Slurry and processing technique of CLBO crystal

WANG Sheng-li(王胜利), LIU Yu-ling(刘玉岭), NIU Xin-huan(牛新环), TAN Bai-mei(檀栢梅)

School of Information Engineering, Hebei University of Technology, Tianjin 300130, China

Received 10 April 2006; accepted 25 April 2006

Abstract: CsLiB₆O₁₀(CLBO) is a new-type nonlinear optical crystal material. CLBO has many good performances, especially the frequency multiplication performance in deep ultraviolet band. CLBO has important application prospect on solid-state UV laser, broad band tunable laser and laser nucleus flame igniter. Though, CLBO will be air slaking and cracking when the ambient humidity is more than 40%, which brings more difficult on CLBO surface finishing. According to the performance and structure characteristic of CLBO crystal, a new water-free slurry applying for CLBO crystal chemical mechanical polishing(CMP) was investigated. The abrasive is SiO₂. The influence of polishing processing parameter on polishing process for CLBO crystal was discussed, and the parameter optimal value of polishing plate speed, pressure, pH value and abrasive concentration were determined. Through such parameters, high efficiency and precision plane polishing was gotten. The CLBO CMP process was studied, the results show that low pressure and high speed can improve the CLBO crystal surface removal rate and flatness.

Key words: CsLiB₆O₁₀ crystal; water-free slurry; chemical mechanical polishing

1 Introduction

CsLiB₆O₁₀(CLBO) is a new-type nonlinear optical crystal, which was found in 1990s[1,2]. CLBO has many good performances, such as higher nonlinearity factor, higher antisun damage threshold, wide transmit range, and stable chemical performance. CLBO is widely  used as tunable laser, full solid state short wavelength laser, especially DUV full solid state laser for its frequency multiplication in ultraviolet band. Otherwise, CLBO also has major temperature bandwidth, angle bandwidth and spectra bandwidth, which leads to the higher stability of CLBO frequency multiplier in practical application[3].

Although CLBO has so many good performances, it has the disadvantage of air slaking and cracking easily under the condition of ambient humidity more than 40%[4]. With the crystal growth defect increasing, cracking is easier to happen. Such character has a strong impact on its practical application, and takes more difficulty for CLBO growth and device processing. So the surface planarization is an very important and difficult problem.

As yet, the outside CLBO planarization technique is still in blocked state, and the reports of such aspect

inland is rarely. ZHANG et al[5, 6] reported the optical characteristic and growth structure of CLBO, and HE et al[7] reported the mechanism of CLBO cracking. In this study, we investigated a new water-free slurry which was applicable for CLBO crystal chemical mechanical polishing(CMP)[8,9]. We discussed the influence of polishing processing parameter on polishing process for CLBO crystal and determined the parameter optimal value of polishing plate speed, pressure, pH value and abrasive concentration. Through such parameters, we got high efficiency and precision plane polishing. We also studied the CLBO CMP process and found that low pressure and high speed can improve the CLBO crystal surface removal rate and flatness. This study can improve the practical application economic benefit of CLBO effectively.

2 Experimental

Because of the air slaking character of CLBO, it is easy to crack when it is placed in air long time and the ambient humidity is more than 40%. So we can’t use the usual slurry for CLBO surface planarization technique. It is important to investigate the water-free slurry, polishing method and polishing condition.

2.1 Preparation of slurry

Slurry is the key factor during the chemical mechanical polishing(CMP) process. The performance of slurry can influence the surface quality of post-polishing material. There are many traditional slurry species, such as magnesia aqua, alumina aqua, titanic alkali slurry, silica alkali slurry, chromic trioxide acidic slurry and cupric ion acidic slurry[10]. But there is water in these slurry, they can’t be used as CLBO slurry. It is required to find a new-type water-free slurry, which should select organic matter as solvent and has good solubility for nano-abrasive.

2.1.1 Selection of nano-abrasive

Al₂O₃, SiC, ZrO₂, SiO₂, B₄C, chromium ion and copper ion are the typical nano-abrasives. SiC, ZrO₂, SiO₂, B₄C, chromium ion and copper ion abrasive have higher hardness, which can cause surface layer damaged deeply. The thermo-chemical fault density can reach 10⁵-10⁶ cm^-2. Otherwise, chromium ion and copper ion can result in ambient pollution and metallic ion contamination. Al₂O₃ may also introduce aluminum ion in CLBO crystal during the CMP process. Comparing the above abrasives, silica abrasive has the advantages as follows: 1) lower hardness(Moths hardness 7); 2) smaller particle size (10-100 nm); 3) shallow damage layer (fault density less than 1×10² cm^-2). Then we chose silica as abrasive to prepare water-free slurry.

2.1.2 Selection of organic solvent

Organic solvent is one category of organic compound with not too large molecular mass, which is used in life and manufacture widely. It is liquid state at room temperature. Organic solvent has many species, such as paraffin, olefine, alcohol, aldehyde, amine, ester, alkone, arene and halocarbon, and most of them have toxicity. But the organic solvent used for water-free slurry must have the characters as follows: 1) liquid, non-poisonous and no pollution; 2) non-volatility; 3) good solubility for SiO₂ abrasive. Firstly, the organic solvent with benzene ring, such as C₆H₅CH₃ and C₆H₄(CH₃)₂, are mostly toxic. Secondly, oiliness organisms, for example petrol, are mostly volatility, and the solubility for SiO₂ abrasive is not good. Considering the economic benefit and character, such as non-poisonous, non-volatile, viscosity moderate and good solubility for SiO₂ abrasive, we can choose low cost polyethylene glycol and glycol or glycerin as the water-free slurry organic solvent.

2.1.3 Selection of alkali

In alkali slurry, it is very important for alkali selecting. We should consider the induction number, such as stable pH value, non-poisonous and dissolution in organic solvent. The usual used solid inorganic base, such as KOH and NaOH, can’t dissolve in organic solvent. In order to satisfy the water-free slurry, we should select water-free organic base as the pH value conditioner. Organic amine is not easily volatile, and has stable pH value and good buffer action, but organic amine used as pH conditioner has preferential character. The preferential character is that the reaction velocities of organic base on crystal surface different orientation is not equal, which can enlarge the sample surface pattern difference. Such performance is disadvantaged for finished sample surface. But we can reduce the preferential character of organic amine by decreasing the alkali concentration. In addition, organic amine also takes the action of chelate, which can decrease metallic ion adsorption and contamination on crystal surface. So we chose organic amine as the pH value conditioner in our experiment.

2.1.4 Selection of surfactant

Surfactant takes important action on CLBO crystal polishing. It affects the dispersivity of abrasive, cleaning complex degree after grain adsorption and metallic ion contamination. The surfactant in slurry can decrease sample surface tension and damage layer thickness. It can lead the adsorbate to physical adsorption state, and then the adsorbate is easy to clear. But the usual used FA/O surfactant is the mixture of organic matter and water, so it is not applicable for water-free slurry. The surfactant of JFC and OP-7 has the performance of decreasing surface tension, improving the finish and osmosis of polishing surface. There are water-free organic matters mutual dissolving with organic solvent. So we chose the surfactant of JFC and OP-7 as the using object in our experiment design.

2.2 CMP process

During the slurry preparing, we put reinforcing silica (SiO₂ abrasive) in organic solvent, then put organic base in it and adjusted pH value to a effective range. Before polishing, we put some certain amount surfactant in the slurry to decrease the surface tension and improve the surface finish. The requirement for slurry is good fluidness, non-precipitation, clumping and good floatability, innocuity, good removal rate and surface quality. In order to realize the requirement of slurry, we chose nano-SiO₂ (particle size 100-200 nm), polyethylene glycol and organic amine alkali as the abrasive, organic solvent and pH value conditioner (concentration more than 85.5%), respectively. After preparing the slurry, we did the CMP experiments under the condition of different component property. The CMP experiment was carried out on C6382I-W/YJ polisher. According to the experiment results, we found that the optimal process parameters for CLBO crystal polishing are that the abrasive concentration is 4%-5%, pH value is 8.5-9.5, down pressure is 50-100 kPa and wheel speed is 60 r/min.

3.1 Effect of abrasive concentration on polishing rate

Fig.1 shows the relation between polishing rate and abrasive concentration. From Fig.1, we can find that the polishing rate increases with the abrasive concentration increasing. The reason is that, with the abrasive concentration increasing, the number of particle taking part in the mechanical action increases, which leads to the improvement of polishing rate. But slurry fluidness and chemical constitution deteriorates with the abrasive concentration increasing. In order to ensure the slurry fluidness and avoid fovea caused by chemical staying longer time on CLBO crystal surface, the abrasive concentration should not be higher, e.g. 4%-5% was available. Such concentration can also be advantage for finishing surface quality.

Fig.1 Relation between polishing rate and abrasive concentra- tion

3.2 Effect of pH value on polishing rate

Fig.2 shows the relation between pH value and polishing rate. From Fig.2, we can find that the polishing rate increases with the pH value increasing. If the pH value is high, then the chemical action is very clear, which can cause not only high polishing rate, but also more surface defect. The pH value chosen is 8.5-9.5. If the organic base concentration is too high, it can cause CLBO crystal surface corrosion seriously. With the alkality increasing, the chemical reaction between organic base and CLBO crystal surface atoms is stronger, which causes unbalanced chemical and mechanical action. Then, the pits appear. If the alkality is weaker, the mechanical action takes the key action, which can cause surface damage. From the experimental results, we can find that the polishing rate between the pH value of 8.5-9.5 is available for application and the surface quality is better.

Fig.2 Relation between pH value and polishing rate

3.3 Effect of polishing pressure on polishing rate

Polishing pressure is also an important factor for CLBO polishing. According to Preston equation[11], the polishing rate should increase with the pressure increasing. The high pressure can improve mechanical action and cause temperature become higher, which is advantageous for chemical action. So higher pressure can cause higher polishing rate. But according to such material physical and chemical character, the pits appear under high polishing temperature and pressure. The polishing pressure chosen is 50-100 kPa.

3.4 Effect of other factors on polishing rate

The process of CMP is a complex physical and chemical process. The factors of influence the surface quality include many aspects，such as temperature, the uniformity of abrasive, slurry flow, operation environment and polishing time. With temperature increasing, the alkality of organic base increase, so temperature should be avoided increasing during the CMP process. The factors influencing temperature rise are wheel speed, polishing time, pressure, and so on. Slower speed is adverse to the mechanical action, so the method of avoiding temperature rise is shorting polishing time and reducing pressure, which can also reduce surface tears of CLBO crystal. According to our experiment, we find that the available process for CLBO CMP is lowering abrasive concentration, weakering alkality, lowering pressure and moderating temperature, which can cause good surface quality. Table 1 shows the comparison of CLBO polishing effect for different process parameters of different slurry. From the result, we can find that the available process for CLBO CMP is lowering abrasive concentration, weakering alkality, lowering pressure and moderating temperature, which can cause good surface quality.

Table 1 Comparison of CLBO polishing effect for different slurry on process parameters

Because the hygroscopy of CLBO crystal at room temperature under the humidity condition of 40%, the free-water slurry with good fluidity, pollution free, non-corrosion, moderate concentration and good floatability is prepared. In such slurry, abrasive is nano-SiO₂, organic solvent is polyethylene glycol, pH value conditioner is organic amine base. According to the adjusting component content, the optimal CMP process parameters gotten are that the abrasive concentration is 4%-5%, pH value is 8.5-9.5, pressure is 50-100 kPa, and wheel speed is 60 r/min.

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(Edited by LI Yan-hong)

Corresponding author: WANG Sheng-li; Tel: +86-22-26564054; Fax: +86-22-60204054; E-mail: shlwang@hebut.edu.cn