) 3104 . [371 M.A . Hartney, D.C . Shaver, M.1. Shepard, I .S. Hub and J. Melngailis, Appl . Phys. Lett. 59 (1991) 485. [381 M.A . Gesley . F.J . Hohn, R.G. Viswanathan and A.D . Wilson, J. Vac. Sci. Technol. B6 (1988) 2014. [391 W. Chu, A. Yen. K. Ismail, M.1. Shepard, II .J . Lezec, C.R . Musil, J. Melngailis, Y.-C. Ku. J.M . Carter and H.I . Smith, J. Vac. Sci . Technol . B7 (1989) 1583 . [401 H. Ryssel, K. Haberger and H. Kranz, J. V,tc. Sci. Technol . 19 (1981) 1358. [411 M. Komuro, N. Atoda and H. Kawakatsu, J. Electroche m. Soc. 126 (1979) 483. 14'1 1 Randall. D.C . Flanders, N.P . Economou, J .P . Donnelly and E.1. Bromley, J. Vac. Sci . Technol . Bl (1983) 1152 [431 S. Mmnui, K. Mori, K. Saigo, T. Shiokawa, K. Toyoda and S. Namba, J. Vac. Sci . Technol. B4 ()986) 845. [441 S. Matsui, K. Mori, T. Shiokawa, K. Toyoda and S. Namba, J. Vac. Sci. Technol . B5 (1987) 853. [451 M.A . McCord, R. Viswanathan, F.J . Hohn and A.D . Wilson (EIPIB Conf. )992) to be published J. Vac. Sci . Technol . B(IVov/Dec 1992) 2764 . [461 M.A . McCord . private communication.
Beam Interactions with Mat.Nils &At...
'Focused , ion beam lithography John Melngailis Massachusetts Institnte of Technoloty, Cambridge, MA, USA
Lithography for microelectronics, that is, the exposure and development of resist, is already being carried out in research laboratories at dimensions well below 0.1 h.m . In production the minimum dimensions are likely to approach 0.1 l :m before the end of the decade. This review will examine the use of focused ion beams for ultrafine lithography . Minimum dimensions down to 0 .015 l..m have been reported as well as exposure of 0.25 ltm thick resist with 0.05 pro linewidth for the making of X-ray lithography masks. At this time there are only two techniques for writing original patterns (as opposed to replicating them) at 0.1 Wm and below : electron beams and ion beams. Electron beams are at a mmure state of development and have aavantages in absence oi shot noise and in fast deflection capability. Ion beams on the other hand have demonstrated absence of proximity effect and high resist sensitivity, i .e . potentially faster writing speed . The development of the gas field ion source promises hundredfold inrrense in current density of light ions (H ', He . . . ) in the beam focal spot. In addition, these light ion beams at high energy can be deflected at the speeds needed for lithography . Thus focused ion beam lithography is a serious candidate for future fine pattern writing . 1. Inttroduction Lithography is universally used in the fabrication of microelectronic devices, and in that sense it is the foundation of our information age . In the microelectronics context lithography consists of the exposure of a radiation sensitive polymer film on the surface of a semiconductor wafer and the subsequent development to produce a fine pattern in the polymer (e.g . the exposed (or unexposed) part is dissolved away). The fabrication steps which follow, such as material removal, implantation, or material addition, then alter the part of the substrate that is not covered by resist . The lithography is repeated with different patterns to build, for example, an integrated circuit. Up to now almost all lithography on integrated circuits is carried out by projecting a demagnified image of a mask onto the substrate using ultraviolet radiation . The mask consists of an opaque film such as chrome on a glass or a quartz substrate . The original pattern on the most advanced of these masks is written by exposure of resist by electron beam lithography . The smallest dii,ensions exposed on the wafer have been shrinking steadily, and now 0.8 p m is typical in advanced production . However, as the minimum dimensions shrink further and approach 0.i Win, alternatives to ultraviolet radiation as the exposure vehicle are being considered . A tool is nrAed to write the original pattern on the mask and another t-)i to replicate the pattern on the semiconductor wafer. The candidates most actively pursued today arc proximity X-ray lithography for the replication and electron beam lithography for the pat0î6h-583X/93/$06.00C~ 1993 - Elsevier
tern writing . Ions have also been considered as an alternative both for pattern writing and for replication over the past decade and a half . But the level of research activity has been much lower . Recent developments, however, which may impact both ion pattern writing and ion pattern, replication, motivate us to review ion lithography. There are three types of ion lithography, i .e. three methods for producing alternate irradiated and unirradiated areas on a surface wiih minimum features of 0 .1 p.m and below . They arc: 1) Focused ion beams. A small intense spot with a current density of 6 .1 to 80 /`./cm 2 is deflected over the surface. This is very similar to electron beam lithography and is the main topic of this review. 2) Ion beam proximity printing (also called masked ion beam lithography). Here a stencil mask with open areas is held in close proximity to a resist covered sample and exposed to light ions such as H+, HZ or He' . The ions can come from a standard ion implanter, ; nd this technique of shadow printing can be very fast. For a review, see ref. [1]. The difficulties of making a mask at the final dimension which is either ion transparent or open have limited the applicability of this technique . Lines and sp, ces down to 80 nm have been demonstrated [2j as well as special applica'i-tns whicc the limitations dfe less important [3]. :1) Ion projection lithography . In this technique a stwicil mask is uniformly illuminated by a beam of light ions, and Men by means of electrostatic optical elements the image of the stencil mask is projected onto a sample surface [4], The demagnification permits the mask spccitications to be relaxed, and the removal of
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Va. NOVEL TECHNIQUES (a)
!. Melngailis / F,x,tured `oelxant lnhography
1272
the mask from the proximity of the sample may simplify step and repeat exposures. Recent progress in controlling the field distortion [5], the image stability [61, and in achieving fine lines [7] make ion projection a promising lithography technique . Regardless of wheiher ions or X-rays are used for future ultrafine a technique is needed for writing the original pattern . We will review developments which indicate that ions may prove to be the preferred technique . But first we will look at the ion-resist interaction in general.
260keV Be FIB
20keV EB
INovolak Based Negative Resist PMMA '100 z Y U >_
PMMA xx-xx y
a'
w L' 50
x
a î zz
2. lua-it,Ast Eateraction
0
The sensitivity of resist to radiation is determined by the number of molecules of resist transformed by the incident radiation . In case of positive resists, like PMMA, the radiation breaks up long molecules, and one speaks of a "chain scission efficiency" . The competing process of cross-linking also takes place . In positive resists the scission efficiency is larger than the cross linking, and in negative (single component) resists the opposite is true [8]. Particularly in the case of ions, many resists can act both positive and negative depending on the dose [9,10]. Light ions, such as H, He, or Be in the 100-200 kcV range, lose all of their energy in traversing about a gm of resist, while electrons penetrate much deeper . Thus the sensitivity of r,si,t in terms of ions/cm2 needed is typically two orders of magnitude higher thin the sensitivity to electrons/cmz . An ion is often visualized a, exposing a cylinder of resist of about 10 nm diameter surrounding the point of entry. This picture has not been directly verified, except by the observation that evesi if the ion beam is focused to diameters of 8 rim, the minimum linewidth obtained is still 12 am [i i].
oil
1012
lo is
Ici .
1 0"
DOSE ( tons, electrons /cme)
Fig . 1 . From ref . [121. Ey!iosure characteristics of PMMA and negative novolak resist . The vertical axis shows the %r thickness remaining after development. Note that PMMA is about two orders of magnitude r,orc .,easitir:c to ions #tat, electrons . By sensitivity we generally mean the minimuin dose needed to expose the resist. Some typical va!ucs of sensitivity to ions arc given in table 1 . The resist sensitivity is in most cases obtained from plots of resist thickness remaining vs dose as shown in fig. 1 . The resist sensitivity depends, in part, on baking, on ,uickness, and on development conditions . The dependence on development time, for example, is seen in fig . 2. 2.2. Shot noise
One of the consequences of the high sensitivity of resist to ions is that statisticL! Guctuatioii iii the number of ions in a given minimun, dimension featoie clement (shot noise) must be considered. This feature element, often called a pixel, can be taken as a square at the minimum dimensions. Ttkus if N ions are on the
Table 1 Examples of organic resist exposure with ion beams Ion H'
Energy (kcV) 120
H' He'
1(M)
Be 2 '
1.
Bc 2 ' Ga' Ga'
280 IOJ 30
lit 14)
Resist PMMA (pos.) PMMA PMMA PMMA Novolak (Shipley SAL 601) (neg .) PMMA PMMA SAL 601 silylated
Min . dose (ions/cm z ) 3x10' 2
Min. line width ( )
Remuiks
Ref.
Thru stencil mask
[40)
2x 10" 3x 10" 1 .8x 10'2
0 .08
Thru stencil mask
[421 [411
2x10"
0.1 (0.6 I
m thick)
Focused ion beam
[121
2x 10" 6x10 11
(t 05
thick) 0.06 (0 .12 Wm thick resist) 0.08
Focused ion beans Focused ion beam Focused ion beam
(391 [431 [351
8x 1Ol'
(0.3
tLm
J. Mehigailis / Focused ion heron lithography ê m c E w N
cY u t tIon Dose Thickness of PMMA remaining vs dose after various development times as indicated in the inset . The highest contrast (steepest drop-off) is for the 30-60s develop ment. The initial thickness of the PMMA is less than the range of Be`' is ns .
Fig. 2. From rcf. [101.
average incident on a pixel, the range over which N can vary statistically is of order FN . For a sensitivity of 10'= ions/cm = each 0.1 Wm x 0.1 I .tm pixel would have 100 ;-- 10 ions incident . (There is, of'_ot :rsv, a nonncgligiblc pre :)ability of having a number of ions outside of this range of t 10.) Experimentally, and somewhat contrary to this statistical argument, well-formed, continuous lines ha-c been demonstrated with the number of ion, per l ., .:I in the range of 25 to 35 [13,141 . Below this number. lines at minimum dimensions become discontinuous . 2.3. Range Ideally the range of the ions should match exactly cite resist thickness so that the desired pattern is devclopcd all the way down to the substrate . The range it ;eases with increasing energy and decreasing ion mass. Some typical values are shown in table 2. If the lithography is to be carried out directly over Table 2 Ion range in PMMA resist (or depth to which exposed resist is developed) ion"H' He' Be Si
Ga
Energy (keV)
Range (wm)
Reference
40 1io 240 40 120 240
0.52 1 .12 1 .85 0.44 0.96 1 .54 0.45 0.72 1 .2 0.65 0.074
[401
IIAi
2tN1 20(1
100
1401 1441 [9,101 (9,101 [131
1273
active devices, for example by exposing transistor gates, then the ion penetration into the substrate may lead to damage or unwanted doping. In some cases matching the resist thickness to the ion energy may be a solution [151 . Thus after the eons pass through the resist and reach the substrate, they have vary little energy left . In general some penetration into the substrate will occur . For light ions, protons or He, the effects may be minimal, while for heavier ions such as Be or Si the damage and the substrate implantation are harder to avoid . NMOS transistors have been fabricated using protons for all levels of lithography. The devices were annealed at 450°C . No statis: :ealiy siguifieant effects of the irradiation were found [; 61 . GaAs MESFETs have also been fabricated where I rotons were used only for the gate lithography which i:. expected to be the most critical . The unwanted effet.ts of proton irradiation could be avoided by any one of four techniques : a sacrificial SiO, layer, the correct resist thickness, a 450°C, 5 s anneal, or removal of 50 nm of the surface by wet etching [171 3. Focused ion beams Focused ion beam systems capable of lithography have been devcioped mainly in the US and in Japan. For reviews of focused ion beams and üppiications, see refs . [18-231. The same systems can also be used for direct maskless implantation of dopants and many unique devices have beer. demonstrated. This has been reviewed elsewhere [231 . The essential system elements arc the focused ion beam column, the precision x-y stage, and the beam and stage control circuitry. 3.1 . lon co/untn An example of an ion column is shown schematically in fig . 3a. A point source of ions is followed by electrostatic lenses which tocus the ions back into a point on the sample . Precise beam deflection is accomplished by applying voltages to electrodes adjacent to the beam path, and beam blanking is done by deflecting the beam out of an aperture and onto the surface of a plate. A crossed electric and magnetic field velocity filter acts to separate masses so that only the desired mass species is propagated down the column. 3.2. Sources - The liquid metal ion source is used in almost all of the focused ion beam systems today . It consists of a needle surrounded by a liquid metal reservoir. See fig, 3b . (Heat may be needed,) The liquid slides dowr the needle and is pulled into a cusp by the electric field Va . NOVEL TECHNIQUES (a)
J. Melngaifis / Focused ion beam lithography
1274
FOCUSED ION SYSTEM
BEAM
(8)
SOURCE
produced by the bias voltage on an extraction electrode. The ions are emitted from the point of this cusp. (For a de=tailed discussion of liquid metal ion source, see ref. [20] .) The most commonly used source metal is Ga . However, Au/St, and Au,/Si/Be alloys have also
been used for lithography because of the lighter mass ions available in these sources . For Ga ions no mass separator is needed and the column can be considerably simpler titan shown in fig. 3a. The remarkable fact is that these ion sources particularly Ga, and Au/Si ope=rate stably for many hours, certainly long enough to do resist exposures. Provided the lenses arc properly designed and precisely rn~whined, and electrical noise
and vibration arc controlled, beam diameters down to 0.05 am can be readily achieved . The two main fundamental performance limitations arc the virtual source size and the energy spread of the ions emitted from the sourrre . The virtual source size (typically 50-100 nm) requires that demagnification I+c used to achieve smaller focal spots on the sample . Focal spot, down to 8 nm have been achieved using
ACCEL LENS
DEFLECTION PLATES ELECTRON DETECTOR
SAMPLE,X " Y STAGE
-- 10 x demagnification [11]. The energy spread of the ions (AE - 5 to 10 eV) leads to chromatic aberration, that is ions of different energies arc focused at diffcrc^t depths . This requires that the cone angle of the ions be reduced if small diameters are to be achieved. Thus the energy spread has the effect of limiting the current density in the focal spot . - The gas field ion source is m-.ire complex but promises higher performance . It has been studied [2429] since the mid 70s, but has not proved practical because the emission was not stable and the structure was cumbcrsumc . Its potential advantages for lithography were, however, recognized [30] . Therefore, the
recent advances in the technology which have resulted in stable operation of 170 it [31] and in the operation of the source in an ion column [32] are exciting developments. A schematic of the gas field ion source is shown in fig . 4. It consists of a cryogenically cooled single, crystal tip, usually tungsten, with gas He, H, or Nc for example, surrounding the tip at a pressure of about ?0 -° Torr. As in the liquid metal ion source, a biased annular extraction electrode faces the tip. The gas
preferentially ionizes on a specially grown surface protrusion (a few nm size) and the ions are emitted in a narrow ronc - 1°, which is well matched to the operation of the ion column . The energy spread of the ions DE is typically - 1 eV anti the virtual source size is estimated to be of order 1 nm . Because of this the angular brightness is estimated to be 1O t° A/cm`sr which is three to four orders of magnitude higher than IONS
Fig. 3. (a) Schematic of the 150 kV column in use at MIT. (b) Schematic of liquid metal ion source with enlarged view of tip showing liqu; d pt,lled into a cusp by the elec=tric field.
for the liquid metal ion sourer .. Accordingly the current density in the focal spot has been calculated from existing data to be 80 A/cm' for F1 2' and 87 A/cm' for He' [33].
J. Melngnihs / Focused ion beam lithography
IONS Fig . 4 . Schematic of the gas field ion source of the type used in refs. [31-33]. The structure holding the emission tip has to be in a gimbal mount so that the tip can be tilted "- 15° and the emission can bu made to go down the axis of the system perpendicular to the extraction electrode. This superior performance is based on two de~elonments : the use of ultraclean conditions, i .e. 10- "' Torr vacuum and purified gases, and the controlled, in situ growth of the reliable small emission tips . The implications of this development for lithography will be easier to appreciate after we review the focused ion beam exposure results obtained so far . But first we will briefly look at the components of the beam writing process . 3.3. Stage and beam control For focused ion beam lithography we must have the ability to deliver a given dose in desired patterns on the surface . To do this, the system must be able to deflect the beam, to turn it on and off (beam blanking), and to align to existing features . In addition, since most patterning is likely to be over entire wafers, or at least over a chip, and since the maximum practical deflection of th,. ion beam is only a few hundred micrometers, :he stage holding the sample must be moved, and the . stage motion and beam deflection must be accurately aligned to one another. - Beam deflection is accomplished by a transverse electric field generated by suitable electrodes . Simple parallel plates or octupoles are typically used, see fig . 3 . Although deflection appears to be straightforward, it is responsible for some of the system limitations such as writing speed and field size . The deflection speed is limited by the ability of the electronics to generate the usually up to 60 V pulses with high precision, low noise, and very fast rise and fall times, and by the transit time of the ions between the deflection electrodes . Some values of the time for °,n ion tc, traverç t: t cot ,^.re ^oiven in t lhln 2 ,. - Beam blanking is usually done further up in the column, where the beam has lower energy. Together
1275
with deflection On th e simple, it is an essential part of any writing. Blanking is done by deflecting the beam above an aperture preferably where the beam has a crossover . For the system depicted in fig. 3 the blanking plates are between the E x B mass separator and the mass separating aperture, and they deflect the beam pcipundicular to the deflection direction of the F_ x B mass separator . Because the blanker has a finite length and displares the beam laterally before sweeping it off th,: apes lure, the beam has "tails" both is time and in space . The blanking pulse needs to rise fw ;t from zero and return cleanly and quickly to zero, but can have a noisy maximum . The transit time between the blanking plates, see table 3, also limits the maximum blanking rate . - Alignment except for the writing of an original mask pattern, alignment to existing features is essential . Alignment is achieved by using, the focused ion beam itself to image . In this mode the machine is used much like on a scanning alPctron microscope (SERO The beam is raster scanned over the sample and secondary electrons emitted from the point of ion incidence are collected by a channel electron multipiier located above the sample (see fig. 3). Since the alignment marks are covered by resist, the image formed is of the resist topography. For 0 .1 wm thick PMMA covering the alignment masks, alignment accuracy of t0 .1 wm has been reported [9] . However, if thicker resist needs to be used, the alignment marks may need to be uncovered, for example . by laser ablating the resist-or by ion milling [9] . - Pattern writing is accomplished by combining the deflection, blanking stage motion, and alignment (if needed). The sample is located on a precimon x-y stage whose position is measured to t 10 nm by a laser interferometer system. The stage position is fed to the computer that controls the beam deflection and blanking . Patterns are usually written in the vector mode, where a blanked (virtual) beam is deflected to the position of a desired feature where it is unblanked and deflected to fill in the feature . The position and size of the features to be lithographically cxpo~ed are in a computer file. This data is Table 3 Time to travel 1 cm [ns] Species Electron Proton H, He
Be Ga
Energy [keV] 30 0.097 4.17 5.90 8.32
12.5
34.7
100
300
0.053 2.28 3.23 4.56 6.84 19.0
0.031 1 .32 1 .86 2.63 3.95 11 .0
Va . NOVEL TECIINIQUES (a)
1276
J. Mchigaihs / Focaacd inn N-arrr hrlugrapha,
converted to beam deflection voltages and appropriate stage commands by the on-board computers.
IonBeam
4. Lithography results The concept of exposing resist with focused ion heenis ,predates the liquid metal ion soué,e [34]. Since the early eighties, when columns began to be developed with good optics and with liquid metal sources, numerous demonstrations of focused ion bean? lithography have been reported (see refs. [9,22,23] and references .herein). In negative resist (SAL 601-ER7) lines down to 0.05 pm width have been demonstrated in thin 0 .12 pm resist and good quality 0.1 Wm lines have been exposed in 11.6 wm thick resist . See fig . 5. The dose needed for exposure is m the range of 1 to 2 x 10 12 ions/cm= . In this resist the exposed portion is not developed away . The minimum features demonstrated arc certainly adequate for future microfabrication needs . The resolution capabilities of the positive resist PMMA are even better. Fig. 6 shows 0.05 p.m gold features which were plated into 0 .3 Wm thick resists for the purpose of making an X-ray mask . These leaturcs were exposed with 280 keV B,: = ' ions. The finest features have been achieved in a specially configured column using Ga' ions and a high degree of dlemagnification. The minimum width lines exposed in PMMA arc in the 12-15 nm range [11,14,36] . This minimum beam diameter in a Ga' beam at 50 kV has also been demonstrated by exposure of InP and subsequent chemical etch [36] . See fig. 7. Another important feature of ion lithography is the absence of proximity effect [12,35,36] . This is illus-
F---f 0 .5 Nm Fig. 5 . From ref . 19] (on left). Perspective view of negative resist lines (SAL Gilt-12117) exposed wilb 260 keV Bc2 ' ions at a dose of 2x 11112 ions/cm= linewidth is 0.1 Wm and resist thickness is 0.6 wm . (On the right) 0.05 wm features in the same resist of thickness 11 .12 wrn "i-ed with 260 keV Be -` ions at I x 10 12 ions /cm2 .
Cold
D,, ,1,, p&Mal,
J
--n
Dissolve PMMA Fig . 6. From ref. [39[. Focused ion beam exposure of PMMA and electroplating of features used to fabricate X-ray lithography masks . First PMMA is exposed with 280 keV Be 2 ' ions at I x 111 1 ' ions/cm` and d nA(1ped then gold « nlawd up from a plating babe (a thin. film of gold under the PMMA). Finally, the PMMA is dissolved resulting in the features sho-n in the photo. 150 riles we ; c exposed with pairs of lines ~) Pro long. trated in fig. 8 where the width of a narrow line is unchanged by the presence or absence of close proximity large area features . This is not true in the case of electrons which scatter widely as they penetrate into a solid and leads to the need for proximity effect correction, i.e . the dose has to be adjusted to account for the variable background produced by the electron scattering. The proximity effect can be -educed by exposing with a high energy 100 keV eicctroi, beam [45], particularly if it is done over a thin membrane as in the fabrication of X-ray lithography masks . But even with 100 keV electrons over a membrane the writing of patterns ~s still proximity corrected for forward scattering [45]. An indirect measure of the proximity effect is the fact that the minimum dose needed to expose a large area in PMMA at 100 keV is 600 WC/cm= when it is over a membrane (the valu . , ussd in table 4) but it is 4(M la,C/cm= when it is over a solid substrate [46]. This indicates that backscattered electrons produce a large background. Nevertheless, impressive 0.075 lam features including lines, spaces, and arrays, have been exposed by the I(Hl keV electrons in 0.75 wm thick resist and gold plated to a thickness of 0.6 Wm [45] . So far we have been discussing exposure of single layer positive and negative resist by ions or elections . An alternate scheme is to use surlace imaged resists combined with directional, reactive ion etching [35,37] .
1277
J, hlr/ngulhs / Focused ion brarrt litltngrnphy 4.1 . Focused ion beasts us electron beasts
Of these two techniques capable of writing original patterns below 0.1 pro which is preferable? The main advantages of ion beams over e-beams arc: - Resist sensitivity (fast exposure) This will be discussed below. - Absence of proximity effect. As shown above in fig . 8 ions deposit their energy where needed and do not produce unwanted scattering . - improved stability. This is less well documented . However, ions are heavier and move more slowly so they are less susceptible to magnetic field disturbances. Also unwanted charging effects may be ab-
Fig. 7 . From ref . [361. A 15 nm line exposed in InP using 50 keV Ga' ions (dose in line - 2 .5 x 10 13 ions/cm 2, dose in pad - 4 x If) 14 ions/cm 2 etched after exposure in ammonium hydroxide and water, 1 : 15, for 1(I to 30 s). 'Phis illustrates some of the finest features exposed by ion beams . (See also refs . [11,14J .)
The process is illustrated schematically in fig. 9. Incidentally this process is also being developed for exposure of resist with deep ultraviolet radiation. 193 nm wavOcagth, and below where the resist is no longer transparent . The silylation process has the advantage that Ga' ions can be used which are available at current densities about ten times higher than Bc2 ' but which fia~e a limited range in the resist . An example of feature,; exposed by this process is shown in fig . 10. Proximity effects have also been found to be absent in the ion exposure of silylated resist (fig . 8) [35,371. Many inorganic films can also act as "resists" . The ion irradiation can be used to modify the solubility of films in wet or plasma etches. For a tabul-ttion of recent results, see ref. [23], see also fig. 7, ref, [361. However, in most cases the dose needed for exposure is about two orders of magnitude higher than it is for organic resists. One important reason for interest in inorganic resists is that they are potentially usable in in situ processing, i .e . deposition, exposure, development, and subsequent processing, all done in vacuum .
tym 0.3 Nm
Fig. 8. (upper) From ref. [351. SAL 601 resist exposed to 240 keV Bee' ions, 4x 10 13 ions/cm 2 , and developed using the silylation process (fig. 9) . No proximity effect is observed, i .e . the width of the exposed line in the center is unchanged by the presence of large exposed areas immediately adjacent. (lower) From ref. [361, Absence of proximity effect illustrated by exposing 60 un thick PMMA with 50 keV Ga - ions and ,ifting off 3:, nm of AuGe . Linewidth is 24 nm and the gap between the pads is 130 nm . Va NOVEL TECHNIQUES (c :
1278
J. Meingailis / Focused ion beam lithography
FOCUSED 10-4 PEAM EXPOSURE
i 11
1411
4111
111
iii
CROSSLINKED
SILTLATION nMSDMA, ti0-t00^c
Fig. l'.). From ref . [351 . Features produced in SAL 601 resist by tht! silylation process (fig 9), exposed -46 'In well Gs" ions. On the left the minimum width lire is seen in a top view to be 0 .08 am. On the right we see vertical side walls produced by "development" in the OZ reactive ion etching,
SILdLATED
Fig .
9.
A schematic of the citylation prtea :., rei. (351 or [37) .
sent because ions "clean off" surfaces that they strike and are less likely to build up charged surface films than electrons . Conversely the main advantage: of electron beams OVCT ion 1 ,eams arc : - f0sence of shot noise effects There are many more (typically a huadred times) electrons than ions needed pe . unit area to produce exposure . Thus statistical fluctuation is less '.ikuly to play a role . - Transit time effects arc unimportant . The deflection and blanking speed of ion systems is ultimai, .y li.a-
ited by the transit time through the column . ElccIrcnc at the same energy travei 43 times faster even than protons. Sce table 3. - Electrons produ,~e no implantation effects. Ttiis is important only fer direct writing on a wafer. - Mature state of development. Historically e-beam development for lithography started about ten years before ion beam development . Therefore, to overcome this head start ion beam lithography would have needed to show very dramatic advantages. Among the listed disadvantages both sides can point to ways of overcoming them, and both camps can show laboratory demonstrations of microfabrication well below 0.1 i,m.
Table 4 Electron beam vs focused it .n beam writing time fo; ultrafine features . Note - in the case of ion beams ti-e minimum linewidth is written with a single pass of the beam . This has been shown (39,35,361 to yield well formed lines. The linewidth can be adjusted by varying the dose . In (he case of electrons the 75 run minimum linewidth features are written with five passes of a 15 nm diameter beam (ref. [381) or three passes of a 28 run diameter beam (refs. [45,461) in order to have good control of the linewidth . If multiple passes are used with ion beams the writing time will be increased, e.g . using tw. passes will increase the writing time by about o factor e,
Source Features written Beam current Resist sensitivity
Writing time
e-beam (ref. [381)
e-beam (ref. [4'5,461)
Ion beam (ref. [391)
Field emission e-gun 25 kV 75 nm lines 1 .17 nA 150 WC/Cm 2 (9x 10 1° e/cm 2 ) (PMMA 0.1 wm thick)
Field emission 100 kV 75 mn lines 5 nA 600 WC/cm2 (PMMA 0.73 Wln thick over thin Si membrane) 12 pCArr,2 (n'ga-c. rhemic3lly amplified 1200 s/r,m 2 PMM. k
Bee' at 280 keV
1300 s/mm`
24 s/mm 2 for negativ_ resist
50 nm lines 20 pA 1 x 10 11 ions/cm2 (PMMA 0 .3 gin thick)
1700 s/mm 2 or 710 s/mm2 if scaled to 75 nrl features
Ion beam 66-wû, ref. [351) Ga * 30 keV
1(n, beam (projected Ref. [32,331)
75 nm lines 176 pA 8X1011 (positive SAL 601 silylation)
75 not lines 3.6 nA 1 .5 x 10 11/ul.' (positve resist fMMA)
7 .6 s/mm 2
H, 100 keV
1 .5x 10 12 (negative resist SAL 601) . .9 ,/m,,2 for PMMA 0 .i9 s/mm2 for negative resist
J. Melngaills / Focused ion beam lithography
One of the fundamentt.l issues which directly affects the utility of a lithographic technique is the exposure time as determined by the resist sensitivity. In table 4 we compare electron beams to various kirni~ of ions. For electron beams we take the brightest reported field emission type sources which will yield the fastest writing time . The results reported so far show that the writing time per unit area (large area sensitivity: ions or electrons per unit area divided by the particle current) is comparable or slightly shorter for 13eZ * ions than for electrons. However, the Be e+ ions come from an alloy source and are only a fraction of the total beam, and, becau-e of the energy spread, the acceptance angle r:as to he limited as discussed above . The writing current is, therefore, only 20 pA . Fer the gas field ion sources being developed the projected current is 3 .6 nA, i.e . 360 times more ions/s in the beam (20 pA Be` + is equivalent to 10 pA singly charged ions) . The writing time with H z ions for example is projected to be 174 times faster than with electrons frt positive resist and 35 times faster for negative. resist . The transit time and shot noise effects also will not limit the lithography with these light ions. A 0.1 x 0.1 h, .m area even for the most sensitive resist (1 .5 x 10" ions/cm') would have 150 ions which is at least three times higher than the measured shot noise limit. As for the transit time effects: at a current of 3 .6 nA, 150 ions are delivered in 6 .9 ns . The 106 keV H, ions traverse 1 cm in 3 .2 ns . The blanking and deflection electrodes can be less than 1 cm in length, or they can be segmented, as in some oscilloscopes, with pulses applied to them in synchronism wkh the passing ion . Thus the transit time will not post a limit to the writing speed fir the light ions. In the case of the surface imaged silylated resist, Ga* ions. can be used. Ga + ion beams are now readily available at low energies (25-50 keV) for repair processes If we were to use these for the surface imaged lithography (see above and table 4), the time to deliver 80 ions needed to expose a 0.1 x 0.1 gm area would be 7() its . A 50 keV Ga* ion traverses 1 cm in 27 ns . Thus again the transit time should not pose a limit, i .e. the current available in the beam is limited by other factors ;anyway.
5 . Conclusions Many examples of successful resist exposure with ions at dimensions well bciutt 0.1 pin leave no doubt that the technique works . In addition, focused ion beam lithography does not puffer from proximity effects and can have many times shorter exposure times than electrons, particularly if gas field ion sources are used . Thus focused ton beam lithography is worthy of
1279
development for the writing of original ultrafine patterns
Acknowledgements The author wishes to thank R.L. Kubena and S. Mastui f^r oroviding copies of figures, and M.A . McCord for pr )viding a preprint and additional enlightenment on e-beam lithography. Although. the writing of this re-I :ew paper is not directly sponsored . the author's work at MIT on foe-ased ion beam lithography is supported by DARPA/ARO and by Sematech/SRC
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J. Mefngnihs / F-,.,d ton beam hthography
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