Ion beam Etching and Reactive Ion beam etching

Ion beam Etching and Reactive Ion beam etchingBy Tim JollyIntroductionIon beam etching is a versatile etch process in which the substrate to beetched is placed in a vacuum chamber in front of the broad-beam ionsource (see Figure 1). Ions (typically argon) are generated inside the ionsource and are accelerated into a broad parallel beam, and to a definedenergy, by the extraction grids on the front of the source. As the ion beam etches thesurface, the substrate is tilted to an angle in the beam and continuously rotated in orderto optimize the smoothness of the etch. If a pattern is being etched by the use of aphotomask, the use of tilt and rotation allows the user adjust the wall angles in theresulting etch. If one uses an inert gas such as argon, the process is relatively slow,(typically 50 - 100nm/minute,) and the heat that is generated must be removed with care.Figure 1sourceOxford Instruments Ionfab300Plus with optional sputterIon beam processing has been transformed in recent years by the introduction by OIPT of arange of inductively-coupled ion sources that in the case of the 35cm ion source allows theetch of substrates up to 8” diameter (see Figure 2). The 35cm ion source uses a 2MHz RFgenerator (see Figure 3) and the 15cm ion source (see Figure 4) uses 13.56MHz. The ionsource is virtually maintenance-free. This is in stark contrast to the earlier “Kaufman” ionsources that were first used in the 1970s and that had filaments that lasted as little as 10hours.October 2003 ⏐ Page 1

Figure 2Typical etch uniformity with 30cm ion beamFigure 3Oxford Instruments 35cm ion sourceOctober 2003 ⏐ Page 2

Figure 4Oxford Instruments 15cm ion sourceIonfab300PlusThe Oxford Instruments Ionfab300Plus represents the state of the art for ion beam etching.It takes the 15cm or 35cm ion source, and has a loadlock so that throughput is greatlyincreased, and processing can be cassette-to-cassette if necessary, see Figure 5. Theloadlock is to standard “MESC” specification, so that this system can be built into clusterswith other process chambers from Oxford Instruments or other vendors.Figure 5Oxford Instruments Ionfab300Plus with loadlockAs an argon ion beam etching system, the 300Plus is used for etching materials such asgold or platinum, cadmium mercury telluride (CMT), indium phosphide (InP) or aluminiumgallium arsenide (AlGaAs) The chief advantage of the process is that it will etch throughall materials. A typical example of its advantage is when etching a multilayer stack ofalternate layers of GaAs and AlGaAs. The ion beam process will etch through all of thelayers with minimum deviation in the wall angle.October 2003 ⏐ Page 3

In an argon ion beam etch, it is inevitable that heat will be generated in the substrate bythe ion beam, as a high energy of ion is needed. In fact, in many materials, the minimumheat is generated by the use of ions with an energy of around 600V. The 300Plus useshelium cooling of the wafer backside to remove the heat.Reactive Ion Beam EtchingA major advantage of the RF ion source, is that you can perform reactive etches by feedinggases such as Cl2, F2, CF4, O2 directly into the ion source. This process is known as ReactiveIon Beam Etching (RIBE). The gases are cracked in the ion source, and the positive speciesare extracted into the ion beam, see Figure 6. Again, unlike the older Kaufman ion source,source reliability is not affected by the use of such gases. The process that results will bepartially chemical, as the ions react with the surface, and partially physical, as the reactionproducts are sputtered away by the energy of the ion flux. An evident advantage of RIBEis that the relative importance of these processes can be changed by adjusting the ionenergy. Typical RIBE processes include CMT, GaAs, InP, see Figure 7.Figure 6 Cracking pattern of SF6 gas in Oxford Instruments ionsource courtesy of Stefan SchneiderFigure 7Ion beam etching a substrateOctober 2003 ⏐ Page 4

FlexibilityIon beam etching systems are uniquely capable of developing new processes, since all ofthe parameters are directly and independently controlled: ion energy; ion flux density;angle of incidence; process chamber pressure; substrate temperature. It is fair to note thatfor high volume production, the process will generally need to be transferred to anothertechnology - generally a plasma system -- in order to reduce the production costs. Hence,for example, almost no mainstream silicon production steps are performed with ion beam.However, for a number of complex processes, where it is too difficult or simply not costeffectiveto attempt to transfer the process to another technology, ion beam etching orRIBE will remain the process of choice.OIPT supply both ion beam systems and plasma etch systems. Our ICP plasma etch systemshave similar capabilities. In the ICP380, for example, a plasma is generated in a chamberabove the substrate that is to be etched, and ions are accelerated down onto the substrateby applying RF power onto the table that holds the substrate. The process is known asReactive Ion Etching (RIE) although in fact it is possible to use argon for an inert etch. Thefollowing table highlights some of the differences between ion beam etching, RIBE andplasma etching.Process ResearchAdvantages of IonBeam EtchingAdvantages of ICP EtchApplicationsIon Beam Etch and RIBEFundamental reseach into processphysics and chemistry in ion beamsystems helps us to understand ICP aswell as ion beam processes. Ion beamremains an important productionprocess in its own right.All parameters can be setindependently for rapid processoptimization.Inert etching with variable incidence tooptimize the etch process. Ion enegycan be adjusted 20eV – 1500eV.Etch process is face-down, which canimprove process cleanliness.Mainstream production of specialist III-V devicies. Mainstream etching ofmaterials such as CMT and V2O5.Production etching of Au and Ptstructures and multilayer devices.ICP EtchProcess developmentproceeds by empiricalresearch and processmodellingProcess parameters areadjustable but cannot beset independently. Forexample, increasing gasflow will affect chamberpressure, plasma density,bias voltage etc.Inert etching possible, butresults limited by the useof normal incidence.Etch process is face-upReactive processing canetch most materials atlower energy and(sometimes) with lowerdamage. Faster speedsAt great expense, certainICP processes have beendeveloped that allowunmatchable results; forexample in the case of the“Bosh” process, deepstructures with verticalwalls.Mainstream processing ofSi for semiconductor andMEMS markets.Mainstream processing ofGaAs etcOctober 2003 ⏐ Page 5