MEMS2.2 Bulk Micromachining(MEMS讲义)
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1. Increasing the temperature increase the reaction rate. 2. The graph shows two straight-line segments.
T < 30oC: Steeper slope => higher activation energy EA The energy is associated with the oxidation reaction => reaction-limited.
(2.2)
primary oxidizing species, OH-, is formed by the
dissociation of water
H2O <---> OH- + H+
(2.3)
3. Reaction of the hydrated Si with complexing agent
(complexant) in the solution
mechanism of silicon wet etching
1. Injection of holes into the semiconductor
Si + 2h+ ---> Si2+
(2.1)
2. Attachment of hydroxyl group OH- to Si2+
Si2+ + 2(OH-) ---> Si(OH)2
However, thick layer are used as a mask anyway , especially for shallow etching, as the oxide is so easy to form and pattern.
2. Photoresists (or Al) cannot stand up to HNO3 and cannot be used as mask material.
directionality of silicon etching
One of the most important characteristics: directionality a) isotropic or nondirectional b) anisotropic or directional
Anisotropic etching of silicon
Commonly used etchant systems: EDP(乙二胺邻苯二酚), KOH, Hydrazine(肼，联氨)
➢Etch rate ratios of about 400:1 for (100) to (111) orientations ➢ The etch rate for (110) surfaces lies between those for (100) and (111) surfaces ➢Masking materials: Silicon dioxide, silicon nitride, and some metallic thin films etc.
T > 30oC: milder slope => lower activation energy EA The energy is associated with the dissolution of oxide => diffusion-limited. Agitation can change the etch profile.
Isotropic etching of silicon
Polishing Slices structure Patterning film
HyCdroofmluomrico(nHlFy),unistreicdaecitdc(hHaNnOt3)saynsdteacmeti:c acHidN(CAH3COOH) Overall reaction of HNA etchant with Silicon:
(b) The top plan view of an anisotropically etched wafer showing the fabrication of a cantilever(悬臂) beam using etch stop layers.
(c) The cross section, A-A', showing the hole, diaphragm and cavity of (a).
Etch stop techniques
It is desired to stop etching in silicon when:
2.2 Bulk Micromachining
Isotropic etching of silicon Anisotropic etching of silicon Dopant dependent etch stop Electrochemical etch stop P-N Junction Etch Stop Dry Etching
Depend on etchant system, structure of substrate, and etching methods
Etchant systems: HNA: hydrofluoric, nitric acid and acetic acid EDP: ethylene-diamine(乙二胺) and pyrocatechol(邻苯二酚) KOH:
3. Dopant concentration < 1017 atoms/cm3 => the etch rate of Si in HNA is reduced by ~150 times, and can be used as an etch stop.
4. Au/Cr or LPCVD silicon nitr ide (etching rate at 10 to 100 A/min) are the preferred masking materials.
Corner Compensation
Etch stop techniques
To control the etch depth by controlling the etch-rate and -time requires monitoring and stabilization of:
➢ uniformity of the silicon wafer thickness (the best wafer quality is about 2 mm. It can be as high as 40 mm in some wafers!) ➢ Etch composition ➢ Etch aging ➢ Etch temperature ➢ Light effect ➢ Surface preparation.
to sculpt micromechanical devices from the silicon substrate
Wet micromachining theory
The fundamental etch processes are electrochemical reactions:
a) oxidation-reduction b) dissolution of the oxidation products
Factors determining etching rate: a) supply of minority carriers (electron-hole pairs) b) crystal orientation c) concentration and species of doping atoms d) lattice defects e) surface structure
4. Dissolution of the reacted product into etchant solution
mechanism of silicon etching
Two kinds of process: a) reaction-rate limited (temperature) b) diffusion-limited (agitation)
Both anodic and cathodic microscopic sites exist at the silicon surface. Oxidation of semiconductor atoms takes place at the anodic sites, while the oxidant is reduced at the cathodic sites.
Anisotropic etching of silicon
(100) silicon substrate etching proceeds along the (100) planes while it is Chemical means of etching may displayprcarcytsictaallllyogsrtoapppheidc along dependence due to bond energy and botnhde (d1e1n1s)itpylaonfeesach plane
Si + HNO3 + 6HF ---> H2SiF6 + H2NO2 + H2O + H2
ISO-ETCH curve foCH curve for HNA system
ISO-ETCH curve for HNA system
Temperature Dependence of Etch Rate in HNA System
Effect of Misalignment on {100} Substrate
Convex corner undercut
If mask pattern exit convex corner s, the silicon will be undercut along (212) planes when the etchant is KOH
Agitation Dependence of Etch Rate in HNA System
Masking for the HNA Etchant
Masking for the HNA Etchant
1. Thermal oxide etches at 300 to 800 A/min. is not a good masking material.
(d) The cross section, B-B', showing the cantilever beam of (b).
Anisotropic etching of silicon
For (100) plane etching:
Wb = W0 – 2hcos(54.75o)
where Wo is the width of the etch mask window on the wafer back surface, h is the etched depth，and Wb is the bottom of pattern.
Basic structure produced in silicon
(a) The bottom plan view of an anisotropic etched wafer showing the fabrication of cavities(腔), diaphragms(膜) and holes(孔).
Classification
Bulk micromachining of silicon:
➢ Wet bulk micromaching or Wet etching ➢ Dry etching
Related techniques:
➢ etch masks ➢ etch stops
Purpose:
T < 30oC: Steeper slope => higher activation energy EA The energy is associated with the oxidation reaction => reaction-limited.
(2.2)
primary oxidizing species, OH-, is formed by the
dissociation of water
H2O <---> OH- + H+
(2.3)
3. Reaction of the hydrated Si with complexing agent
(complexant) in the solution
mechanism of silicon wet etching
1. Injection of holes into the semiconductor
Si + 2h+ ---> Si2+
(2.1)
2. Attachment of hydroxyl group OH- to Si2+
Si2+ + 2(OH-) ---> Si(OH)2
However, thick layer are used as a mask anyway , especially for shallow etching, as the oxide is so easy to form and pattern.
2. Photoresists (or Al) cannot stand up to HNO3 and cannot be used as mask material.
directionality of silicon etching
One of the most important characteristics: directionality a) isotropic or nondirectional b) anisotropic or directional
Anisotropic etching of silicon
Commonly used etchant systems: EDP(乙二胺邻苯二酚), KOH, Hydrazine(肼，联氨)
➢Etch rate ratios of about 400:1 for (100) to (111) orientations ➢ The etch rate for (110) surfaces lies between those for (100) and (111) surfaces ➢Masking materials: Silicon dioxide, silicon nitride, and some metallic thin films etc.
T > 30oC: milder slope => lower activation energy EA The energy is associated with the dissolution of oxide => diffusion-limited. Agitation can change the etch profile.
Isotropic etching of silicon
Polishing Slices structure Patterning film
HyCdroofmluomrico(nHlFy),unistreicdaecitdc(hHaNnOt3)saynsdteacmeti:c acHidN(CAH3COOH) Overall reaction of HNA etchant with Silicon:
(b) The top plan view of an anisotropically etched wafer showing the fabrication of a cantilever(悬臂) beam using etch stop layers.
(c) The cross section, A-A', showing the hole, diaphragm and cavity of (a).
Etch stop techniques
It is desired to stop etching in silicon when:
2.2 Bulk Micromachining
Isotropic etching of silicon Anisotropic etching of silicon Dopant dependent etch stop Electrochemical etch stop P-N Junction Etch Stop Dry Etching
Depend on etchant system, structure of substrate, and etching methods
Etchant systems: HNA: hydrofluoric, nitric acid and acetic acid EDP: ethylene-diamine(乙二胺) and pyrocatechol(邻苯二酚) KOH:
3. Dopant concentration < 1017 atoms/cm3 => the etch rate of Si in HNA is reduced by ~150 times, and can be used as an etch stop.
4. Au/Cr or LPCVD silicon nitr ide (etching rate at 10 to 100 A/min) are the preferred masking materials.
Corner Compensation
Etch stop techniques
To control the etch depth by controlling the etch-rate and -time requires monitoring and stabilization of:
➢ uniformity of the silicon wafer thickness (the best wafer quality is about 2 mm. It can be as high as 40 mm in some wafers!) ➢ Etch composition ➢ Etch aging ➢ Etch temperature ➢ Light effect ➢ Surface preparation.
to sculpt micromechanical devices from the silicon substrate
Wet micromachining theory
The fundamental etch processes are electrochemical reactions:
a) oxidation-reduction b) dissolution of the oxidation products
Factors determining etching rate: a) supply of minority carriers (electron-hole pairs) b) crystal orientation c) concentration and species of doping atoms d) lattice defects e) surface structure
4. Dissolution of the reacted product into etchant solution
mechanism of silicon etching
Two kinds of process: a) reaction-rate limited (temperature) b) diffusion-limited (agitation)
Both anodic and cathodic microscopic sites exist at the silicon surface. Oxidation of semiconductor atoms takes place at the anodic sites, while the oxidant is reduced at the cathodic sites.
Anisotropic etching of silicon
(100) silicon substrate etching proceeds along the (100) planes while it is Chemical means of etching may displayprcarcytsictaallllyogsrtoapppheidc along dependence due to bond energy and botnhde (d1e1n1s)itpylaonfeesach plane
Si + HNO3 + 6HF ---> H2SiF6 + H2NO2 + H2O + H2
ISO-ETCH curve foCH curve for HNA system
ISO-ETCH curve for HNA system
Temperature Dependence of Etch Rate in HNA System
Effect of Misalignment on {100} Substrate
Convex corner undercut
If mask pattern exit convex corner s, the silicon will be undercut along (212) planes when the etchant is KOH
Agitation Dependence of Etch Rate in HNA System
Masking for the HNA Etchant
Masking for the HNA Etchant
1. Thermal oxide etches at 300 to 800 A/min. is not a good masking material.
(d) The cross section, B-B', showing the cantilever beam of (b).
Anisotropic etching of silicon
For (100) plane etching:
Wb = W0 – 2hcos(54.75o)
where Wo is the width of the etch mask window on the wafer back surface, h is the etched depth，and Wb is the bottom of pattern.
Basic structure produced in silicon
(a) The bottom plan view of an anisotropic etched wafer showing the fabrication of cavities(腔), diaphragms(膜) and holes(孔).
Classification
Bulk micromachining of silicon:
➢ Wet bulk micromaching or Wet etching ➢ Dry etching
Related techniques:
➢ etch masks ➢ etch stops
Purpose: