Developing an undispersed VUV beamlinefor large area surface processing
Joachim Janes and Norbert Lutz
A versatile windowless beamline for undispersed VUV radiation in the energy range between 10 eV and 100 eV
is presented. An adjustable light guiding assembly with two glass capillary arrays as vacuum interfaces is
constructed to assure high optical transmission (30%) of white synchrotron light. Three differential
pumping stages reduce the pressure of a reaction gas from 1 mbar to 5 X 10-9 mbar. Estimate of the optical
transmission of the system are given. The beamline was designed to irradiate a large area of a semiconductor
surface for synchroton photon enhanced etching and deposition processes.
1. Introduction
The future fabrication of VLSI elements will requirestrong boundary conditions for the processes neededfor structuring surfaces.1 In particular, high direc-tionality of processes to improve the anisotropy ofetching processes is a vital requirement. The ratio ofvertical to lateral etching rates has to be large enoughto assure that the underetching of structures is smallerthan all structural deviations caused by lithography.Etching processes used for the structuring of semicon-ductor surfaces to produce VLSI elements consist ofthe many physical and chemical steps found in com-monly adopted dry etching plasmas.2
Particle excitation, e.g., electron impact ionization,as well as the absorption of photons in a reaction gas isresponsible for the production of reactive radicals orreactive ions which can reach the surface by any kind ofacceleration, e.g., diffusion or electric fields. On thesurface, those radicals or ions react with surface parti-cles, resulting in an ablation of the surface. In thispaper we present a VUV beamline for the use of syn-chrotron photon enhanced etching and deposition(SPEED) processes.
The production of reactive particles for etching pro-cesses is not limited to gas phase reactions. Photo-induced processes are better than than plasma pro-cesses to evoke etching reactions on the interfacebetween an adsorbed layer and the surface to be struc-tured. With respect to the enhancement of etch rates,the much higher particle density in an adsorbed layeris advantageous compared with gas phase reactions.
The authors are with Institute fur Mikrostrukturtechnik derFraunhofer-Gesellschaft, 53 Dillenburgerstrasse, 1000 Berlin 33,
In contrast to the commonly used plasma inducedstructuring processes, photo-induced reactions caneven be started by absorbing high energy photons inthe surface itself. The variety of these processesranges from photo stimulated desorption to core levelexcitation of surface atoms3 causing an enhancedchemical reaction rate for particle ablation from thesurface. The photo-induced or photo stimulated pro-cesses introduced above demand a photon energyrange extending from -1 eV to some 100 eV. Addi-tionally, the photon source should have a low diver-gence of emittance providing the directionality of pho-tons for highly anisotropic processes. Bothrequirements are simultaneously met in synchrotronsources.
11. Apparative Setup
The use of VUV synchrotron radiation for large areasurface reactions requires a special beamline to couplethe reaction chamber to the source which is in our case,the electron storage ring BESSY in Berlin.4 The pur-pose of this beamline is to guide VUV photons withoutany spectral differentiation with high optical trans-mission and 2-cm2 cross section onto a semiconductorsurface. Simultaneously, the beamline has to separatethe reaction chamber from the storage ring assuring asufficient pressure reduction from the wafer surface tothe ring.
In the storage ring a pressure of p < 10-8 mbar has tobe maintained to minimize Coulomb scattering of thestored electrons with the residual gas atoms.5 If thepressure in the ring rises to about 10-7 mbar, the stor-ing time of the electrons is reduced to only a fewminutes. An even higher pressure of p > 10-5 mbarwill lead to a break down of the ring because the iongetter pumps will refuse to operate. On the otherhand, pressures of the process gases in the vicinity ofthe wafer surface should be allowed to range up to 1mbar. Consequently, the beamline has to guarantee a
Fig. 3. Schematic diagram showing the relationships of pressure,conductance, and required pumping speed for the reaction chamberand the three pumping stages separated by two GCAs and the baffle
tube.
pressure reduction of eight powers of magnitude. Abeamline fulfilling the conditions described above canbe achieved by consecutive differential pumpingstages. In Fig. 1, an overview of the configuration ofthe beamline and the reaction chamber is shown. Thethree pumping stages and the reaction chamber pumpare demonstrated as well as the location of the gatevalves necessary for vacuum security. The other de-tails found in Fig. 1 are described below.
A. Vacuum Interfaces
Glass capillary arrays (GCAs) used as vacuum inter-faces assure efficient pressure reduction between twopumping stages.6 7 The GCAs disposed in the beam-line consists of an array of parallel capillaries in sodalime glass (Galileo Electro-Optics Corp.). The chan-nel diameter is 10 ,gm and the length of a channel is 1mm. A photography of the GCA surface taken with amicroscope is shown in Fig 2. As shown in this figurethe geometry of the pores is in general not circular andthe arrangement of the capillaries is not free of distor-tions. Nevertheless, all calculations and estimationsconcerning the GCA were made assuming an idealcircular geometry. The open area ratio of the GCA isabout 60%. Using convenient vacuum technology cal-culations, the conductance of this GCA with a 2-cm2
cross section is found to be 0.2 liter/sec.In Fig. 3 the arrangement of three differential
pumping stages to couple the reaction tube to theBESSY storage ring is shown schematically. In thereaction tube the pressure is 1 mbar. With a conduc-tance of 0.2 1/s through the GCA and a pumping speedof at least Seff = 500 liter/s a pressure of 4 X 10-4 mbaris attained in the first pumping stage. Separated byanother GCA with 0.2 liter/s conductance a pumpingspeed of at least Seff = 500 liter/s is necessary to reducethe pressure in the second pumping stage to 2 X 10-7mbar. The vacuum interface between second andthird pumping stages consists of a rectangular pipe oflength = 45 cm and 2-cm2 cross section serving as abaffle tube with a conductance of 2 liter/s. With aneffective pumping speed of at least 100 liter/s onereaches a final pressure of 5 X 10-9 mbar, which issufficient for vacuum stability in the storage ring.
B. System Description
The fluctuations of the plane of the electron orbit inthe ring requires an optimization of the optical axis ofthe beamline with respect to the momentary directionof the synchrotron radiation. The optical axis of thesystem is defined by the alignment of the channels ofthe two GCAs and the center of the baffle tube. Aconvenient adjustment of the optical axis is made pos-sible by connecting these elements to a rigid intrava-cuum light guiding assembly (LGA). This LGA isinstalled in the vacuum chambers with an externalaccess for adjustment to assure optimal transmissionof the synchrotron radiation.
Fig. 4. Schematic of the intravacuum light guiding assembly(LGA). On the left hand side, the position of the wafer relative tothe open front of the reaction tube is shown. The adjustment of the
wafer with the micromanipulator is indicated by arrows.
The modular setup of the LGA is shown in Fig. 4.The synchrotron radiation enters the aperture of thebaffle tube. The perforated cylinder and tube allows agas flow from the LGA to the outer vacuum chambersand to the pumps. These gas outlets are separated bya flange including a GCA. Also mounted in a flange,the second GCA separates the reaction tube from theLGA. The front side of the reaction tube is open witha diameter of 50 mm. Facing it, the wafer stands -0.1mm away. A gate valve is installed to maintain thevacuum security of the storage ring. The LGA is nest-ed in the vacuum chambers by two flanges besides thegate valve and a flexible bellows.
The combination of the LGA and the outer vacuumchambers is shown in Fig. 5. Mounted on an x-ymicromanipulator with a rotary vacuum leadthrough,the aperture of the LGA is adjustable normal to thedirection of the synchrotron radiation. A flexibleleadthrough between the first and second pumpingstages is given by a bellows. Between the first pump-ing stage and the reaction chamber the LGA comes outof the vacuum chambers and is sealed from the atmo-sphere with a set of bellows. In this way externalangular adjustment of the optical axis of the lightguiding assembly is possible.
The gate valve in the LGA keeps the pumping stagesevacuated during service ventilation of the reactionchamber. The first and second pumping stages areseparated by a bellows and the GCA inside the LGA.Both pumping stages are equipped with turbomolecu-lar pumps with a pumping speed of 1000 liter/s (Ley-bold Turbovac 1000 C). In the same way the separa-tion between the second and third stages is controlledby the baffle tube and a small bellows inside the x-ymicromanipulator. Two gate valves separate thethird pumping stage from the beamline and the ring.Because of the low base pressure in the third pumpingstage, an ion getter pump with 400 liter/s (Leybold IZ400) guarantees secure vacuum conditions for couplingthe beamline to the electron storage ring BESSY.Equipped with a turbomolecular pump with a pump-ing speed of 1500 liter/s (Leybold Turbovac 1500 C)the reaction chamber contains the reaction tube andthe wafer, and it offers the possibility for experimentalinvestigations, e.g., mass spectroscopy or photon fluxmeasurements.
C. Reaction Tube
Fed into the reaction tube and separated from thepumping stages by the GCA, the process gas flow is
2IH]Ji K> Fig. 5. Schematic of the beamline. The locationVjU of the light guiding assembly embedded in the out-
er vacuum chambers is shown. The VUV radiation
comes in on the right hand side of the beamline.
go
directed onto the wafer surface. Unused process gasand reaction products can leave the reaction columnthrough the slit between the reaction tube and thewafer. The wafer is mounted on a high precision ma-nipulator giving rise to the exact positioning of thewafer in front of the open side of the reaction tube.Regulating the distance between wafer and reactiontube varies the ratio of conductance of the slit and theGCA. This configuration regulates the process gasflow so that the main part of the gas leaves the reactiontube via the slit between wafer and tube, thus support-ing the dynamic flow of reactive components towardthe wafer surface. Figure 6 shows the reaction tubewhere the process gas supply is done by a toroidal tube.Channels with a diameter of about 200 um in thetoroidal tube feed in the process gas directly towardthe wafer surface; these channels can easily be built asnozzles. A molecular jet can be directed onto thewafer surface, reducing the pressure in the reactiontube with the distance variation between wafer andtube. Depending on the heat capacities and tempera-ture differences, the process gas can be used as a medi-um for transporting process heat from the wafer sur-face.
GAS SUPPLY _
WAFERX \ \\X~~~~ R~EACTION TUBE
\ XV -~~ADJUSTABLE SLIT
Fig. 6. Schematic of the reaction tube. The location of the torodialnozzle tube for gas supply is shown. Facing the open front of the
reaction tube, the wafer is positioned with a micromanipulatorwhose directions of adjustment are indicated by arrows.
The optical transmission of the beamline describedabove is determined by two factors. First, intensityreductions caused by the purely geometric design ofconsecutive GCAs has to be taken into account. Esti-mates are performed assuming the channels to be sta-tistically distributed over the surface of the GCA. Us-ing consecutive GCAs, each with an open area of about60%, the total transmission of the system is reduced toabout 40%. On the other hand, intensity losses bydiffraction have to be considered as an important con-tribution to photon flux reduction. The diffractionintensity losses, which strongly depend on the GCApore size, can be split into two: The first is an innerchannel mechanism caused by the entrance aperture ofeach channel. Diffracted toward the walls a part ofthe radiation is absorbed in the GCA material. Thesecond mechanism results from the consecutive ar-rangement of the GCAs. The divergence of the radia-tion diffracted in the first GCA leads to absorptionbecause of the limited angular acceptance of the chan-nels of the second GCA.
The inner channel intensity losses are estimatedusing a calculation based on the Huygens principle tocompute intensity distributions of diffracted lightwith elementary waves. Using Kirchhoffs approxi-mation8 the amplitude of a electromagnetic wave in anarbitrary point P is described by:
-ik (eikrb(P) =-2 I-cos(h - )v ds. (1)
The meaning of parameters and variables occuring inEq. (1) is easily found in Fig. 7 where the situation at asingle capillary is considered. For the evaluation ofEq. (1) the distance r is usually expanded into a series.Depending on how many terms of this expansion aretaken into account one receives the Fraunhofer orFresnel approximation of the diffraction problem.Based on the Fresnel approximation a structural fac-tor9 can be defined to predict the type of diffraction
APERTUREAREA S)
plane wave v P
wave veca oor
Fig. 7. Ray diagram showing the parameters and variables neces-sary to calculate the diffraction patterns by a circular capillaryaperture irradiated by a plane wave v with wave vector . A capil-
lary with length I and aperture area S is indicated.
Table I. Structural Factors for Different VUV Wavelength and ApertureDiameters
X lOO m 30 m 15n nd
8 IL 1.1 2.1 2.9
10 L 1.4 2.6 3.6
12 ju 1.7 3.1 4.4
20 2.8 5.2 7.3
pattern to be expected in certain geometric conditions.The structural factor is given by
(2)
where d is the linear dimension of the diffracting aper-ture, X the wavelength, and I the distance between theaperture and the plane of observation, which is in ourcase the length of the capillary. For u < 1.0 a far fielddiffraction pattern and for u > 1.0 a near field diffrac-tion pattern can be expected, respectively.9 In TableI, the structural factors are listed for different VUVwavelengths and aperture diameters with a 1-mm cap-illary length. For GCAs with a 10 Am capillary diame-ter used in our LGA one can see in Table I that, in therange of X = 10-100 nm, a near field approximation hasto be used to calculate inner channel absorption losses.
As an example, the intensity distribution of VUVlight (X = 509 nm) diffracted by the entrance apertureof a 10 m capillary is shown in Fig. 8. Two parallellines indicate the diameter of the capillary. Lightintensities diffracted to the inner capillary wall will beeither absorbed or reflected. The wavelength depen-dent part of light diffracted onto the walls normalizedto the incident intensity is given in Fig. 9 for differentcapillary diameters; the capillary length is 1 mm. Inthe 10-gm diameter capillaries of the GCAs used in ourLGA the amount of light diffracted onto the wallsarises from 16% at X = 15 nm to 55% at X = 100 nm. Inthis wavelength range the diffraction toward the innerwall of the capillary leads to a nearly grazing angle ofincidence. The coefficient of this reflection is -80%resulting in an effective absorption of 3% and 10%, for X= 15 nm and X = 100 nm, respectively. As seen in Fig.9, GCAs with pore diameters d > 20 m exhibit ratherlow absorption losses in the depicted wavelengthrange. On the other hand, pore diameters d < 8 Amlead to wavelength dependent absorption losses thatcannot be neglected, giving rise to a spectrally differen-tiated transmission. In particular, it is necessary tofind a compromise between optical transmission andgas flow conductance for individual applications metby the user.
Fig. 8. Normalized intensity distribution of diffracted light by a
circular aperture on a line perpendicular to the axis of the capillary;
aperture diameter is 10 ,m. Depending on the capillary length, thedistance of the aperture to the plane of observation is 1 mm. Thephoton wavelength is 50 am. Two parallel lines indicate the inner
wall of the capillary.
0 ooooooo 0o
oO COOID _
0ugng S0SRT
To estimate intensity losses contributed by the sec-ond mechanism mentioned earlier in this section, onehas to calculate the Fraunhofer diffraction pattern ofthe first GCA. In this far field approximation thecapillaries of GCA are assumed to be apertures, theirlengths being neglected. If the apertures are regularlydistributed, the diffraction pattern must be calculatedby a superimposition of elementary waves from allapertures. On the other hand, a purely statisticaldistribution of identical apertures will result in aFraunhofer diffraction pattern analogous to that of asingle aperture. 8 The design of our GCAs produces adiffraction pattern composed of both marginal cases.Using the statistical approach, the angles of diffractionof this intensity pattern are estimated basing onFraunhofer diffraction of a circular aperture with a 10-,gm diameter. The angle of diffraction of the firstintensity minimum for VUV photons with 12-eV ener-gy is calculated to be 12 mrad. At this photon energythe central intensity maximum produces a spot on thesurface of the second GCA with a diameter of 14 mm.Intensities of maxima of higher order than zero aresmall compared with the central maximum. The an-gular acceptance of the channels is 10 mrad; thatmeans that nearly all diffraction angles found in thecentral maximum are accepted by the second GCA.Thus we assume the intensity losses caused by the
Phton Energy (V)
Fig. 9. Wavelength dependence of the transmitted intensity nor-malized to the initial intensity on the aperture for capillaries with
length 1 = 1 mm and for four different aperture diameters.
,.h Fig. 10. Examples of SPEED processes for semi-
conductor surface structuring: (a) gas phase reac-tions; (b) processes in adsorbed layers; (c) absorp-
tion in bulk material.
limited angular acceptance of the second GCA lie inthe same order of magnitude as the inner channellosses discussed above.
Taking into account the inner channel absorptionlosses in the second GCA, we calculate a total transmis-sion of VUV photons of about 33% at X = 15 nm and26% at X = 100 nm.
IV. Uses
The beamline is assumed to stimulate SPEED pro-cesses on semiconductor surfaces.10 In Fig. 10 threepossible interactions of photons with gas phase parti-cles, adsorbed layers, and the surface itself are shown.The typical photon flux of an undulator in the energyrange between 10 eV and 100 eV of the storage ringBESSY II, in Berlin, is -1014 Phot/(s X 0.1% BW).Therefore, the undulator is an ideal candidate to in-duce SPEED processes. If, for example, the reactiongas over the surface were CF4, the ionization potentialof the lowest molecular orbital fi is 16.2 eV"t and themaximum ionization cross section is 24 Mb at a photonenergy of about 30 eV. Although these values do notallow an estimate of photo-induced ion assisted etchrates, they nevertheless demonstrate that gas phaseprocesses need high photon fluxes to yield ion densitiesfound in commonly used RIE plasmas.2 ComparableSPEED processes running on Cl2have been performed
successfully with synchrotron radiation from a 750-MeV storage ring.'2 Even photoinduced ablation ofphotoresists using synchrotron radiation is discussedas a future application.'3 Photoinduced processes fol-lowed by chemical reactions in an adsorbed layer on asurface might play a major role in future wafer process-ing. In particular deposition of metallic compoundsfor metallization processes, e.g., filling of contact holes,can turn out to be an improvement to plasma inducedprocesses. Although fast energy dissipation will leadto a heating of the semiconductor surfaces, the absorp-tion of photons in semiconductor bulk material canalso result in inner shell excitations of surface atoms.In which way these excitations could enhance a chemi-cal reaction rate with reactive particles with respect toan improvement of etch rates is a matter of futureresearch.
SPEED processes seem to be a promising alternativeto laser-induced wafer processing'4 which is consid-ered to be a gentle fabrication technique for semicon-ductor materials.15 However, photochemical surfacereactions with lasers have to deal with limited resolu-tions caused by diffraction as the pattern size de-creases.' 6 These diffraction effects can be overcomeby the use of shorter wavelengths, such as synchrotronradiation. Diffraction by long wavelength photons inwhite VUV sources can be limited by using appropriatepore sizes of GCA channels. The future developmentof high flux and high brilliance sources of VUV radia-tion'0 and a beamline allowing photochemical surfacereactions of combined processes, e.g., dissociation andionization and inner shell excitation of gas phase andsurface particles, could lead to structuring rates ofsemiconductors comparable with commonly used reac-tors.
V. Conclusion
Because of its versatility the beamline described inthis paper may provide an attractive alternative forVUV processes to present etching and depositionmethods for metal semiconductors and insulators.Further experimental investigations should revealwhether the experimental conditions are an improve-ment for the etching and deposition rates and thestructural quantities demanded in microelectronic en-gineering.
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The authors would like to acknowledge W. Henkefor his helpful discussions and instructive commentson the theoretical parts of the work.
This work was supported in part by the FederalDepartment of Research and Technology of the Feder-al Republic of Germany.
