Lecture 10 Wet Etching - Can be very useful for making...

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Wet Etching and Bulk Micromachining Fundamentals of Micromachining Dr. Bruce K. Gale (WFKLQJ±&KHPLVWU\ ±7KH±HWFKLQJ±SURFHVV±LQYROYHV² ± 7UDQVSRUW±RI±UHDFWDQWV±WR±WKH±VXUIDFH ± 6XUIDFH±UHDFWLRQ ± 7UDQVSRUW±RI±SURGXFWV±IURP±WKH±VXUIDFH ±.H\±LQJUHGLHQWV±LQ±DQ\±ZHW±HWFKDQW² ± 2[LGL]HU ±H[DPSOHV²±+±³±2±³±´±+12±µ ± $FLG±RU±EDVH±WR±GLVVROYH±R[LGL]HG±VXUIDFH ±H[DPSOHV²±+±³±62±¶±´±1+±¶±2+ ± 'LOOXWHQW²PHGLD²WR²WUDQVSRUW²UHDFWDQWV²DQG²SURGXFWV²WKURXJK ²H[DPSOHV³²+²´²2µ²&+²¶²&22+ .H\±7HFKQRORJLHV±RI±:HW±(WFKLQJ 3URILOHV ±²,VRWURSLF²DQG²$QLVRWURSLF $SSOLFDWLRQV ±²6LOLFRQ³²6LOLFRQ²1LWULGH³²6LOLFRQ² 'LR[LGH³²0HWDO &RQWUROV ±²'RSLQJ³²(OHFWURFKHPLFDO³²)LOP²4XDOLW\³² 0DVN²0DWHULDOV :HW±(WFKLQJ ,VRWURSLF±HWFKLQJ – Same etch rate in all directions – Lateral etch rate is about the same as vertical etch rate – Etch rate does not depend upon the orientation of the mask edge $QLVRWURSLF±HWFKLQJ – Etch rate depends upon orientation to crystalline planes – Lateral etch rate can be much larger or smaller than vertical etch rate, depending upon orientation of mask edge to crystalline axes – Orientation of mask edge and the details of the mask pattern determine the final etched shape • Can be very useful for making complex shapes • Can be very surprising if not carefully thought out • Only certain “standard” shapes are routinely used
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LVRWURSLF± HWFKLQJ DQLVRWURSLF± HWFKLQJ 0DVN 6XEVWUDWH )LOP 6LOLFRQ±(WFKLQJ 6LOLFRQ±(WFKLQJ 6LOLFRQ (WFKLQJ
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6LOLFRQ±(WFKLQJ 6LOLFRQ±(WFKLQJ ,VRWURSLF ´µ¶²+)±+12 · ±&+ · &22+±+ ¸ ¹²²´¸¶²+) ´·¶²+)±1+ º ) $QLVRWURSLF ´µ¶².2+²²²´¸¶²('3² ´(WK\OHQHGLDPLQH²3\URFDWHFKRO¶ ´·¶²&V2+²²´º¶²1D2+²²²²²´»¶²1 ¸ + º ¼+ ¸ ¹²´+\GUD]LQH¶ 0DVNLQJ²0DWHULDOV ´µ¶²3KRWRUHVLVW²´$FLGV²2QO\¶²²²²²´¸¶²6L · 1 º ´·¶²6L2 ¸ KOH Etching Etch Rate (110) > (100) > (111) (100) > (110) > (111) w/ IPA Varies with Temperature and Concentration (see appendix C in Madou) 54.7 ° <100> Wafer Cross-section Top View [ ] [ ] kT E a e KOH O H k R = 4 1 4 2 0
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R. B. Darling / EE-527 Hydroxide Etching of Silicon Several hydroxides are useful: KOH, NaOH, CeOH, RbOH, NH 4 OH, TMAH: (CH 3 ) 4 NOH Oxidation of silicon by hydroxyls to form a silicate: Si + 2OH + 4h + Si(OH) 2
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