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Ion and plasma assisted etching of holographic gratings
Vacuum/volume Printed in Great
36/numbers Britain
1-3/pages
Ion and plasma gratings D A Darbyshire,
A P Overbury
University
London,
College
55 to 60/l
986
assisted
0042-207X/86$3.00 Pergamon
etching
and C W
Torrington
+ .OO Press Ltd
of holographic
Pitt, Department of Electronic Place, London WClE 7JE, UK
and Electrical
Engineering,
Fine geometry grating patterns of <0.3pm periodicity may be produced in photoresist by optical interference methods. Although wet chemical etching can be used to transfer the interferometric grating pattern into the substrate, this process tends to degrade the grating geometry by undercutting and by solution saturation effects. Ion beam milling and reactive ion plasma etching have proved effective in transferring photoresist patterns into the underlying solid substrate with reduced aberrations. Freon- 14 (CF,! in a plasma has been utilized to etch gratings in soda-lime glass and lithium niobate (LiNbOs). Carbon tetrachloride (Ccl,) plasmas have similarly been used to transfer gratings into aluminium thin films on the substrate. The Al film pattern is then converted into an Al oxide mask by either milling in a 20% 0,/80% Ar ion beam or by reactive ion etching in the presence of oxygen. The oxide acted as an in-contact mask with enhanced lifetime so that deeper gratings were transferred into the substrate by subsequent ion etching.
1. Introduction Laser interference (holographic) techniques are used in many cases where periodic patterns, often gratings, are required. Typically the intensity distribution across the interference plane of two intersecting laser beams may be recorded by a photoresist coating on a substrate. The photoresist, when developed, will contain a modulated surface relief which may then be used as the grating. This photoresist pattern is however, somewhat fragile and may be far from ideally suited to the intended application. It is, therefore, often necessary to transfer the pattern into the substrate by an etching process. Recent attention has been focused on establishing dry etching processes suitable for etching a range of materials for both electronic and optical device fabrication’,‘. These techniques are now being used to produce high component density structures which utilize fine geometry patterns, which are in several respects similar to the interference gratings described above. We have developed an improved procedure for the etching of fine-geometry patterns with sub-micron periodicity, the overall process necessitating the careful etching of polymer photoresist, Al thin films, soda-lime glass and LiNbO,, while maintaining a high degree of selectivity between mask and substrate. There is considerable previously reported work on etching most of these materials; several of the earlier experiments have been followed up in some detail 3-5.1 5 and incorporated in the new process. However, other materials such as LiNbO,, have not been extensively examined; we feel that the capability to etch these newer materials may enable the fabrication of devices suitable for integrated optics. This paper presents our initial attempts at etching LiNbO,, together with photoresist, Al and glass, and describes their use in
producing integrated
useful interferometric process strategy.
gratings
by means
of an
2. The production of gratings by interference techniques We have previously explored the use of gratings as guided wave beam expanders for integrated optic device&‘, particularly in the acousto-optic spectrum analyser. In this device it is necessary to record the interference pattern on an optically waveguiding substrate (titanium indiffused LiNbO,), by means of a resist layer. The pattern is then etched through the resist mask into the substrate so that the guided optical wave may interact with the grating. For our applications, the parameters of most importance are the grating pitch, p, and the amplitude of the grating pattern when transferred into the substrate, d,. The interference pattern produces a grating of varying amplitude, d,, in the photoresist, and we need some control over the grating profile during etching. There are many different arrangements suitable for recording the interference gratingsg. We have selected the arrangement in Figure 1, a design which uses the substrate itself as one of the interferometer mirrors, and so has to be metallized before being coated with photoresist (Figure 2). A major advantage of this arrangement is that the pattern is independent of the substrate refractive index; however, because there is no length symmetry in the interferometer arms the laser must have a sufficiently long coherence length. We use an Ar ion laser running in the violet (457.9 nm) with a Fabry-Perot etalon in the cavity to select a single longitudinal mode and increase the coherence length. The laser operates in the TEM,, mode and the beam is expanded x 10 and filtered with a 10 Grn dia pinhole; after expansion the effects of 55
DA
Darbyshlre,
A P Overbuy
and C W Pitt:
Reflecting
Ion and plasma
assisted
etchmg
of holographrc
gratmgs
Mirror
Metallised Substrate ,’
Collimating
/’
Resist
With Coating
Expanding
Mirror
Lens ~~~I:==++
~
Argon
457.9 nm Laser Beam
O
Pinhole
Figure I. Interferometer
used for recordmg
grating
Figure 2. Grating
Resist
800
1000
(‘ontours
armulation of’ intcrferencc of constant intensit)
pattern
for 53X nm prtch
Exposure
And
After
Etching
replication
process.
‘j
Development
the Gaussian intensity distribution are minimized, giving a relatively uniform pattern across the substrate. This degree of uniformity is needed in order to assess the etching techniques used. In any interference pattern the intensity of illumination in the depth ofthe photoresist layer is a function of the distance from the substrateiphotoresist interface. This may be appreciated by considering that not only the two incident beams take part in the interference, but also the two beams reflected from the substrate: photoresist interface”, and all of the beams interact to form a vertical standing wave in the photoresist layer. The effect of this standing wave is that the light intensity in the photoresist is a periodic function with depth. It would be ideal if the photoresist received a uniform exposure right down to the substrate. In this case the photoresist grating could be developed to its full depth. and would then act as a direct mask for etching the metal layer. However, the metal layer imposes a standing wave intensity minimum at the substrate/photoresist interface, and so the pattern is unlikely to develop down to the surface of the substrate (the metal thin film in this case). Hence, the depth modulation in the developed photoresist is superposed on a layer of undeveloped photoresist. Further, the substrate-normal intensity of the standing wave pattern at the surface of the photoresist has to be considered. Using a computer simulation lo, we have predicted the form of the standing wave pattern. It may be seen (Figure 3) that unless the photoresist depth is chosen carefully. the surface of the 56
600
Along Substrate
I
I
After
grating.
Exposure
400
nm
patterns.
Figure 3. Computer Before
200
Distance
photoresist receives no exposure and so development is inhibited. It is not clear whether the standing wave minima in the photoresist can totally block the development of the pattern under all conditions, but we have observed that the photoresist development or erosion technique employed is important in transferring the lightly modulated pattern through the undeveloped photoresist layer to the metallization. Following exposure. differential-rate etching techniques are then employed to etch the pattern into the metal which may then be used as a mask for etching into the substrate. Obviously the particular metal chosen for the substrate metallization vvill affect not only the photoresist grating pattern, but also the etch characteristics of the system. We have chosen Al for our needs because it has high reflectivity, and the oxide has a low sputter yield during reactive ion beam etching; by judicious selection of the photoresist and metallization parameters, and by optimizing the etching technique, the transfer of the pattern into the substrate may be enhanced both in depth and in uniformity of profile. 3. Etching experiments Three processes have been examined for transferring gratings from photoresist to optical waveguiding layer: (I) using ion beam milling (IBM) and reactive ion beam etching (RIBE); (7) by ion assisted plasma etching, also termed reactive ion etching (RIE) and (3) by a combination of RIE and RIBE. 3.1. lon beam milling and reactive ion beam etching. Ion beam milling and reactive ion beam etching were performed in a VEECO Microetch system (Figure 4(a)) working at ion energies of 250 eVl.0 keV and beam current densities in the range 0.25 mA cm 2 to 1.0 mA cm ‘. These parameters. together with vacuum base pressure down to 3 x 1O- ’ Pa, provide a contamination free environment and a precisely controlled ion milling action over an effective beam diameter of 8 cm. The non-uniformity of etching associated with beam inhomogeneity has been eradicated by using a water-cooled, rotating substrate table; the table and substrate may be inclined with respect to the beam”. A shallow grating, i.e. an exposed photoresist layer with depth of modulation small compared with the thickness of photoresist was completely developed by using a 400 eV, 0.4 mA cm ’ Ar beam. The low energy and current density was selected to avoid photoresist degradation due to thermal effects. The appearance of the Shipley Microposit 1350 positive photoresist pattern at this
D A Darbyshire, A P Overbury and C W Pitt; Ion and plasma
assisted
gas inlet
I
anode
I
I
Figure 4(a). Schematic system.
representation
of the micro-etch
pumping
ion beam milling
gas/
vapour inlet
upper
electrode II
u
cylindrical ---Pyrex chamber. electrode
I
vacuum
pumping I
Figure 4(b). Planar
reactor
chamber
configured
of holographic
gratings
OJAr gas mixture, the metal film grating was converted into an Al,O, mask giving greatly enhanced mask lifetime as a result of the lower sputter yield of the oxide. The milling of the glass or LiNbO, was maintained to the point where the AI,O, mask was completely removed. The results are reported in Section 4.
heated spiral I cathode
vacuum
etching
3.2. Ion assisted plasma etching. The reactive ion etching experiments were conducted in a Plasma Technology PE80 planar etching system shown schematically in Figure 4(b). The reactor comprises two parallel, anodized Al electrodes 24 cm in diameter, the upper electrode being grounded. Substrates were positioned on the lower electrode which is coupled to a 13.56 MHz rf generator through an impedance matching network. Experiments were performed with electrode spacings of 336 cm and operating gas/vapour pressures of 2.&14.0 Pa, to establish etch rates for Microposit 1350 photoresist, Al films, glass and LiNbO,. The plasma etch rate of ca 1 pm thick photoresist layers spincoated onto Al film on glass, was determined for varying reactor parameters using Ar, 0, and ratios of0, to Ar of 1: 1 and 2: 3. The etch rate tests performed on the Al films used Ar bubbled through liquid Ccl,. The substrate table was heated to 55+ 5°C. Pre-mixing a carrier gas, such as Ar, with Ccl, vapour prior to entering the system presents the additional problem ofcontrolling the ratio of Ar to Ccl,. For adequate process control the ratio of Ccl,: Ar must be held to better than & 5%. Having achieved transfer of the grating into the metal film, the chemical reactivity of CF, with glass and LiNbO, was utilized to impress the grating pattern from the metal film into the substrate. The basic plasma etching gas was CF, mixed with l-10% 0,; each gas was introduced through a separate line and the flowrate monitored by means of rotameters. The electrode spacing and substrate temperature were fixed at 4 cm and 25°C respectively. 3.3. End point detection. A laser end-point detection instrument has been incorporated into the RIE system in order to monitor the dry development of the photoresist, and the etching of the Al film prior to transferring the grating to the substrate. In principle, it relies on the change in grating diffraction efficiency observed having etched through one layer and then encountering a second layer. The procedure adopted, as shown in Figure 5, is as follows.
r.f. generator
for reactive ion etching.
stage, is that of stripes of photoresist some several hundred Angstroms thick, approximately 0.34.4 pm wide and separated by stripes of exposed metal film of similar width. The ion beam was then accelerated to a relatively high energy (750 eV) for short duration (2&30 s), to remove the native oxide layer present on the Al film surface. The beam energy and current density were then again reduced to 400 eV and 0.4 mA cm- *, respectively, to avoid excessive heating and to improve the etch-rate selectivity between photoresist and the Al thin film. The Al film was etched to the point of total photoresist mask removal, by which time the photoresist grating had been transferred into the metal film completely. The third stage of the process, the transfer of the metal grating into the optical guiding layer in the substrate, depends on the substrate material. For etching glass and LiNbO, using the Al mask, experiments were conducted using both Ar and O,/Ar beams and the substrate gratings were compared. By using the
I 3 mW He-Ne
laser
Figure 5. Laser end point detection 57
D A Darbyshire,
A P Overbury
and C W Pitt: Ion and plasma
assisted
etching
of holographic
gratings BEAM
A laser beam from a 3 mW He-Ne laser is directed into the chamber and onto the surface of the grating device which is positioned so as to extract a first-order diffracted beam. The power intensity of the forward-reflected and first-order-diffracted beams emerging from the chamber can be measured by positioning photodetectors in the path of each beam. Upon etching down to the interface between two adjacent layers a rather large increase in diffraction efficiency can be observed. resulting in an increased power reading for the first-order-diffracted beam and a corresponding reduction for the forward-reflected beam. The point of maximum diffraction efficiency indicates that the process parameters may be changed so as to etch the lower layer. The new process continues until the next layer interface is reached and so on.
CURRENT
DENSITY
:
PRESSURE
X
SODA-
ImA cm-zJ
1.5 x10-*
LIME
Pa
GLASS
4. Results and discussion The ion milling rates for Microposit
1350 photoresist,
Al films.
energy and beam current density are shown in Figure 6(a). For ion beam energies and current densities of 500 eV and 0.5 mA cm ’ respectively, the selectivity is found to be almost unity for the photoresist. Al, glass and LiNbO,. This poor selectivity becomes further exaggerated at higher ion energies and current densities. for instance, the selectivity of a photoresist mask on Al at I keV beam energy is only about 0.75. The implication is that photoresist and Al films are less attractive as masking materials for glass and LiNbO,. The data on RIBE with an 02/Ar gas mixture, as illustrated in Figure 6(b), indicate an improvement in selectivity at all energies when using Al,O, (Al film converted to oxide mask) as compared with Al or photoresist (Figure 6(a)). At 750 eV. the selectivity of AI,O, on glass reaches about 5. a significant improvement compared with the other masks. For the RIE process, Figures 7(a) and 7(b) show the variation in soda-lime
glass
and LiNbO,,
BEAM
as a function
CURREN:
DENSITY
of Ar ion beam
ION
BEAM
ENt R(,‘l
ir~v)
Figure 6(b). Reactive ion heam etch rates l’or pIas
LiNhO,
and Al,0 I 111
20X, 0, ‘X0”,;,Ar. erosion rate of Microposit I350 photoresist with gas prcssurc and feed gas composition respectively, using Ar. 0: and Ar’O, mixtures. It is evident that by varying the O2 to AI- etchant gas ratio. a controllable erosion can be achieved in situations where the depth of optical exposure:development modulation in the photoresist layer is shallow compared to the total thickness of the photoresist.
ImA cme21
1 o MICROPOSIT
1350
D ALUMINIUM X SODA-LIME
GLASS
RESIST
700.
-7 600.G E “4 500-
0,IAr
Ratio
1.1
: lx 4005 5 300 -
B
100 t
100
,/,
,
, /,
.5
0
ION
BEAM
,
ENERGY
0
(KeV)
Figure 6(a). Ion beam milling etch rates for photoresist, film. glass and LiNbO, in argon. 58
I” J
1.0
3 GAS
aluminium
thin
6 PRESSURE
Figure 7(a). Erosion rate of Microposit plasma etching.
Parenthesis I#
9
12
I
15
(Pal
1350photoresist
using ion assisted
DA
Darbyshire,
A P Overbury
and C W Pitt:
Ion and plasma assisted etching of holographic gratings
RX
0.2 wcm-2
LiNbO.
-
100
R.F.
0.1 wcln-2
8
0 100 % 02 FEED
50%
o2
50%
Ar
GAS
10 0 % Ar
on the etch rate of photoresist
300
250
^ ‘i .;
MICROPOSIT
200
1350
ml w
150
2 z =
100
50 R.F.
0’
0.1 WC,+ I
3
6
9
GAS I VAPOUR
Figure 7(c). Etch rates in a CClJAr
PRESSURE
0
0 3
1
12
15
1PO 1
plasma.
Selective etching of Al by RIE was used to transfer the finegeometry grating patterns of ~0.3 pm periodicity from the photoresist mask. Figure 7(c) shows the differential etching rates achievable by processing the grating with the relatively slow etch rates caused by using a large electrode spacing (6 cm), reduced rf power density (0.1 W cm-‘) and low substrate temperature (20°C). The combined effect of these measures is to reduce thermal
_
A’203 9
6
GAS PRESSURE
Figure 7(d). Etch rates in a CFJO,
COMPOSITION
Figure 7(b). Effect of feed gas composition using ion assisted plasma etching.
I
12
15
(Pai
plasma.
degradation of the photoresist layer, thus prolonging the mask lifetime, while maintaining the Al etch rate. However, other measures are necessary to further improve selectivity. The reasons for poor selectivity are twofold: (i) the introduction of Ar, used as the carrier gas for CCI, vapour, increases the sputter rate of photoresist and (ii) the native oxide layer present at the Al surface initially impedes the chemical reaction. An effective means of removing the oxide layer is by sputter etching. Although N, could be employed to reduce the sputter rate of resist, it is too slow in eroding the Al oxide. Experiments were conducted using Ar for short durations, until the oxide had been removed, then N, in place of Ar to maximize resist lifetime. The use of O,, at this stage of the process, to remove carbon from the plasma, thus supplying additional chlorine to the reaction with Al, must be avoided for two reasons; firstly, it is evident that any 0, present seriously degrades the photoresist mask (illustrated by the results in Figure 7(b)) and secondly, it converts the Al thin film to an oxide, actually impeding further chemical etching. However, H,, for example, may be used in place of 0,. Having formed the Al mask on the surface of the substrate, the conversion to an Al oxide mask by the introduction of 0, into the plasma is now very beneficial. The improved selectivity achieved by etching glass and LiNbO, with Al,O, masking is displayed in Figure 7(d). This oxide film is reformed continuously at the surface of the Al layer by the 0, content in the CF,+S%O, mixture used in the etching plasma. In addition to the enhanced mask lifetime, there is evidence that a plasma gas comprising 0, mixed with CF, has two other advantages: (a) the 0, effects the removal of carbon from the plasma reaction byproducts creating a higher concentration of free fluorine for the reaction with glass and LiNbO, (refs 12, 13), and (b) the 0, also partially overcomes the problem of carbon-rich film growth during etching. The production of carbon layers in carbon-compound plasmas and its role in impeding the reaction between the Halogens and the substrate material is well knownr6. 59
D A Darbyshire,
A P Overbury
and C W Pift:
Ion and plasma
assisted
5. Conclusions The work on etching Microposit 1350 photoresist is directed towards a totally dry plasma development process for optically exposed photoresist films. To date the capability to control the etch rate, and the noted difference in etch rates between exposed and unexposed photoresist, suggest that such a process is within sight. Etch rate tests conducted on single material films. by the techniques described in the preceding sections. reveal the usefulness of employing a dual process incorporating both ion milling and RIE. It was found that in some material systems RIE had advantages over milling techniques, mainly as a result of the improved selectivity. Results obtained by combining the use of RIE to plasma etch Al through a photoresist mask with RTBE to etch glass and LiNbO,, have proved particularly encouraging. The etch rates of glass and LiNbO, using plasmas of CF, with 0, and for the system parameters chosen, were slow by comparison with ion milling. Other fluorine based gas etchants, such as C,F,, CHF, and SF, together with a wider range of process parameters are currently being investigated to improve the etching rate while maintaining a high degree of selectivity. The optical gratings produced by these processes have been incorporated into devices such as guided wave beam expanders” and have exhibited much improved performance compared with devices manufactured by the processes previously used14.
60
etching
of holographic
gratmgs
The underlying strategy that of using a sub-micron geometry device, the interference grating, as the target device for the process research has produced a further benefit. The developed process operates entirely satisfactorily for super-micron structures as well as the sub-micron devices.
References
’ P J Revel1
and G F Goldspink,
Chcuurn. 34. 455 (19X4).
J E Curran. Vucuum, 34, 343 (19841. .1M Cantagrel, J P’uc.Sci T~hnol, 12, I340 (I 975). a M Cantagrel and M Marchal, J Mu~erial Sci. 8, 17 I I (1973). ’
’ P M Schaible. W C Metzger and J P Anderson. .I 1’~. .%I 7uc~hnol. IS. 334 (1978). ’ V Neuman, C W Pitt and L M Walptta. Procrr&y\ r!f the Firsf Europrun Cor$nwc~ WI Intryrarrd Optic,.s, p 89 (September 1981). ’ C W Pitt and L M Walpita. Optics C‘ommun.s. 52, 241 (1984). ’ C W Pitt and L M Walpita, ilppl Optics, 23, 3434 (1984). ’ M C Hutley, D@uction Grutinqs. Techniques of Physics: h. Academtc Press. New York (1982). to E Kapon and A Katzir, J uppl Phys, 53. 1387 (19821. I ’ S Hosaka and S Hashimoto. J C’U Sci Twknr~/, IS, I71 2 ( 197X). I2 B A Raby. J I’ac Sci Ted~nol. 15, 205 (197X). I3 A J van Roosmalen, J’ucuum, 34,429 (1984). ” V Neuman, C W Pitt and L M Walpita. Elec,rrort Lrrrs. 17, IhS (19X1 I. I’ S P Singh and C W Pitt. 2nd IPAT Proceedmys. p 37 (1979). Ih M Sato and H Nakamura. J F’u~,Scr Tec~hnol. 20, IX6 (19x2).
36/numbers Britain
1-3/pages
Ion and plasma gratings D A Darbyshire,
A P Overbury
University
London,
College
55 to 60/l
986
assisted
0042-207X/86$3.00 Pergamon
etching
and C W
Torrington
+ .OO Press Ltd
of holographic
Pitt, Department of Electronic Place, London WClE 7JE, UK
and Electrical
Engineering,
Fine geometry grating patterns of <0.3pm periodicity may be produced in photoresist by optical interference methods. Although wet chemical etching can be used to transfer the interferometric grating pattern into the substrate, this process tends to degrade the grating geometry by undercutting and by solution saturation effects. Ion beam milling and reactive ion plasma etching have proved effective in transferring photoresist patterns into the underlying solid substrate with reduced aberrations. Freon- 14 (CF,! in a plasma has been utilized to etch gratings in soda-lime glass and lithium niobate (LiNbOs). Carbon tetrachloride (Ccl,) plasmas have similarly been used to transfer gratings into aluminium thin films on the substrate. The Al film pattern is then converted into an Al oxide mask by either milling in a 20% 0,/80% Ar ion beam or by reactive ion etching in the presence of oxygen. The oxide acted as an in-contact mask with enhanced lifetime so that deeper gratings were transferred into the substrate by subsequent ion etching.
1. Introduction Laser interference (holographic) techniques are used in many cases where periodic patterns, often gratings, are required. Typically the intensity distribution across the interference plane of two intersecting laser beams may be recorded by a photoresist coating on a substrate. The photoresist, when developed, will contain a modulated surface relief which may then be used as the grating. This photoresist pattern is however, somewhat fragile and may be far from ideally suited to the intended application. It is, therefore, often necessary to transfer the pattern into the substrate by an etching process. Recent attention has been focused on establishing dry etching processes suitable for etching a range of materials for both electronic and optical device fabrication’,‘. These techniques are now being used to produce high component density structures which utilize fine geometry patterns, which are in several respects similar to the interference gratings described above. We have developed an improved procedure for the etching of fine-geometry patterns with sub-micron periodicity, the overall process necessitating the careful etching of polymer photoresist, Al thin films, soda-lime glass and LiNbO,, while maintaining a high degree of selectivity between mask and substrate. There is considerable previously reported work on etching most of these materials; several of the earlier experiments have been followed up in some detail 3-5.1 5 and incorporated in the new process. However, other materials such as LiNbO,, have not been extensively examined; we feel that the capability to etch these newer materials may enable the fabrication of devices suitable for integrated optics. This paper presents our initial attempts at etching LiNbO,, together with photoresist, Al and glass, and describes their use in
producing integrated
useful interferometric process strategy.
gratings
by means
of an
2. The production of gratings by interference techniques We have previously explored the use of gratings as guided wave beam expanders for integrated optic device&‘, particularly in the acousto-optic spectrum analyser. In this device it is necessary to record the interference pattern on an optically waveguiding substrate (titanium indiffused LiNbO,), by means of a resist layer. The pattern is then etched through the resist mask into the substrate so that the guided optical wave may interact with the grating. For our applications, the parameters of most importance are the grating pitch, p, and the amplitude of the grating pattern when transferred into the substrate, d,. The interference pattern produces a grating of varying amplitude, d,, in the photoresist, and we need some control over the grating profile during etching. There are many different arrangements suitable for recording the interference gratingsg. We have selected the arrangement in Figure 1, a design which uses the substrate itself as one of the interferometer mirrors, and so has to be metallized before being coated with photoresist (Figure 2). A major advantage of this arrangement is that the pattern is independent of the substrate refractive index; however, because there is no length symmetry in the interferometer arms the laser must have a sufficiently long coherence length. We use an Ar ion laser running in the violet (457.9 nm) with a Fabry-Perot etalon in the cavity to select a single longitudinal mode and increase the coherence length. The laser operates in the TEM,, mode and the beam is expanded x 10 and filtered with a 10 Grn dia pinhole; after expansion the effects of 55
DA
Darbyshlre,
A P Overbuy
and C W Pitt:
Reflecting
Ion and plasma
assisted
etchmg
of holographrc
gratmgs
Mirror
Metallised Substrate ,’
Collimating
/’
Resist
With Coating
Expanding
Mirror
Lens ~~~I:==++
~
Argon
457.9 nm Laser Beam
O
Pinhole
Figure I. Interferometer
used for recordmg
grating
Figure 2. Grating
Resist
800
1000
(‘ontours
armulation of’ intcrferencc of constant intensit)
pattern
for 53X nm prtch
Exposure
And
After
Etching
replication
process.
‘j
Development
the Gaussian intensity distribution are minimized, giving a relatively uniform pattern across the substrate. This degree of uniformity is needed in order to assess the etching techniques used. In any interference pattern the intensity of illumination in the depth ofthe photoresist layer is a function of the distance from the substrateiphotoresist interface. This may be appreciated by considering that not only the two incident beams take part in the interference, but also the two beams reflected from the substrate: photoresist interface”, and all of the beams interact to form a vertical standing wave in the photoresist layer. The effect of this standing wave is that the light intensity in the photoresist is a periodic function with depth. It would be ideal if the photoresist received a uniform exposure right down to the substrate. In this case the photoresist grating could be developed to its full depth. and would then act as a direct mask for etching the metal layer. However, the metal layer imposes a standing wave intensity minimum at the substrate/photoresist interface, and so the pattern is unlikely to develop down to the surface of the substrate (the metal thin film in this case). Hence, the depth modulation in the developed photoresist is superposed on a layer of undeveloped photoresist. Further, the substrate-normal intensity of the standing wave pattern at the surface of the photoresist has to be considered. Using a computer simulation lo, we have predicted the form of the standing wave pattern. It may be seen (Figure 3) that unless the photoresist depth is chosen carefully. the surface of the 56
600
Along Substrate
I
I
After
grating.
Exposure
400
nm
patterns.
Figure 3. Computer Before
200
Distance
photoresist receives no exposure and so development is inhibited. It is not clear whether the standing wave minima in the photoresist can totally block the development of the pattern under all conditions, but we have observed that the photoresist development or erosion technique employed is important in transferring the lightly modulated pattern through the undeveloped photoresist layer to the metallization. Following exposure. differential-rate etching techniques are then employed to etch the pattern into the metal which may then be used as a mask for etching into the substrate. Obviously the particular metal chosen for the substrate metallization vvill affect not only the photoresist grating pattern, but also the etch characteristics of the system. We have chosen Al for our needs because it has high reflectivity, and the oxide has a low sputter yield during reactive ion beam etching; by judicious selection of the photoresist and metallization parameters, and by optimizing the etching technique, the transfer of the pattern into the substrate may be enhanced both in depth and in uniformity of profile. 3. Etching experiments Three processes have been examined for transferring gratings from photoresist to optical waveguiding layer: (I) using ion beam milling (IBM) and reactive ion beam etching (RIBE); (7) by ion assisted plasma etching, also termed reactive ion etching (RIE) and (3) by a combination of RIE and RIBE. 3.1. lon beam milling and reactive ion beam etching. Ion beam milling and reactive ion beam etching were performed in a VEECO Microetch system (Figure 4(a)) working at ion energies of 250 eVl.0 keV and beam current densities in the range 0.25 mA cm 2 to 1.0 mA cm ‘. These parameters. together with vacuum base pressure down to 3 x 1O- ’ Pa, provide a contamination free environment and a precisely controlled ion milling action over an effective beam diameter of 8 cm. The non-uniformity of etching associated with beam inhomogeneity has been eradicated by using a water-cooled, rotating substrate table; the table and substrate may be inclined with respect to the beam”. A shallow grating, i.e. an exposed photoresist layer with depth of modulation small compared with the thickness of photoresist was completely developed by using a 400 eV, 0.4 mA cm ’ Ar beam. The low energy and current density was selected to avoid photoresist degradation due to thermal effects. The appearance of the Shipley Microposit 1350 positive photoresist pattern at this
D A Darbyshire, A P Overbury and C W Pitt; Ion and plasma
assisted
gas inlet
I
anode
I
I
Figure 4(a). Schematic system.
representation
of the micro-etch
pumping
ion beam milling
gas/
vapour inlet
upper
electrode II
u
cylindrical ---Pyrex chamber. electrode
I
vacuum
pumping I
Figure 4(b). Planar
reactor
chamber
configured
of holographic
gratings
OJAr gas mixture, the metal film grating was converted into an Al,O, mask giving greatly enhanced mask lifetime as a result of the lower sputter yield of the oxide. The milling of the glass or LiNbO, was maintained to the point where the AI,O, mask was completely removed. The results are reported in Section 4.
heated spiral I cathode
vacuum
etching
3.2. Ion assisted plasma etching. The reactive ion etching experiments were conducted in a Plasma Technology PE80 planar etching system shown schematically in Figure 4(b). The reactor comprises two parallel, anodized Al electrodes 24 cm in diameter, the upper electrode being grounded. Substrates were positioned on the lower electrode which is coupled to a 13.56 MHz rf generator through an impedance matching network. Experiments were performed with electrode spacings of 336 cm and operating gas/vapour pressures of 2.&14.0 Pa, to establish etch rates for Microposit 1350 photoresist, Al films, glass and LiNbO,. The plasma etch rate of ca 1 pm thick photoresist layers spincoated onto Al film on glass, was determined for varying reactor parameters using Ar, 0, and ratios of0, to Ar of 1: 1 and 2: 3. The etch rate tests performed on the Al films used Ar bubbled through liquid Ccl,. The substrate table was heated to 55+ 5°C. Pre-mixing a carrier gas, such as Ar, with Ccl, vapour prior to entering the system presents the additional problem ofcontrolling the ratio of Ar to Ccl,. For adequate process control the ratio of Ccl,: Ar must be held to better than & 5%. Having achieved transfer of the grating into the metal film, the chemical reactivity of CF, with glass and LiNbO, was utilized to impress the grating pattern from the metal film into the substrate. The basic plasma etching gas was CF, mixed with l-10% 0,; each gas was introduced through a separate line and the flowrate monitored by means of rotameters. The electrode spacing and substrate temperature were fixed at 4 cm and 25°C respectively. 3.3. End point detection. A laser end-point detection instrument has been incorporated into the RIE system in order to monitor the dry development of the photoresist, and the etching of the Al film prior to transferring the grating to the substrate. In principle, it relies on the change in grating diffraction efficiency observed having etched through one layer and then encountering a second layer. The procedure adopted, as shown in Figure 5, is as follows.
r.f. generator
for reactive ion etching.
stage, is that of stripes of photoresist some several hundred Angstroms thick, approximately 0.34.4 pm wide and separated by stripes of exposed metal film of similar width. The ion beam was then accelerated to a relatively high energy (750 eV) for short duration (2&30 s), to remove the native oxide layer present on the Al film surface. The beam energy and current density were then again reduced to 400 eV and 0.4 mA cm- *, respectively, to avoid excessive heating and to improve the etch-rate selectivity between photoresist and the Al thin film. The Al film was etched to the point of total photoresist mask removal, by which time the photoresist grating had been transferred into the metal film completely. The third stage of the process, the transfer of the metal grating into the optical guiding layer in the substrate, depends on the substrate material. For etching glass and LiNbO, using the Al mask, experiments were conducted using both Ar and O,/Ar beams and the substrate gratings were compared. By using the
I 3 mW He-Ne
laser
Figure 5. Laser end point detection 57
D A Darbyshire,
A P Overbury
and C W Pitt: Ion and plasma
assisted
etching
of holographic
gratings BEAM
A laser beam from a 3 mW He-Ne laser is directed into the chamber and onto the surface of the grating device which is positioned so as to extract a first-order diffracted beam. The power intensity of the forward-reflected and first-order-diffracted beams emerging from the chamber can be measured by positioning photodetectors in the path of each beam. Upon etching down to the interface between two adjacent layers a rather large increase in diffraction efficiency can be observed. resulting in an increased power reading for the first-order-diffracted beam and a corresponding reduction for the forward-reflected beam. The point of maximum diffraction efficiency indicates that the process parameters may be changed so as to etch the lower layer. The new process continues until the next layer interface is reached and so on.
CURRENT
DENSITY
:
PRESSURE
X
SODA-
ImA cm-zJ
1.5 x10-*
LIME
Pa
GLASS
4. Results and discussion The ion milling rates for Microposit
1350 photoresist,
Al films.
energy and beam current density are shown in Figure 6(a). For ion beam energies and current densities of 500 eV and 0.5 mA cm ’ respectively, the selectivity is found to be almost unity for the photoresist. Al, glass and LiNbO,. This poor selectivity becomes further exaggerated at higher ion energies and current densities. for instance, the selectivity of a photoresist mask on Al at I keV beam energy is only about 0.75. The implication is that photoresist and Al films are less attractive as masking materials for glass and LiNbO,. The data on RIBE with an 02/Ar gas mixture, as illustrated in Figure 6(b), indicate an improvement in selectivity at all energies when using Al,O, (Al film converted to oxide mask) as compared with Al or photoresist (Figure 6(a)). At 750 eV. the selectivity of AI,O, on glass reaches about 5. a significant improvement compared with the other masks. For the RIE process, Figures 7(a) and 7(b) show the variation in soda-lime
glass
and LiNbO,,
BEAM
as a function
CURREN:
DENSITY
of Ar ion beam
ION
BEAM
ENt R(,‘l
ir~v)
Figure 6(b). Reactive ion heam etch rates l’or pIas
LiNhO,
and Al,0 I 111
20X, 0, ‘X0”,;,Ar. erosion rate of Microposit I350 photoresist with gas prcssurc and feed gas composition respectively, using Ar. 0: and Ar’O, mixtures. It is evident that by varying the O2 to AI- etchant gas ratio. a controllable erosion can be achieved in situations where the depth of optical exposure:development modulation in the photoresist layer is shallow compared to the total thickness of the photoresist.
ImA cme21
1 o MICROPOSIT
1350
D ALUMINIUM X SODA-LIME
GLASS
RESIST
700.
-7 600.G E “4 500-
0,IAr
Ratio
1.1
: lx 4005 5 300 -
B
100 t
100
,/,
,
, /,
.5
0
ION
BEAM
,
ENERGY
0
(KeV)
Figure 6(a). Ion beam milling etch rates for photoresist, film. glass and LiNbO, in argon. 58
I” J
1.0
3 GAS
aluminium
thin
6 PRESSURE
Figure 7(a). Erosion rate of Microposit plasma etching.
Parenthesis I#
9
12
I
15
(Pal
1350photoresist
using ion assisted
DA
Darbyshire,
A P Overbury
and C W Pitt:
Ion and plasma assisted etching of holographic gratings
RX
0.2 wcm-2
LiNbO.
-
100
R.F.
0.1 wcln-2
8
0 100 % 02 FEED
50%
o2
50%
Ar
GAS
10 0 % Ar
on the etch rate of photoresist
300
250
^ ‘i .;
MICROPOSIT
200
1350
ml w
150
2 z =
100
50 R.F.
0’
0.1 WC,+ I
3
6
9
GAS I VAPOUR
Figure 7(c). Etch rates in a CClJAr
PRESSURE
0
0 3
1
12
15
1PO 1
plasma.
Selective etching of Al by RIE was used to transfer the finegeometry grating patterns of ~0.3 pm periodicity from the photoresist mask. Figure 7(c) shows the differential etching rates achievable by processing the grating with the relatively slow etch rates caused by using a large electrode spacing (6 cm), reduced rf power density (0.1 W cm-‘) and low substrate temperature (20°C). The combined effect of these measures is to reduce thermal
_
A’203 9
6
GAS PRESSURE
Figure 7(d). Etch rates in a CFJO,
COMPOSITION
Figure 7(b). Effect of feed gas composition using ion assisted plasma etching.
I
12
15
(Pai
plasma.
degradation of the photoresist layer, thus prolonging the mask lifetime, while maintaining the Al etch rate. However, other measures are necessary to further improve selectivity. The reasons for poor selectivity are twofold: (i) the introduction of Ar, used as the carrier gas for CCI, vapour, increases the sputter rate of photoresist and (ii) the native oxide layer present at the Al surface initially impedes the chemical reaction. An effective means of removing the oxide layer is by sputter etching. Although N, could be employed to reduce the sputter rate of resist, it is too slow in eroding the Al oxide. Experiments were conducted using Ar for short durations, until the oxide had been removed, then N, in place of Ar to maximize resist lifetime. The use of O,, at this stage of the process, to remove carbon from the plasma, thus supplying additional chlorine to the reaction with Al, must be avoided for two reasons; firstly, it is evident that any 0, present seriously degrades the photoresist mask (illustrated by the results in Figure 7(b)) and secondly, it converts the Al thin film to an oxide, actually impeding further chemical etching. However, H,, for example, may be used in place of 0,. Having formed the Al mask on the surface of the substrate, the conversion to an Al oxide mask by the introduction of 0, into the plasma is now very beneficial. The improved selectivity achieved by etching glass and LiNbO, with Al,O, masking is displayed in Figure 7(d). This oxide film is reformed continuously at the surface of the Al layer by the 0, content in the CF,+S%O, mixture used in the etching plasma. In addition to the enhanced mask lifetime, there is evidence that a plasma gas comprising 0, mixed with CF, has two other advantages: (a) the 0, effects the removal of carbon from the plasma reaction byproducts creating a higher concentration of free fluorine for the reaction with glass and LiNbO, (refs 12, 13), and (b) the 0, also partially overcomes the problem of carbon-rich film growth during etching. The production of carbon layers in carbon-compound plasmas and its role in impeding the reaction between the Halogens and the substrate material is well knownr6. 59
D A Darbyshire,
A P Overbury
and C W Pift:
Ion and plasma
assisted
5. Conclusions The work on etching Microposit 1350 photoresist is directed towards a totally dry plasma development process for optically exposed photoresist films. To date the capability to control the etch rate, and the noted difference in etch rates between exposed and unexposed photoresist, suggest that such a process is within sight. Etch rate tests conducted on single material films. by the techniques described in the preceding sections. reveal the usefulness of employing a dual process incorporating both ion milling and RIE. It was found that in some material systems RIE had advantages over milling techniques, mainly as a result of the improved selectivity. Results obtained by combining the use of RIE to plasma etch Al through a photoresist mask with RTBE to etch glass and LiNbO,, have proved particularly encouraging. The etch rates of glass and LiNbO, using plasmas of CF, with 0, and for the system parameters chosen, were slow by comparison with ion milling. Other fluorine based gas etchants, such as C,F,, CHF, and SF, together with a wider range of process parameters are currently being investigated to improve the etching rate while maintaining a high degree of selectivity. The optical gratings produced by these processes have been incorporated into devices such as guided wave beam expanders” and have exhibited much improved performance compared with devices manufactured by the processes previously used14.
60
etching
of holographic
gratmgs
The underlying strategy that of using a sub-micron geometry device, the interference grating, as the target device for the process research has produced a further benefit. The developed process operates entirely satisfactorily for super-micron structures as well as the sub-micron devices.
References
’ P J Revel1
and G F Goldspink,
Chcuurn. 34. 455 (19X4).
J E Curran. Vucuum, 34, 343 (19841. .1M Cantagrel, J P’uc.Sci T~hnol, 12, I340 (I 975). a M Cantagrel and M Marchal, J Mu~erial Sci. 8, 17 I I (1973). ’
’ P M Schaible. W C Metzger and J P Anderson. .I 1’~. .%I 7uc~hnol. IS. 334 (1978). ’ V Neuman, C W Pitt and L M Walptta. Procrr&y\ r!f the Firsf Europrun Cor$nwc~ WI Intryrarrd Optic,.s, p 89 (September 1981). ’ C W Pitt and L M Walpita. Optics C‘ommun.s. 52, 241 (1984). ’ C W Pitt and L M Walpita, ilppl Optics, 23, 3434 (1984). ’ M C Hutley, D@uction Grutinqs. Techniques of Physics: h. Academtc Press. New York (1982). to E Kapon and A Katzir, J uppl Phys, 53. 1387 (19821. I ’ S Hosaka and S Hashimoto. J C’U Sci Twknr~/, IS, I71 2 ( 197X). I2 B A Raby. J I’ac Sci Ted~nol. 15, 205 (197X). I3 A J van Roosmalen, J’ucuum, 34,429 (1984). ” V Neuman, C W Pitt and L M Walpita. Elec,rrort Lrrrs. 17, IhS (19X1 I. I’ S P Singh and C W Pitt. 2nd IPAT Proceedmys. p 37 (1979). Ih M Sato and H Nakamura. J F’u~,Scr Tec~hnol. 20, IX6 (19x2).