Optimized condition for etching fused-silica phasegratings with inductively coupled plasma technology

Shunquan Wang, Changhe Zhou, Huayi Ru, and Yanyan Zhang

Polymer deposition is a serious problem associated with the etching of fused silica by use of inductivelycoupled plasma (ICP) technology, and it usually prevents further etching. We report an optimized etchingcondition under which no polymer deposition will occur for etching fused silica with ICP technology.Under the optimized etching condition, surfaces of the fabricated fused silica gratings are smooth andclean. Etch rate of fused silica is relatively high, and it demonstrates a linear relation between etcheddepth and working time. Results of the diffraction of gratings fabricated under the optimized etchingcondition match theoretical results well. © 2005 Optical Society of America

OCIS codes: 050.1380, 220.4000, 230.1950.

1. Introduction

Micro-optics has received great attention in recentyears, and micro-optical elements are becoming in-creasingly more widely used in optical systems.1–5

The procedure for fabrication of micro-optical ele-ments takes advantage of microelectronics technolo-gies. First a mask is fabricated by an e-beam facilityor a pattern generator. Then a lithographic processtransfers the pattern from the mask into a photore-sist (PR) layer upon a substrate. The last step is theprocess during which the substrate is etched with aphotoresist layer or a layer of another material toserve as a protective layer. Obviously, the last step iscrucial for the successful fabrication of micro-opticalelements.

Wet chemical etching6 (WCE) or dry etching meth-ods can be used. Generally speaking, dry etchingmethods have more merit than does WCE. WCE ischeap and fast. The depth of etching in the substratecannot be accurately controlled during chemical so-lution etching. Moreover, the undercut effect of WCEgreatly affects the vertical etching profile, which de-termines the performance of micro-optical phasegratings. Dry etching is achieved in a plasma envi-ronment through chemical reactions or physical re-

actions between an ionized erosive gas and asubstrate. Dry etching methods include reactive ionetching,1–5,7 ion milling,8,9 electron cyclotron reso-nance, and use of inductively coupled plasma (ICP)technology.10–17 ICP is a high-density plasma at lowpressure, which can achieve a high etch rate andanisotropic etching. An independent radio frequencypower is used to control the self-bias voltage of thesubstrate. Thus, ion bombardment energy on the sub-strate can be controlled. Because of its merits as de-scribed above, ICP technology for etching high-precision micro-optical elements should be studied.There have been some reports of the fabrication ofmicro-optical elements with ICP and reactive ionetching technologies. But, of the reports about etch-ing fused silica with ICP technology,10–17 none hasincluded detailed methods for avoiding polymer dep-osition. To fabricate high-quality micro-optical ele-ments it is crucial to obtain optimized etchingconditions to prevent polymer deposition on opticalsurfaces.

In the field of optics, fused silica is an ideal opticalmaterial with a wide transmitting spectrum rangingfrom deep ultraviolet to far infrared. Furthermore,fused silica is of high optical quality. Gratings madefrom fused silica are useful in high-precisionoptics.18–21 One serious problem associated with theetching of fused silica by use of ICP technology is thedeposition of polymer on the surface of the substrate,which usually prevents further etching. In the worstcase, polymer deposition can completely stop theetching process. According to our experience, thepolymer adheres firmly to the surface of the sub-strate. Heating to 800 °C or strongly corrosive chem-

The authors are with the Shanghai Institute of Optics and FineMechanics, Academia Sinica, P.O. Box 800-211, Shanghai 201800,China. C. Zhou’s e-mail address is chazhou@mail.shcnc.ac.cn.

Received 16 September 2004; revised manuscript received 27February 2005; accepted 7 March 2005.

0003-6935/05/214429-06$15.00/0© 2005 Optical Society of America

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icals cannot wipe the polymer off. Therefore, findingways to optimize experimental parameters to preventpolymer deposition completely has become an impor-tant issue for etching of fused silica with ICP tech-nology. In this paper we report an optimized ICPetching condition under which fused-silica phasegratings were fabricated. The surfaces of the fabri-cated gratings were clean, and no polymer was ob-served. The diffraction of the gratings matches thetheoretical results well.

2. Experimental Setup

ICP (ICP-98A) made by the Microelectronics R&DCenter, Chinese Academy of Science, is used in theexperiment. Figure 1 is a schematic diagram of theICP etching system. The system has two independentradio frequency induction sources (RF1 and RF2) withthe same frequency, 13.56 MHz. Sources RF1 andRF2 are used to generate a high-density plasma andto introduce a self-biased electrical field, respectively.The plasma in the chamber is generated mostly byinductive coupling of RF1 through the dielectric win-dow. A gas feed ring is located just below the inneredge of the cap. The other independent source, RF2, isused to generate the self-bias voltage, and thereforeone can independently control the ion energy to bom-bard the substrate by adjusting the RF2 power. Theplasma chamber is pumped by a 500 l�s turbomolecu-lar pump. During etching, the sample holder is watercooled to keep the sample near room temperature.Water cooling is used to prevent any thermally in-duced damage to the sample and to maintain a lowtemperature for a relatively high etch rate.

A mixture of CHF3, Ar, and O2 is used in the ex-periments. CHF3 is the main working gas that pro-duces most of the ions and erosive neutrals; O2 is usedmainly to decompose the polymer to reduce the poly-mer deposition rate. Ar is used as a buffer gas to keepthe plasma stable; also, it can increase the etchingselectivity of fused silica to photoresist.22 The fused-silica wafer used in the experiments has a diameter of2 cm, and its thickness is 1.5 mm.

Before etching, the steps in fabricating a sample of

fused silica with an Al mask are as follows: (1) de-positing an Al layer onto the surface of a clean, dryfused-silica wafer; (2) coating a positive PR film(Shipley Model s1818) onto the Al layer. (3) Trans-ferring the mask pattern to the PR layer by photoli-thography. (4) Developing. (5) Using a chemicalsolution to form a pattern on the Al layer. (6) Using achemical solution to remove the PR layer. (7) Baking.Then the wafer is put into the vacuum chamber of theICP equipment for etching. The fabrication of a fused-silica wafer with a PR mask is easier; steps (1), (5),and (6) are omitted.

3. Optimized Condition

A. Two Effective Factors in Etching

Polymer deposition is a frequently encountered prob-lem in the fabrication of micro-optical elements byICP technology. At the beginning of our research pro-gram, experimental results indicated that polymerdeposition will happen if the total gas flow rate is lessthan 100 SCCM, where SCCM denotes cubic centi-meters per minute at STP. Etching and polymer dep-osition occur at the same time when a fluorocarbonplasma is used for the etching. If the chamber’s en-vironment is more favorable for polymer depositionthan for etching, polymer deposition will occur. Thepolymer deposition rate has a close relationship tofactors such as self-bias voltage, rf power, chambertemperature, and gas flow rate.

The gas flow rate directly affects the time � (Ref. 13)during which the gas resides in the chamber, which isan important parameter in the experiment, as de-scribed by

� � P V�Q, (1)

where P, V, and Q are the pressure, the volume of thechamber, and the gas flow rate, respectively. Equa-tion (1) indicates that, if the gas flow rate is low, thetime during which the gas resides in the chamber willbe long; thus polymer deposition will be more likely tooccur. Accordingly, a high gas flow rate was employedin the following experiments.

In addition, through extensive experiments wefound that the proportion of O2 in the mixed gasgreatly affected the etch rate of the fused-silica sub-strate with the Al mask that was used as the protec-tive layer. Figure 2 shows the relation between theetched depth and the gas flow rates of O2 and Ar.From Fig. 2 we know that the etch rate of fused silicawith an Al mask is much more sensitive to the pro-portion of O2 than of Ar. The result is helpful forproper control of the gas mixing ratio to get a highetch rate of fused silica.

B. Optimized Condition and Etched Results

According to the discussion above, extensive experi-ments were carried out in which different gas mixingratios, gas flow rates, and RF powers were used.From the experiments we obtained the following op-timized parameters: 200 SCCM of CHF3, 20 SCCM of

Fig. 1. Schematic diagram of the ICP equipment.

4430 APPLIED OPTICS � Vol. 44, No. 21 � 20 July 2005

Ar, 5 SCCM of O2, 700 W of RF1 power, and 200 W ofRF2 power. Under the optimized etching conditionswe successfully fabricated phase gratings that wereetched into the surface of fused silica with an Almask. The etched profile of a grating with a period of40 �m and duty cycle 1:2 was measured with Taylor–Hobson equipment (Form Talysurf Series 2), and theresult is shown in Fig. 3: The surfaces of the fabri-cated gratings were clean, and no polymer was ob-served.

The etch rate of fused silica with an Al mask dem-onstrates a fine linear relation between etched depth(up to 4 �m) and working time, as shown in Fig. 4.Compared with the conditions of previous research,10

in which the optimal etching condition was not soclear, the etch rate under the optimized conditionhere is higher and the linear relation between theetched depth and the working time fits better. So wecan control the etching time accurately to acquire theprecise depth that we need. Accurate control of theetched depth is important in the fabrication of high-precision optical elements.

Uniformity of the etched depth in the whole largesample holder is an important parameter of ICP etch-ing technology for mass production. It is determinedby many factors, such as the size of the sample, theposition of the sample in the holder, the cleanness ofthe sample’s surface, and the operating conditions ofthe etching system. All these factors will directly or

indirectly affect the uniformity of the etched depth. Inthe experiment, five samples were placed up, down,left, right, and centrally, upon a 4 in. (10.16 cm) cir-cular holder. After the experiment the etched depthswere measured; the values were 0.92, 0.88, 0.95, 0.90,and 0.92 �m, respectively. The average depth errorwithin the 4 in. zone was �3.83%. The uniformity ofetching in the samples was quite good.

Process repeatability is another important param-eter for mass production. It is affected by many fac-tors such as chamber leakage rate, pump deliveryrate, chamber pressure at a constant gas flow rate,and RF power stability. The repeatability experi-ments were carried out for five consecutive days. Af-ter the experiments the etched depths weremeasured, and the values were 0.72, 0.73, 0.75, 0.72,and 0.72 �m in a time sequence. The average deptherror was �1.92%. The process repeatability wasgood, and the result is important to the mass produc-tion of micro-optical elements in fused silica by ICPtechnology.

4. Etching of Fused Silica with PR Masks

A fused-silica wafer that uses a PR mask as a pro-tective layer was also etched under optimized condi-tions. Experimental results indicate that thereported optimized etching condition is also suitablefor etching gratings in fused silica with PR masks. Agrating with period of 40 �m and a duty cycle of 1:2was fabricated, and its etched profile is shown in Fig.5. We can see that the surface of the grating was

Fig. 2. Experimental relation between etched depth and the gasflow rates of O2 and Ar.

Fig. 3. Profile of one phase grating with a period of 40 �m and aduty cycle of 1:2 that was etched into fused silica with the mea-sured (with Taylor–Hobson surface equipment) depth of 4 �m.

Fig. 4. Relation of etched depth to working time.

Fig. 5. Profile of one phase grating with a period of 40 �m and aduty cycle of 1:2 that was etched into fused silica with a PR mask(measured with Taylor–Hobson Surfacet equipment).

20 July 2005 � Vol. 44, No. 21 � APPLIED OPTICS 4431

clean, and no polymer was observed. Comparing thisresult with that shown in Fig. 3, we know that an Almask is more suitable for deep etching of fused silica.A high-density holographic grating with a surface-relief profile was also fabricated on fused silica with aPR mask under the optimized etching condition. Thedensity of the grating was 600 lines�mm. Figures6(a) and 6(b) are the cross-sectional image and thetop-surface image that we obtained with a scanning-electron microscope. The surface of the holographicgrating was clean, and no polymer was observed.From Fig. 6(a) we know that the etched depth of thegrating is 0.5 �m. High-density gratings are widelyused in high-density optical storage and optical com-munication.

The experimental results described above havesupported the view that a relatively small time forthe gas to remain in the chamber and a proper gasmixing ratio can prevent polymer deposition to a cer-

tain extent. However, the exact relation among thebasic plasma parameters such as the number of ionsand neutral radicals, the electron density, the elec-tron temperature, and the time � during which thegas resides in the chamber is not so clear.13 Thereforea mechanism for controlling the effect on polymeriza-tion of the length of time that the gas resides in thechamber has not been established. To obtain such amechanism will require many more extensive stud-ies.

5. Optical Evaluation of the Etched Gratings

To evaluate the optical performance of the fabricatedgratings, we took the gratings shown in Fig. 5 as anexample and measured its diffraction pattern. In theexperiment, a laser beam with a wavelength of488 nm from an Ar-ionized laser was used as theincident light. The diffracted pattern generated bythe grating was captured with a CCD camera, asshown in Fig. 7(a). The intensities of the diffractedspots shown in Fig. 7(b) were measured by a lightpowermeter. For a relatively shallow-etched phasegrating such as the grating shown in Fig. 5, use of ascalar Fourier optics theory is correct for explanationof its optical diffraction. We calculated the diffractionefficiencies of the first-order diffraction spot of thegrating versus its etched depth, by using the scalartheory and a rigorous coupled-wave theory23 thattook into account TE-polarized incident light andTM-polarized incident light, as shown in Fig. 8. Thegrating calculated in Fig. 8 has a period of 40 �m andduty cycle of 1:2. From the figure we know that, for agrating with a small depth-to-period ratio, the dif-fraction performances predicted by the scalar opticaltheory and the rigorous coupled-wave theory are al-most the same. Thus the performance of the gratingshown in Fig. 5 can be well explained by the simple

Fig. 6. (a) Cross-sectional and (b) top-surface images of a high-density surface-relief grating of 600 lines�mm captured with ascanning electron microscope.

Fig. 7. (a) The diffraction spots generated by the etched gratingand (b) the intensities of the diffraction spots.

4432 APPLIED OPTICS � Vol. 44, No. 21 � 20 July 2005

scalar optics theory. However, when the ratio ofdepth to period is near 1:1 or even larger, rigorousvector theory should be applied to analyze the diffrac-tion results for different polarizations of the incidentlight.

A theoretical scalar relation between the diffrac-tion spots is given by

Im � 4 sin2�

2sin2 �ma

(m�)2 , m an integer, m � 0, (2)

and the intensity of the zero-order diffraction spotcan be described by

I0 � cos2(��2), (3)

where

� � 2�(n � 1)h

, (4)

which represents the phase difference between thetop and the bottom of the grating. Among the param-eters, m is the number of the diffraction order, a is theduty cycle of a grating, n is the refractive index offused silica, h is the depth from the top to the bottomof the groove, and � is the wavelength of the incidentlight.

As for the gratings whose fabrication is describedin this paper, refractive index n is 1.46 for an incidentlight wavelength � 488 nm. From Fig. 5 we knowthat duty cycle a is 1:2 and that etched depth h is1.36 �m. With Eqs. (2) and (3), we obtained theoret-ically the ratio of first-order diffraction spot intensityto zero-order diffraction spot intensity, I1�I0 � 1.49,which is very close to the experimental result ofI1�I0 � 1.47, as could be deduced from Fig. 7(b). Thus

the diffraction result of the grating matches the the-oretical result very well and further demonstratesthe effectiveness of the optimized etching condition.The reported optimized etching condition is helpfulfor the fabrication of fused-silica gratings by ICPtechnology.

6. Conclusions

ICP technology and reactive ion etching technologyare becoming increasingly widely used in the fabri-cation of fused-silica micro-optical elements. Becauseof its advantages such as relatively high etching rateand vertical etching profile, ICP technology is receiv-ing more attention. However, polymer deposition hasalways been a serious problem, and few publicationsin the field have reported methods to prevent polymerdeposition. Through extensive experiments we haveachieved an optimized etching condition under whichno polymer deposition will occur.

We have reported the optimized etching conditionfor fabrication of phase gratings in fused silica by ICPtechnology. Under the optimized condition, no poly-mer was observed on the surface of a fused-silicagrating. The etch rate is relatively high, and theetched depth (up to 4 �m) has a regularly linear re-lationship to the working time. The etching unifor-mity and the process repeatability for massproduction of micro-optical phase gratings by ICPtechnology are quite good. The diffraction results ofthe gratings match the theoretical results well, whichdemonstrates the effectiveness of the optimized etch-ing condition. It should be noted that a deeply etchedoptical grating �depth�period 1� is also a usefulelement for concentrating diffraction energy into asingle order to achieve a highly efficient Braggvolume-grating performance. The performance mustbe predicted by use of a rigorous vector optical theorythat takes into account the polarization state of inci-dent light. Determining ways to fabricate deeplyetched gratings is a subject for future research. Webelieve that the reported optimized condition shouldbenefit the fabrication of various micro-optical ele-ments in fused silica by ICP technology.

The authors acknowledge support from the Na-tional Outstanding Youth Foundation of China(grant 60125512) and the Shanghai Science andTechnology Committee (grant 036105013) under theShanghai Subject Chief Scientist program (grant03XD14005).

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