+;bv Pev. -pola T" Demonstration of a Nanolithographic Technique Using a Self-assembled Monolayer Resist for Neutral Atomic Cesium

This experiment demonstrates the formation of nanometer-scale features in a gold substrate using a self-assembled monolayer (SAI{) of nonanethiolate as a resist for a patterned beam of neutral cesium atoms. The mask used to pattern the atomic beam rvas a si l icon nitr ide membrane perforated with nmand plm-scale holes. A dose of ^,3 monolayers of cesium damaged the SAI,I suff iciently to al lorv penetrat ion of the damaged regions by an aqueous etching solut ion. Etching transferred the pattern of damage in the SAN,I laver in to the under ly ing gold substrate. Features of -70-nm s ize rvere etched in to the gold substrate. Invest igat ions of the ref lect iv i ty of samples exposed to the atomic beam rvi thout a mask and subsequent l r , 'e tched revealed that the res is t -etch svstem exhib i ts a minimum threshold dose of cesium for damage; at doses lower than ^,3 monola-vers, the damage was insuf f ic ient to a l lorv penetrat ion of the SAI I { by the etch ing solut ion. Conventional ithographic processes incorporate three basic elements: 1) u damaging agent ( r .S.photons, ions, or e lec t rons) ,2) u pat tern ing mechanism ( r .g .a mask or focus ing opt ics), and 3) u resist that is sensi t ive to the damaging agent ( typical ly a -1-pm-thick organic layer). The relatively recent development of optics for neutral atomic beamsl has yielded a patterning mechanism for l i thographic techniques that use neutral atoms.2-4 There is, therefore, interest in nelv resist systems that are sensit ive to exposure to neutral atoms.s-8 The experiment described in this paper demonstrates a new method of making nanostructures that uses a patterned beam of neutral cesium atoms to damage a ^, 1.2-nm-thick sel f -assembled monolayer (SA\1) resist of alkanethiolates on golds'r0 (see Figure 1). The atomic beam rvas not patterned opt icai ly, but rv i th a contact mask consist ing of a ;-rm-scale holes. 40 pm x 40pm si l icon ni t r ide (Si3i{4) membrane perforated with nmand Exposure to the cesium altered the SAN,{ layer sufficiently to allow penetrat ion of a rvet -chemical e tch. resul t ing in ^ ,70-nmlv ide features etched in to a ^,20-nm-thick gold la.ver on a si l icon substrate. With an appropr iate mask and a broad atomic beam. patterning over larger areas (^,cm2) should be possible rv i thout increasing exposure t imes or feature size. The pr imarv appl icat ion for th is technique is the fabr icat ion of nm-scale structures, however, this method might also be used for high-resolution detection of atomic density. Lithography using neutral atoms has several desirable features: the theoreticaliy achievable image resolution of patterns formed with neutral atoms is l imited by the size of the atom (^,0.3 nm),11 and because neutral atoms are not subject to Coulombic forces, th is technique avoids some of the instabi l i t ies inherent in ionand electron-beam l i thography. Cesium has several attractive features for atomic l i thographv: 1) Cesium has cycling atomic resonances with large electronic susceptibil i t ies in regions of the spectrum (852 and 895 nm) that are easiiy accessible by relatively high powered diode (-100 m!V) and solid-state (^,1 W) lasers; it can. therefore, be easily and inexpensively manipulated using laser l ight . 2) A rvide variety of atomic optical elements have already been developed for use in patterning atomic cesium, and in preparing high-quality beams.t 31 Thermal effusive sources of cesium are in widespread use; thel/ are relatively inexpensive and simple to fabricate. Current techniques for neutra l a tom l i thography fa l l in to t rvo c lasses: i ) an opt ica l ly pa t te rned a tomic beam ( r .g .sod ium.2 ch romium,3 and a luminum{ ) i s depos i ted d i rec t l v onto a substrate; or 2) the in ternal energ-v s tored in exc i ted metastable s tates of neutra l atoms ( r .S.argonS and hel iumt ' t ) is used to damage a SAIU. l l le tastable argon can a lso be used to create a res is t on the substrate us ing contaminant vapor in the vacuum system.6 In pat tern ing a gold substrate by image t ransfer f rom a res is t o f SAIvI exposed to neutra l ces ium atorns, lve demonstrate a r le \v technique for neutra i a tom l i thography that uses a fundamenta l ly ' d i f ferent phy 's ica i mechanism to pat tern a res is t . L in l ike exper iments us ing the in ternal energy of metastable atoms ( -10 eV) to damage the res is t , the damage process for cesium is l ikely to be chemical because the energy stored in the thermally occupied states in the ground state hyperf ine manifold 1^,4 peY ) and the kinetic energy (-50 meV) together should be insuff icient to effect col l is ionai damage to the chemical bonds in the res is t ( ^ ,2 eV) .12 This resist-based technique potentiaily incorporates several advantages of resist systems for appl icat ion to neutra l -atom l i thographl , and h igh-resolut ion atomic detect ion: 1) Unlike direct deposit ion schemes that bui ld features up one atom at a t ime, the etching process used to transfer the pattern from cesium into gold can ampli fy the depth of the image, so a few monolayers of material can be used to make a tal ler structure (20 nm) in Iess t ime. 2) Conventional resists are thick (^,1 pm), and therefore would not be chemically sensitive to monolayer-level doses of neutral atoms. Because typical beams of neutral cesium are relat ively low in f lux (^,3 min monolayer t ime), the thinness of SAMs (-1.2 nm), and therefore thei r sensi t iv i ty to low doses of ces ium, rvas cr i t ica l to accomplishing exposures in real ist ic t ime scales. 3) I f the dose profi le of a feature inciudes some undesi red background, res is ts that exhib i t a min imum dose threshold for damage can be used to narrorv a feature and irnprove i ts contrast after image transfer. We observed evidence of such a threshold at a dose of ^,3 monolavers for mm-scale features formed in samples exposed to a broad cesium beam rvithout a mask. 4) I f the amount of gold remaining after etching depends l inearl l ' on dose,, and i f this l inearity is observabie at the nm scale, carefu l exposure of a SAl l l res is t in th is dosing regime could detect atomic densi ty rv i th nm-scale resolut ion: the re lat ive l ,v iner t pat terns created in the res is t or substrate could be imaged outs ide of vacuum using convent ional techniques of high-resolut ion microscopy. For mm-scale features,, there was a region, betrveen ̂ "3 and ^,7 monolayers of cesium, where such a l inear response lvas observed. Figure 1 shorvs the processing steps used in this experiment. 20-nm-thick gold f i lms were deposited by electron-beam evaporation onto a Si (100) substrate rvith a ^,2-nm-thick native oxide la-ver using 1.5 nm of Ti as an adhesion promoter. The SAlvls were formed on a solut ion of ̂ ,1 mN'I nonanethiol (CHr(CHr)rSH) in the gold bv overnight immersion in absolute ethanol ( t r toH ). After the several t imes rvith EIOH, and rvere samples \\ 'ere remove(l from solut ion, they \\ 'ere r insed subsequently dried using nitrogen gas rvith part iculates