aluminium etching - MicroChemicals

1 AluminiumEtchingRevised: 2013-11-07 , wafers, plating solutions, etchants and solvents ..Phone: +49 731 977343 0 chemicals concentrations mentioned in this chapter with a * refer to a conventional concen-tration listed in the last section of this has a density of g/cm3 and thereforebelongs to the light metals. Its crystal structure is cu-bic face to its high electric conductivity, aluminium is usedfor conductors in microelectronics where it is often al-loyed with copper in order to prevent electro migra-tion, or with silicon in order to prevent the formationof (silicon-consuming) aluminium -silicon a standard potential of V, aluminium doesnot belong to the noble metals. However, the forma-tion of a very thin (few nm) Al2O3 film makes it very inert in many substances. Therefore, Aletchants require at least compounds for dissolving Al2O3 as well as for etching (or, respectively,oxidizing) etching MechanismTypical aluminium etchants contain mixtures of 1-5 % HNO3* (for Al oxidation), 65-75 %H3PO4* (to dissolve the Al2O3), 5-10 % CH3 COOH* (for wetting and buffering) and H2O dilutionto define the etch rate at given etching is highly exothermic, an (inevitable, since isotropic etching ) underetchingof the resist mask causes local heating (increased etch rate) and super-proportional under- etching of the mask as a consequence, if no agitation is H2-bubbling reduces etch homogeneity.

2 Improvements of the etch rate homogeneitycan be achieved if the etching will be interrupted every approx. 30 seconds by a short dip intoDI water. hereby, the H2 bubbles temporarily , etching starts after the dissolution (by H3PO4) of few nm Al2O3 film present on everyAl surface. For this reason, the pho-toresist processing impacts on theAl etching : The alkaline developerspreferentially dissolve the Al2O3where the resist is primarily through-developed (at regions with lowerresist film thickness, near the edgesof cleared structures, or belowcleared structures with larger fea-tures).Dependant on the extent of (desiredor undesired) over-developing aswell as delay between developmentand Al- etching , the process param-eters may lead to a spatial inhomo-geneous Al etching : cubic face centred Fotolack Substrat Al 2 O 3 Entwicklerangriff Fotolack Substrat Al O 3 Entwicklerangriff Al2O3 Developer AttackResistSubstratePhotoresists, wafers, plating solutions, etchants and solvents.

3 Phone: +49 731 977343 0 GmbH - aluminium etching with PhotoresistsEtch Rate and SelectivityThe Al etch rate of H3PO4/HNO3 mixturesstrongly depends on the temperature ( for a certain composition, afterthe removal of the Al2O3-layer on the Alsurface) and doubles approx. all 5 alloyed with few % of Siliconshows a comparable etch rate as comparedto pure Al etchants attack copper significantlymore then aluminium . The nickel etch rateis lower then the aluminium etch rate. Ti-tanium, chromium and silver are hardlyetched, noble metals such as gold or plati-num are not aluminium EtchOur aluminium etchant ANPE 80/5/5/10 has the following composition:H3PO4 : HNO3 : CH3 COOH : H2O = 73 %: % : % : %We supply this mixture in L sales volumes in VLSI quality. Other grades/sales volumesavailable on Photoresists and their Processing for Al EtchingAll AZ and TI resists are suited and sufficiently stable as mask for etching few m of Alu-minium.

4 Generally, we recommend the usage of resists with optimized adhesion such as theAZ 1500 series (resist film thickness range approx. m via the AZ 1505, 1512 HS,1514 H, and 1518), or the AZ 4533 (3-5 m).The deeper Al has to be etched, the ticker the resist film should be. If this requires a highaspect ratio, we recommend the high-resolution AZ ECI 3000 series (resist film thicknessrange approx. m).Since most NaOH-, KOH-, or TMAH-based developers attack aluminium at a rate of nm/min, sensitive processes require an Al-compatible developer such as the AZ Developer with very low Al order to improve the resist adhesion, a hardbake after development can be beneficial. Werecommend 140-145 C for 5-10 minutes. Since the resist film hereby embrittles, the coolingdown to room temperature should not take place abruptly in order to prevent the formation resist removal after etching , the AZ 100 Remover can be used as long as no dilution orcontamination with water (even in traces!)

5 Occurs, otherwise Al will strongly be attacked. Incase of sensitive processes, the pH-neutral organic solvents NMP or DMSO are more resists and ancillaries mentioned in this section are distributed by us and more detailed inthe document Photoresists, Developers, and Grade of the Substances Mentioned in this DocumentHCl* = 37% HCl in H2 OHNO3* = 70% HNO3 in H2OH2SO4* = 98% H2SO4 in H2 OHF* = 49% HF in H2OH2O2* = 30% H2O2 in H2OH3PO4* = 85% H3PO4 in H2 ONH4OH* = 29% NH3 in H2 OCH3 COOH* = 99% CH3 COOH in ml H3PO4* ml HNO3* ml H2O etch rate(103 /min)etch temperature ( C)Photoresists, wafers, plating solutions, etchants and solvents ..Phone: +49 731 977343 0 GmbH - aluminium etching with PhotoresistsDisclaimer of WarrantyAll information, process guides, recipes etc. given in this brochure have been added to thebest of our knowledge. However, we cannot issue any guarantee concerning the accuracy ofthe assume no liability for any hazard for staff and equipment which might stem from theinformation given in this speaking, it is in the responsibility of every staff member to inform herself/himselfabout the processes to be performed in the appropriate (technical) literature, in order tominimize any risk to man or and the AZ logo are registered trademarks of AZ Electronic Materials (Germany) GmbH.