張照云，唐彬，彭勃，陳穎慧，熊壯，蘇偉

(1.中國(guó)工程物理研究院電子工程研究所，四川綿陽(yáng)621999; 2.中國(guó)工程物理研究院超精密加工重點(diǎn)實(shí)驗(yàn)室，四川綿陽(yáng)621999)

一種犧牲層工藝中金屬電極保護(hù)方法研究*

張照云1，2*，唐彬1，2，彭勃1，2，陳穎慧1，2，熊壯1，2，蘇偉1，2

(1.中國(guó)工程物理研究院電子工程研究所，四川綿陽(yáng)621999; 2.中國(guó)工程物理研究院超精密加工重點(diǎn)實(shí)驗(yàn)室，四川綿陽(yáng)621999)

為了在犧牲層工藝中保護(hù)金屬電極不被腐蝕，提出了一種利用鋁層保護(hù)金屬電極的方法。在試驗(yàn)中采用了兩種鋁保護(hù)層厚度300 nm和1μm，在兩種腐蝕液體4NH4F∶1HF∶2甘油和1HAC∶1NH4F中進(jìn)行了試驗(yàn)，通過(guò)試驗(yàn)表明，鋁金屬層在兩種腐蝕液體中都能夠有效保護(hù)金屬電極，延長(zhǎng)犧牲層的腐蝕時(shí)間。最后，通過(guò)SOI微加速度計(jì)加工工藝進(jìn)一步驗(yàn)證了該方法的有效性。

微電子機(jī)械系統(tǒng);微加速度計(jì);犧牲層刻蝕;金屬電極保護(hù)

In recent years，the SOI(Silicon on Insulator) material which has a monocrystalline structure layer is fast gaining prominence in the development of MEMS devices［1-6］.In the SOIMEMSmicromachining process，the release of the structures is the last processing step.Any patterning with the structures freestanding is difficult to implement，due to the destructive effects of the spinning of resist on the delicate structures.Moreover，any deposited materials can get lodged underneath the freestanding structures.Therefore，when the sacrificial etching takes place，the metal pad，needed to connect the electronic components and the micromechanical structures，will have to be present on the wafer.

The hydrofluoric acid(HF)or buffered hydrofluoric acid(BHF)is the most commonly used etchant for silicon dioxide sacrificial layer etching，because these etchants provide a high selectivity between the sacrificial oxide and the siliconmechanical layers and a high etch rate，which is necessary to perform large underetching in a reasonable period of time.

A problem arises，as the etchants HF and BHF etch themetal pad aswell as the sacrificial oxide.During the etching of silicon dioxide sacrificial layer，many pinholes can be found on the metal pad after a certain etching time.As the etching time increases，the number of pinholes increases.When the time is further extended，some electrode shed［7］.Therefore，themetal pad must be protected.

Various approaches have been brought forward to deal with this problem.Literature［8］introduced usingPECVD silicon nitride to protect aluminum interconnect，this layer can be deposited at 300℃.However，this method is too complicated，which needs two extra depositions and two extramasks，and PECVD silicon nitride shows many mechanical defects in the form of pinholes，through which the etchant could reach the aluminum and ruin the aluminum interconnect.Photoresist is usually used to protect themetal pad，but photoresist is not strong enough and limiting the maximum etch time to around 10min.For the long wet etches，the resist needs a complete postbake.And the adhesion of the photoresist layer to the surface of the chip is the another problem.A weak bond can cause the resist to separate from the surface，allowing etchant to seep in underneath and attack the underlying aluminum［7-8］.From literature［9-11］，by the addition of glycerin or glycerol to the BHF can increase the selectivity between aluminum and the sacrificial layer.For example，literature［11］add glycerol to BHF solution with the proportion of 4NH4F∶1HF∶2glycerol in volume and heating the etchant solution to 55℃～65℃while constantly agitating it with a magnetic stirring bar，has significantly increased the selectivity，resulting in a etching selectivity of 600 to 1 between silicon dioxide and aluminum.Enlightened by this，this paper investigated the method of using aluminum layer to protect metal pad during the etching of sacrificial silicon dioxide layer.

1 Experiment

Because the etching rate of silicon dioxide layer and aluminum and the selectivity between aluminum and silicon dioxide was investigated thoroughly in literature［11］.So in this paper，we just studied the metal pad propertieswith the aluminum protection layer.

Two single-side-polished(110)oriented 100 mm silicon wafers were used to carry out the experiments.Two waferswere fist used lift-offmethod to producemetal pad，which was Tiw/Au with 20 nm Tiw and 200 nm Au.And then one wafer was sputtered with 300 nm aluminum layer，the other wafer was sputtered with 1 000 nm aluminum layer.Two wafer were then manually cleaved into dozens of small pieces about1 cm by 2 cm in dimensions for further experiments.

There were generally two series of experiments.One was the etchant solution of 4NH4F∶1HF∶2glycerol and with the aluminum protection layer of 300 nm and 1 000 nm.The otherwas used the solution with the proportion of 1HAC∶1NH4F(40%)and with the aluminum protection layer of 300 nm and 1 000 nm.The etchant solutions for both groupswere fullymixed.

From literature［11］，the selectivity between aluminum and the silicon dioxide sacrificial layer was interrelated with temperature and agitation situation.So during the etching，the etchant solutions agitated with amagnetic stirring bar throughout the process and with a temperature controllable water-bath.

2 Results and Discussion

Selectivity is crucial for MEMSstructures to be left unaffected by the sacrificial layer etch.In the case of poor etch selectivity，the structures are attacked in two possible ways:continuous，homogeneous chemical etching and local dissolution(pitting，intergranular corrosion).There are some methods to inhibit chemical etching of the MEMS structures by the sacrificial layer etchant［12］:

(1)introducing a protection layer，

(2)allowing chemical build-up of a native passivation layer，

(3)adjusting the pH to enhance the selectivity or

(4)adding chemical substances produced in the attack reaction in order to shift the chemical equilibrium

During the sacrificial layer etch，corrosion can occur by the formation of local electrochemical cells，the formation of a porous passivation layer or by propagation of voids and cracks.Some methos to inhibit corrosion are［12］:

(1)improvement of the protection quality of the passivation layer e.g.by adding chemical oxidants to the solution，

(2)suppression of adsorption of reactantmolecules e.g.by addition of surfactants(discussed below in the case of aluminum attacked)，

(3)addition of complexing or gelating agents to peptize corrosion products(discussed below)，or

(4)reduced stress in the structural layer to avoid stress corrosion.

Based on the above theory，we choose two etchants to increase the selectivity:4NH4F∶1HF∶2glycerol etchant and 1HAC∶1NH4F etchant.2.1 4NH4F∶1HF∶2glycerol etchant

This etchant solution started boiling at about 72℃，and started to become instable around 70℃，resulting in the HF beginning to escape from the system.There is no obvious phenomenon even when heated to 67℃［11］，so the temperature is controlled at 65℃.

The experiment results are showed in the Fig.1 and Fig.2.Fig.1 is the results of 300nm aluminum protect layer.From the resultswe can see that:(1)after 30min etching，the aluminum protect layer above themetal pad is etched off;(2)after 60 min etching，the whole aluminum protect layer is etched off and themetal pad is kept in good condition.Fig.1 is the results of 1 000 nm aluminum protect layer.From the results we can see that: (1)after a certain etching time，there is air bubble under the aluminum protect layer above themetal pad; (2)when the etching time increase，the air bubble become bigger;(3)after 60 min etching，the aluminum protect layer is not etched off and themetal pad is kept in good condition after removing the aluminum layer.

Fig.1 The results of 300 nm alum inum protect layer with etchant of 4NH4F∶1HF∶2glycerol

Fig.2 The results of 1 000 nm alum inum protect layer with etchant of 4NH4F∶1HF∶2glycerol

From Comparing the two experiment，we can see that the aluminum protect layer can protectmetal pad and prolong the etching time.And the thicker aluminum layer can have a longer etching time.

2.2 1HAC∶1NH4F etchant

It is dangerous to heat up the etchant of 4NH4F∶1HF∶2glycerol.So we use another etchant of 1HAC∶1NH4F to etching silicon oxide sacrificial layer.This etchant can be used in normal temperature and with high selectivity between silicon oxide and aluminum，but this etchant have a lower etching rate of silicon oxide sacrificial layer.

The experiment results are showed in the Fig.3 and Fig.4.Fig.3 is the results of 300 nm aluminum protect layer and Fig.4 is the results of 1 000 nm aluminum protect layer.From the results we can see that:(1)after a certain etching time，there is air bubble under the aluminum protect layer above themetal pad;(2)when the etching time increase，the air bubble become bigger;(3)after 60min etching，the aluminum protect layer is not etched off and themetal pad can be kept in good condition.

Fig.3 The results of 300 nm alum inum protect layer with etchant of 1HAC∶1NH4F

Fig.4 The results of 1 000 nm alum inum protect layer w ith etchant of 1HAC∶1NH4F

From the two experiment A and B，we can see that the aluminum protect layer can protectmetal pad and prolong the etching time in both etchants.And the thicker aluminum layer can have a longer etching time.The 1HAC∶1NH4F etchant has a lower etching rate compared to the etchant of 4NH4F∶1HF∶2glycerol.

3 Application

To test the actual effect of thismethod，a SOI accelerometer device was examined.The devices were released in the solution of 4NH4F∶1HF∶2glycerol.The main fabrication process of using thismethod to produce SOIaccelerometer is shown in Fig.5:(a)The structural layer is patterned with a 3μm thick positive photoresist，then sputter metal film of Tiw/Au on the structure layer，after stripping the photoresist by acetonesolvent，themetal electrode is obtained;(b)By ashing in oxygen plasma to remove the remaining organic residues，the structural layer is sputtered 1 000 nm aluminum layer coveringmetal pad;(c)A 3μm thick positive photoresist is patterned on the aluminum layer，and then etch the aluminum to form the accelerometer pattern;(d)Use DRIE to etch the silicon deep to the oxide layer;(e)Remove the photoresist;(f)Use the solution of 4NH4F∶1HF∶2glycerol to etch the silicon oxide sacrificial layer;(g)Remove the rest aluminum layer.

Fig.5 Themain fabrication process of using this method to produce SOI accelerometer

As the photoresist film was not removed after the structure etching process with DRIE(step(e))，it will serve as an additional etch barrier during the silicon oxide etching process.Because the photoresist without completely postbake to remove all the sovents present in the resist，leaving it relatively soft and porous.To prolong the resist etch time，the resist need a complete postbake after DRIE process.

Fig.6 SEM images of a part SOI accelerometer with good metal pad using 4NH4F∶1HF∶2glycerol etchant to etch the silicon oxide sacrificial layer

Through above method，the devices of SOImicroaccelerometers are obtained successfully.Fig.6 is a SEM image of a part SOIaccelerometer with good metal pad.So thismethod is tested effective through SOI accelerometer fabrication.

4 Conclusion

The paper proposed a method of using aluminum layer to protect metal electrode in sacrificial etching process.By adding glycerol to BHF solution with the proportion of 4NH4F∶1HF∶2glycerol and heating the etchant to 65℃while constantly agitating it throughout the process，the selectivity between silicon dioxide and aluminum can be increased significantly.So using the 4NH4F∶1HF∶2glycerol to etch the silicon dioxide sacrificial layer while protecting the metal pad using aluminum layer，themetal pad can be kept in good condition after release the MEMS structures.When the undercut length needed to etch is of a few microns，we can use the etchant of 1HAC∶1NH4F to etch the silicon dioxide sacrificial layer.This etchant is not need to heat and has a high selectivity between silicon dioxide and aluminum，but has a lower etching rate.And also can use aluminum layer to protectmetal pad when using 1HAC∶1NH4F to etch the silicon dioxide sacrificial layer.At last，to test the actual effect of thismethod，a SOIaccelerometer device was examined and it is tested effective through SOIaccelerometer fabrication.

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張照云(1984-)，男，四川南充人，工程師，主要研究方向?yàn)槲T性器件設(shè)計(jì)、加工技術(shù)研究。

A M ethod of M etal Electrode Protection in Sacrificial Etching Process*

ZHANG Zhaoyun1，2*，TANG Bin1，2，PENG Bo1，2，CHEN Yinhui1，2，XIONG Zhuang1，2，SUWei1，2

(1.Institute of Electronic Engineering，China Academy of Engineering Physics，Mianyang Sichuan 621900，China; 2.Laboratory of Precision Manufacturing Technology，China Academy of Engineering Physics，Mianyang Sichuan 621900，China)

In order to protect themetal electrode in sacrificial etching process，amethod of using is aluminum layer to protectmetal electrode is proposed.Two thick aluminum layers(one is 300 nm，the others is 1μm)to protect metal electrode were studied in two etchants，one is 4NH4F∶1HF∶2 glycerol and the other is 1HAC∶1NH4F.Through the experiment，themetal pad can be kept in good condition after a long etching time through the protection of aluminum layer.Finally，thismethod is tested effective through SOIaccelerometer fabrication.

MEMS;Micro-accelerometer;sacrificial etching;metal electrode protection.

C:2575

10.3969/j.issn.1005－9490.2017.01.001

TN305.7

:A

:1005－9490(2017)01－0001－05

項(xiàng)目來(lái)源:The Laboratory of Precision Manufacturing Technology of CAEP under Grant(ZZ14007，ZZ16003)

2016-02-04修改日期:2016-02-18