SOFT X-RAYS FOR BIOLOGICAL AND INDUSTRIAL PATTERN REPLICATIONS

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SOFT X-RAYS FOR BIOLOGICAL AND

INDUSTRIAL PATTERN REPLICATIONS

E. Spiller, R. Feder, J. Topallan

To cite this version:

E. Spiller, R. Feder, J. Topallan. SOFT X-RAYS FOR BIOLOGICAL AND INDUSTRIAL

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JOURNAL DE PHYSIQUE Colloque C4, supplkment au no 7 , Tome 39, Juillet 1978, page C4-205

SOFT X-RAYS FOR BIOLOGICAL AND INDUSTRIAL PATTERN

REPLICATIONS

E. SPILLER, R. FEDER and J. TOPALIAN

IBM T. J. Watson Research Center, Yorktown Heights, New York 10598, U.S.A.

RCsumk. - L'utilisation de rayons-X de faible Cnergie pour la fabrication de microcircuits et pour la microscopic des specimens biologiques est passCe en revue. On a obtenu une rksolution qui approche les limites imposees par la diffraction. Les ameliorations rCcentes suggerent la possibilite de produire des ClCments focalisants de haute r6solution.

Abstract. - The use of soft X-rays for the fabrication of microcircuits and for the microscopy

of biological objects is reviewed. A resolution close to the X-ray diffraction limit has been obtained. Recent advances suggest that high resolution focusing elements might also be developed.

1. Introduction. - X-rays can produce images of higher resolution than visible light due to their shorter wavelength. The most successful and most widely used X-ray imaging technique is still contact X-ray micrography as originally introduced by Goby in 1913 [l]. Spears and Smith demonstrated in 1972 [2] that the same technique can also be used for the fabrication of electronic microcircuits and thus esta- blished X-ray lithography. As Goby's X-ray micrography, X-ray lithography is a two step process. X-rays provide only a 1 : 1 imaging, magnification or demagnification has to be provided by another method. Goby used an optical microscope for magnified viewing of the X-ray picture which was recorded on photographic film and was in this way limited in resolution. The first mask for X-ray lithography is fabricated by the scanning electron microscope [3] and this mask is replicated by X-rays to produce secondary masks and devices.

The organic polymer films (resists) which are used as the recording medium in X-ray lithography can have substantially higher resolution than photographic materials, and since the developed resist image is suitable for viewing in a scanning electron microscope, the high resolution of this instrument is now also available for X-ray contact microscopy and a resolution better than l00

W

becomes possible [4].

The development of focusing elements for X-rays with a similar resolution capability will require considerable work in the future. Grazing incidence mirror instruments are used extensively in X-ray astronomy but have technical difficulties in the fabrication of the elements and have not permitted a higher resolution than that obtainable with visible light [5-81.

One can hope that the high resolution fabrication

techniques of electron beam and X-ray lithography can be used to produce high resolution zone plates [9-121 as imaging devices. Another contender for high resolution imaging devices is a normal incidence mirror with a multilayer coating to give increased reflectivity [l 3, 141.

2. X-ray lithographic exposure system.

-

Figure 1 shows two simple arrangements for the replication of a mask on a wafer by X-rays. In figure l a X-rays are generated by electrons which are accelerated towards the water cooled target of a small commercially available electron gun [15]. Evaporation of the target limits the input power to about 500 W which corres- ponds to a total X-ray flux of about 10 mW. A resist of high resolution and low sensitivity requires an exposure of about l J/cm2 for

A

-, 10

A

(see section 4) ;

therefore an area of 1 cm2 can be exposed in about 100 S. Practical throughputs are somewhat lower

/ I ELECTRON TRAJECTORIES t AXIS OF a ROTATIONAL SYMMETRY

MASK WAFER MOUNT MOVING THROUGH EXPOSURE AREA

b FIG. 1. - X-ray exposure stations using a conventional X-ray source ( a ) or a synchrotron radiation source (6). The target material determines the wavelength in (a) a s for example P d ( i = 4.3

a).

AI(). = 8.3 A). Cu(1 = 13 A) o r C(,? = 44.8

A)

while the exposure

spectrum in (b) is determined by the electron energy.

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C4-206 E. SPILLER, R. FEDER A N D J. TOPALIAN

(about a factor 2-5) due to absorption losses in the mask, in possible vacuum windows and most important due to the fact that the wafer intercepts only a fraction of the total cone of emitted X-rays.

Two geometrical distortions prohibit bringing the mask wafer combination too close to the target in order to intercept a large &action of the emission cone. First, penumbra1 blurring is due to the finite size of the X-ray source and limits the resolution to

where s and D are the mask-wafer separation and distance from X-ray source respectively, and d, is the diameter of the X-ray source. Second, the non-normal incidence of the X-rays at the edge of the wafer produces a displacement (run out) 6, of the pattern given by

6, = stang O , (2) where O is the position dependent angle of incidence (see Fig. 1). Both distortions are reduced by increasing the distance between the mask wafer combination and the source. The resulting increase in exposure time does not have to decrease the throughput of the system if a correspondingly larger number of wafers is exposed simultaneously.

An exposure system which uses synchrotron radiation (Fig. 16) is in practice free from any geometrical distortion due to the good collimation of synchrotron radiation sources. Due to this collimation large wafers have to move through the exposure area to give a uniform exposure over the entire wafer. Exposure times are drastically shortened with synchrotron radiation sources [16, 171 and the throughput obtainable with a dedicated storage ring is presently much higher than any foreseeable need.

The optimum X-ray wavelength for the replication of a mask is determined by the available absorption values. An X-ray mask consists of an X-ray transparent substrate on which the circuit pattern is defined by an X-ray absorber (Fig. 2). The absorption coefficients of different materials extend only over a factor of 100 in the X-ray region. With the exception .

of the jumps at absorption edges, the absorption increases with increasing wavelength (Fig. 3). There- fore, the optimum wavelength selection is a compro- mise between the requirements to have a reasonably thick mask substrate as transparent as possible (shorter wavelength preferred) and the requirement to have the pattern as opaque as possible (longer wavelengths preferred).

WAVELENGTH X

(H)

FIG. 3. - Absorption coefficients of some selected materials in the soft X-ray region. Compounds of the light elements can serve as mask substrates, gold or other heavy materials are suitable to define the device pattern. PMMA (polymethylmethacrylate) is a widely used X-ray resist. Data obtained from the mass absorption coefficients of the elements. For nitrogen a density of I g/cm3 is assumed.

In the wavelength region ;l = 4-50

A

materials consisting of light elements are still transparent for thicknesses of several pm, and can be used as a mask substrate while heaw materials as for example gold attenuate soft X-rays sufficiently to provide enough contrast of the mask for thicknesses below l i m {see Fig. 3). The shorter wavelengths of this range are more convenient for many applications because thicker substrates and windows can be used. The longer wavelengths give higher mask contrast and are required for the replication of extremdy high resolution patterns made of thin absorbing materials.

3. X-ray resists and resolution. - All presently used resist materials are organic polymers which al, modified by the incident radiation in such a way that

FIG. 2. -X-ray mask of 3 cm dlall~eter fa~ricated by the vector-S-37. 1 subsequent development vrocess can distinguish

W

electron beam System [31 (a) and micrograph of a small detail (b). _{between unexposed and exposed regions removing}

The mask substrate is polyimide supported by a ring of silicon;

the absorber pattern consists of electroplated gold with a linewidth one and leaving the other intact. The remaining

of 1 um. (In collaboration with

_.

.. E. Castellani. L. Romankiw and resist then serves as a protective coating for the

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SOFT X-RAYS FOR BIOLOGICAL AND INDUSTRIAL PATTERN REPLICATIONS C4-207

areas are removed in the developer are called positive, those where the unexposed areas are removed are called negative resists. A positive resist produces a positive replica of a metal mask if the oldest processing step, chemical etching of a metal is used for the device fabrication. Additive fabrication processes like electroplating [l81 or the lift-off technique [36] reverse the polarity of a mask in the replication with positive resist.

The exposure of a resist film by X-rays is very similar to the exposure of the resist by an electron beam. An absorbed X-ray photon produces a shower of secondary electrons and these secondary electrons interact strongly with the resist material and are responsible for most of the chemical changes in the resist [2].

The effective range of these 'secondary electrons represents a limit for the resolution of a resist material. The highest resolution of all resists has been obtained with polymethylmethacrylate (PMMA) [19]. Figure 4

shows that the effective range of the secondary electrons generated by X-rays in PMMA increases with the photon energy [20]. Diffraction effects, on the other hand, are less limiting for higher photon energies and figure 4 shows that the highest resolution of about 50

A

can be expected with ultrasoft X-rays with wavelengths around ;1 = 50

A.

It is a very favorable coin- cidence that about the same wavelength region also gives the highest contrast for very high resolution replication.

-

5 5 U ) 4 w s W + 2 W a z U W -1 W W

c

g

W l,. l,. W l 1 ' ' I " " I I I - RESOLUTION

-

I 1 1 1 1 1 11 1 I 0.2 0 . 5 I 2 3 PHOTON ENERGY (keV)

FIG. 4. - EfTective range of secondary electrons generated in X-ray resist (PMMA) by X-ray photons of energy E and corresponding wavelength I, with the region of optimum resolution around the

crossover point of the two curves.

The sensitivity of a resist material is theoretically limited by the shot noise of the absorbed photons. PMMA is very close to this theoretical limit for wavelengths around ,l = 50

A

[21]. However, only a small fraction of the incident X-ray photons are

absorbed in a very thin film (thickness x resolution) of PMMA and less exposure would be required if one could increase the absorption for X-rays. Resists doped with heavy materials in order to increase the X-ray absorption have been described [22-241 and higher sensitivities have been obtained. However, all known doped resists have a considerably lower resolution than PMMA and have a much poorer performance than would be allowed by shot noise limitations.

4. Applications of X-ray lithography. - Numeroi~s laboratories are presently using X-ray lithography, but no commercial applications exist at the present time. The Smallest commercial microcircuits have a linewidth of 2.5 pm and it cannot be expected that further miniaturization approaching the resolution limits of X-ray liihography will occur within a short time. The complexity of most devices prohibits very fast progress. Together with a high resolution lithography, compatible high resolution processing steps have to be developed and circuit designs and materials have to be modified. The mechanical stability of masks and wafers may pose a serious limitation if large area patterns have to be overlayed with high precision.

In the laboratory X-ray lithography has been used' for the fabrication of acoustic surface wave devices, C2, 251, diffraction gratings [26] magnetic bubble 'devices [27-291 and silicon diodes and transistors,

[30, 311.

Magnetic bubble devices seemed very well suited for a first test of X-ray lithography because some of the circuits can be fabricated as single level devices eliminating the need for a precise alignment system [32]. The mask in figure 2 is the electron beam fabricated first mask for such a bubble device, where the pattern is defined by electroplated gold (Fig. 2b). Figure 5 is the resist pattern (PMMA) on a wafer obtained by the replication with AlKa(,l = 8.3

A)

X-rays. This resist pattern can serve as the masking

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C4-208 E. SPILLER, R. FEDER AND J. TOPALIAN

pattern for a subtractive processing step like ion milling [33-351 to fabricate the device. Magnetic bubble devices have also been produced with positive resists by using a mask of the opposite polarity than figure 2b and by using electroplating for the deposition of the permalloy structure in the device.

Figure 5 demonstrates one important advantage of X-rays over electron beams. X-rays are practically not scattered in resist and therefore patterns can be replicated in thick layers of resist with high aspect ratio. It would be very difficult to produce a resist pattern like that in figure 5 directly with an e-beam system [36, 371. The slight slope in the resist pattern of figure 5 is due to the low contrast of the electron- beam produced mask shown in figure 2 which allows for some exposure even under the opaque parts of the mask. This limits the dissolution rate ratio between unexposed and exposed resist areas in the copy; while the exposed areas are removed, the unexposed areas are also attacked from the sides such that the resist walls are slowly receeding during development. More vertical resist walls than those in figure 5 can and have been obtained by using either masks of higher contrast or higher exposure levels 1281. It is possible to improve the contrast of an X-ray mask by copying the e-beam fabricated mask with X-rays and then using the copy for the device fabrication. Bernacki and Smith [30] have demonstrated that X-ray lithography can be used for the fabrication of semiconductor devices and have shown that the X-ray induced damage in MOS devices can be completely repaired by annealing the devices at a temperature of 500 OC. Their data also suggest that X-ray lithography can produce a higher yield than photolithography which is expected because organic dust is very transparent to X-rays. A crude optical alignment technique has been used for their experiment with a typical align-

ment error of -( 1 pm. In principle optical alignment

techniques are able to produce an overlay with a precision which is much better than 1 000 and such a precision has also been demonstrated [38, 391. Systems which use X-rays for the alignment have also been discussed [40]. It is not clear at the present time 'over what area such a precision alignment can be obtained and how different processing steps affect the precision of an overlay.

A method which eliminates the overlay problem

has been proposed by Henderson and Pease [41] for electron beam exposure. In this method different mask levels are incorporated into one exposure as different intensity levels. In a first development process the resist is first opened for the highest exposure level for a first processing step. Subsequently a second development step opens the second exposure level for the second processing step and so on.

It is clear that this method requires tighter control of exposure and development than a multi-exposure method. In X-ray lithography exposure and development tolerances are usually very loose and it seems that the method is much more suitable for X-ray lithography than for electron beam lithography.

Figure 6 gives an example for the application of this technique. In a bubble memory two layers of metal- lization are used, a gold conductor for the switching and sensing circuits and a permalloy pattern to guide and sense the magnetic bubbles [42]. how eve^, under the magnetoresistive bubble sensor no gold should be deposited because this gold layer would represent a short for the sensor signal. Figure 6a shows in the mask the additional transmission values, represented by an additional thin film of gold covering the entire sensor area. After a first development the resist is completely removed in the open areas of the mask and gold can be plated in these areas (Fig. 6b). The

FIG. 6 . - X-ray mask with two transmission values in the open part G" the gold pattern ; full transmission in the lower part of the photo and partial transmission (upper part) produced by an additional thin film of gold (a). After exposure in a first development step the resist in the X-ray copy is only removed in the lower part for an electroplating of gold ; in a second development step the top part is also fully developed to allow electroplating of permalloy in both areas. Perfect alignment between the first and the second plating process can be

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SOFT X-RAYS FOR BIOLOGIC& AND INDUSTRIAL. PATTERN REPLICATIONS C%-209

second development opens the sensor area for the plating process and the entire pattern is now plated with permalloy.

5. X-ray microscopy of biological objects. - X-rays have been widely used for microscopy over the last 50 years although the resolution has not been signi- ficantly higher than that of the optical microscope [43,44]. With a resist like PMMA as recording medium a relief structure of an object can be obtained and this relief structure can be viewed in a scanning electron microscope (Fig. 7). Figure 8 shows as an example the resist relief obtained from a diatom with

CKa

(1

= 44.8

A)

radiation. Other objects investi- gated up to now are chromosomes [4], tissue cultures of human central nervous system turnors (in collaboration with L. Manueudis, Yale University), heart of chick embryo cells and retina sections [4, 451. Figure 9 demonstrates the resolution which has been obtained, the smallest feature visible in the micrograph have details with dimensions under 100

A

[4]. Many of the features which are made visible by

X-RAYS SPECIMEN

SUPPORTING

WAFER

I

1

FIG. 7. - Contact X-ray microscopy with a positive resist as a recording medium. The higher elevation in the developed resist image correspond to higher absorption values in the specimen. The developed resist is metallized and inspected in a scanning

electron microscope.

FIG. 9. - X-ray replica of melanin granule In the retina of an Ame- rican Bullfrog obtained with synchrotron radiation from DESY, Hamburg with an effective exposure spectrum from I = 25-44 A.

The smallest details have dimensions below 100 A (from Ref. [4]).

X-rays cannot be seen by any other microscopic technique and one can expect that X-rays will be widely used to complement the information which can be obtained by electron microscopy. X-rays can reveal the internal structure of a specimen much easier than electron beams ; in addition X-rays offer the potential of less radiation damage to the specimen [46].

6. Focusing elements for X-rays. - Soft X-rays can be focused by grazing incidence reflectors [5-81, zone plates [9-121 and by normal incidence reflectors

using multilayer coatings [13, 141. None of these focusing elements will probably ever reach a resolution which is better than that already obtained by contact X-ray microscopy with the scanning electron microscope for magnified viewing. The main payoff for X-ray focusing elements would be their use in a scanning X-ray microscope for biological applications [47]. Such an instrument, where a focused spot scans the object while an X-ray counter monitors the transmitted intensity (or the fluorescence) can produce considerably less radiation damage to the specimen than the contact method, with the resist as detector, for the same image fidelity. The reason is that the X-ray counter can practically count all transmitted photons while the very thin resist film required for high resolution absorbs only a small fraction of the incident photons and requires a correspondingly higher exposure. In addition, the scanning system can easily be buiIt as a quantitative measuring instrument. By storing the image information in a digital memory, the image is immediately available for digital image processing. An important example . -

8. replica in resist (PMMA) of a diatom obtained would be a measurement of the cal&m distribution

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C4-210 E. SPILLER, R. FEDER AND J. TOPALIAN

obtained with a wavelength just above and the other with a wavelength just below a calcium absorption edge (see Fig. 3).

Grazing incidence focusing devices are presently the workhorses in X-ray optics and have been extensively discussed [5-81; their image resolution capability is still poorer than that of optical instruments using visible light. Reflectors near normal incidence have smaller aberrations than grazing incidence instruments and the nearly spherical surfaces of normal incidence reflectors can be fabricated with much greater precision than the off-axis paraboloids or ellipsoids required for grazing incidence instruments. However, no reflector material has sufficiently high normal incidence reflectivity for wavelengths

a

<

200

A.

Theoretically a normal incidence reflectivity near R = 30

%

can be obtained at any wavelength in the X-ray region by using multilayer coatings consisting of alternating layers of high and of low absorption constants [14]. Figure 10a shows the increase in the

0.08 Au: n.0.829 k=0.193 C: nz0.948 k.0.02 0.06 THICKNESS nd

(a)

.-

FIG. 10. - Calculated reflectivity of a 9 layer coating for 1 = 200 A

at normal incidence as a function of the total thickness during deposition (a) and soft X-ray intensity distribution inside the

finished mirror (b).

reflectivity during the deposition of a 9 layer coating of gold and carbon calculated from published optical constants [48] and figure lob gives the intensity distribution inside the finished coating. The reflectivity of this coating has been maximized by positioning the strong gold absorber near the nodes of the standing wave thus minimizing the absorption losses. Haelbich and Kunz 1131 demonstrated first that carbon is a

9 LAYER Re W-C

FIG. 11. - Measured reflectivity of a 9 layer coating made of a ReW

alloy and carbon wavelength around 1 = 200 A.

is a measured reflectivity curve for a 9 hyer system with an amorphous ReW-alloy as the strong absorber and carbon as the spacer material, obtained in collaboration with Haelbich and Kunz. The measured peak reflectivity of 6

%

should be compared to a theoretical value of 8

%

calculated from optical constants of Re [49]. Tight control of the film deposition and supersmooth substrates are required to fabricate multilayer coatings for wavelengths below 100

A

; it appears, however, that these requirements can be met.

Zone plate lenses for soft X-rays have been produced holographically [l21 and by electron beam systems [9, 101 and have reached the resolution of the optical microscope [12]. One can hope that the present advances in the microfabrication technique will make the fabrication of zone plates of higher quality possible. Figure 12 is a section of an electron beam fabricated zone plate which has been copied by X-rays in PMMA to increase the thickness of the absorber

- -

suitable spacer layer for these rnultilayer coatings FIG. 12. -X-ray replica in resist of a section of a zone plate obtained

because it stable boundaries with all heavy _{first zone plate was made of 800}with carbon radiation (1 = 44.8 A). _A_{thick gold, the resist thickness}The electron beam fabricated has low for _{in the copy is 0.6 pm. (First zone plate courtesy of M. Hatzakis}

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SOFT X-RAYS FOR BIOLOGICAL AND INDUSTRIAL PATTERN REPLICATIONS C4-2 1 1

pattern for higher contrast. The large depth of focus order [26]. Considerable effort is still required to of X-ray lithography allows one to produce blazed proceed from. a p,hotograph like figure 12 to a finished zone plates with tilted zones for the concentration optical element with a resolution that matches the of most of the incident power into a single diffraction resolution of the pattern fabrication process.

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