半导体硅片的化学清洗技术

对太阳能级硅片有借鉴作用
一. 硅片的化学清洗工艺原理
　　硅片经过不同工序加工后，其表面已受到严重沾污，一般讲硅片表面沾污大致可分在三类： A. 有机杂质沾污：可通过有机试剂的溶解作用，结合超声波清洗技术来去除。
　B. 颗粒沾污：运用物理的方法可采机械擦洗或超声波清洗技术来去除粒径 ≥ 0.4 μm颗粒，利用兆声波可去除 ≥ 0.2 μm颗粒。  　　C. 金属离子沾污：必须采用化学的方法才能清洗其沾污，硅片表面金属杂质沾污有两大类：
　a. 一类是沾污离子或原子通过吸附分散附着在硅片表面。
    b. 另一类是带正电的金属离子得到电子后面附着（尤如“电镀”）到硅片表面。
硅抛光片的化学清洗目的就在于要去除这种沾污，一般可按下述办法进行清洗去除沾污。
　　a. 使用强氧化剂使“电镀”附着到硅表面的金属离子、氧化成金属，溶解在清洗液中或吸附在硅片表面。  
　　b. 用无害的小直径强正离子（如H+）来替代吸附在硅片表面的金属离子，使之溶解于清洗液中。
　　c. 用大量去离水进行超声波清洗，以排除溶液中的金属离子。
自1970年美国RCA实验室提出的浸泡式RCA化学清洗工艺得到了广泛应用，1978年RCA实验室又推出兆声清洗工艺，近几年来以RCA清洗理论为基础的各种清洗技术不断被开发出来，例如：
⑴ 美国FSI公司推出离心喷淋式化学清洗技术。
⑵ 美国原CFM公司推出的Full-Flow systems封闭式溢流型清洗技术。
⑶ 美国VERTEQ公司推出的介于浸泡与封闭式之间的化学清洗技术（例Goldfinger Mach2清洗系统）。
⑷ 美国SSEC公司的双面檫洗技术（例M3304 DSS清洗系统）。  
⑸ 曰本提出无药液的电介离子水清洗技术（用电介超纯离子水清洗）使抛光片表面洁净技术达到了新的水平。
⑹ 以HF / O3为基础的硅片化学清洗技术。
　　
目前常用H2O2作强氧化剂，选用HCL作为H+的来源用于清除金属离子。
SC-1是H2O2和NH4OH的碱性溶液，通过H2O2的强氧化和NH4OH的溶解作用，使有机物沾污变成水溶性化合物，随去离子水的冲洗而被排除。
由于溶液具有强氧化性和络合性，能氧化Cr、Cu、Zn、Ag、Ni、Co、Ca、Fe、Mg等使其变成高价离子，然后进一步与碱作用，生成可溶性络合物而随去离子水的冲洗而被去除。
为此用SC-1液清洗抛光片既能去除有机沾污，亦能去除某些金属沾污。
SC-2是H2O2和HCL的酸性溶液，它具有极强的氧化性和络合性，能与氧以前的金属作用生成盐随去离子水冲洗而被去除。被氧化的金属离子与CL-作用生成的可溶性络合物亦随去离子水冲洗而被去除
在使用SC-1液时结合使用兆声波来清洗可获得更好的效果。

二. RCA清洗技术

传统的RCA清洗技术：所用清洗装置大多是多槽浸泡式清洗系统
清洗工序： SC-1 → DHF → SC-2
1. SC-1清洗去除颗粒：
⑴ 目的：主要是去除颗粒沾污（粒子）也能去除部分金属杂质
⑵ 去除颗粒的原理：
　　硅片表面由于H2O2氧化作用生成氧化膜（约6nm呈亲水性），该氧化膜又被NH4OH腐蚀，腐蚀后立即又发生氧化，氧化和腐蚀反复进行，因此附着在硅片表面的颗粒也随腐蚀层而落入清洗液内。
① 自然氧化膜约0.6nm厚，其与NH4OH、H2O2浓度及清洗液温度无关。
② SiO2的腐蚀速度，随NH4OH的浓度升高而加快，其与H2O2的浓度无关。
③ Si的腐蚀速度，随NH4OH的浓度升高而快，当到达某一浓度后为一定值，H2O2浓度越高这一值越小。  
④ NH4OH促进腐蚀，H2O2阻碍腐蚀。
⑤ 若H2O2的浓度一定，NH4OH浓度越低，颗粒去除率也越低，如果同时降低H2O2浓度,可抑制颗粒的去除率的下降。
⑥ 随着清洗洗液温度升高，颗粒去除率也提高，在一定温度下可达最大值。
⑦ 颗粒去除率与硅片表面腐蚀量有关，为确保颗粒的去除要有一 定量以上的腐蚀。  
⑧ 超声波清洗时，由于空洞现象，只能去除 ≥ 0.4 μm 颗粒。兆声清洗时，由于0.8Mhz的加速度作用，能去除 ≥ 0.2 μm 颗粒，即使液温下降到40℃也能得到与80℃超声清洗去除颗粒的效果，而且又可避免超声洗晶片产生损伤。
⑨ 在清洗液中，硅表面为负电位，有些颗粒也为负电位，由于两者的电的排斥力作用，可防止粒子向晶片表面吸附，但也有部分粒子表面是正电位，由于两者电的吸引力作用，粒子易向晶片表面吸附。
　　⑶. 去除金属杂质的原理：
① 由于硅表面的氧化和腐蚀作用，硅片表面的金属杂质，将随腐蚀层而进入清洗液中，并随去离子水的冲洗而被排除。
② 由于清洗液中存在氧化膜或清洗时发生氧化反应，生成氧化物的==能的绝对值大的金属容易附着在氧化膜上如：Al、Fe、Zn等便易附着在自然氧化膜上。而Ni、Cu则不易附着。
③ Fe、Zn、Ni、Cu的氢氧化物在高PH值清洗液中是不可溶的，有时会附着在自然氧化膜上。
④ 实验结果：
据报道如表面Fe浓度分别是1011、1012、1013 原子/cm2三种硅片放在SC-1液中清洗后，三种硅片Fe浓度均变成1010 原子/cm2。若放进被Fe污染的SC-1清洗液中清洗后，结果浓度均变成1013/cm2。
b. 用Fe浓度为1ppb的SC-1液，不断变化温度，清洗后硅片表面的Fe浓度随清洗时间延长而升高。
对应于某温度洗1000秒后，Fe浓度可上升到恒定值达1012~4×1012 原子/cm2。将表面Fe浓度为1012 原子/cm2硅片，放在浓度为1ppb的SC-1液中清洗，表面Fe浓度随清洗时间延长而下降，对应于某一温度的SC-1液洗1000秒后，可下降到恒定值达4×1010~6×1010 原子/cm2。这一浓度值随清洗温度的升高而升高。
从上述实验数据表明：硅表面的金属浓度是与SC-1清洗液中的金属浓度相对应。晶片表面的金属的脱附与吸附是同时进行的。
即在清洗时，硅片表面的金属吸附与脱附速度差随时间的变化到达到一恒定值。
以上实验结果表明：清洗后硅表面的金属浓度取决于清洗液中的金属浓度。其吸附速度与清洗液中的金属络合离子的形态无关。
c. 用Ni浓度为100ppb的SC-1清洗液，不断变化液温，硅片表面的Ni浓度在短时间内到达一恒定值、即达1012~3×1012原子/cm2。这一数值与上述Fe浓度1ppb的SC-1液清洗后表面Fe浓度相同
　　这表明Ni脱附速度大，在短时间内脱附和吸附就达到平衡。
⑤ 清洗时，硅表面的金属的脱附速度与吸附速度因各金属元素的不同而不同。特别是对Al、Fe、Zn。若清洗液中这些元素浓度不是非常低的话，清洗后的硅片表面的金属浓度便不能下降。对此，在选用化学试剂时，按要求特别要选用金属浓度低的超纯化学试剂
例如使用美国Ashland试剂，其CR-MB级的金属离子浓度一般是：H2O2 ＜10ppb 、HCL ＜10ppb、NH4OH ＜10ppb、H2SO4＜10ppb
⑥ 清洗液温度越高，晶片表面的金属浓度就越高。若使用兆声波清洗可使温度下降，有利去除金属沾污。
⑦ 去除有机物。
由于H2O2的氧化作用，晶片表面的有机物被分解成CO2、H2O而被去除。
⑧ 微粗糙度。
晶片表面Ra与清洗液的NH4OH组成比有关，组成比例越大，其Ra变大。Ra为0.2nm的晶片,在NH4OH: H2O2: H2O =1:1:5的SC-1液清洗后，Ra可增大至0.5nm。为控制晶片表面Ra，有必要降低NH4OH的组成比，例用0.5:1:5
⑨ COP（晶体的原生粒子缺陷）。
CZ硅片经反复清洗后，经测定每次清洗后硅片表面的颗粒 ≥2 μm 的颗粒会增加，但对外延晶片，即使反复清洗也不会使 ≥0.2 μm 颗粒增加。据近几年实验表明，以前认为增加的粒子其实是由腐蚀作用而形成的小坑。在进行颗粒测量时误将小坑也作粒子计入。小坑的形成是由单晶缺陷引起，因此称这类粒子为COP（晶体的原生粒子缺陷）。
介绍直径200 mm 硅片按SEMI要求：
256兆 ≥ 0.13 μm，＜10个/ 片，相当COP约40个。
　2.DHF清洗
a. 在DHF洗时，可将由于用SC-1洗时表面生成的自然氧化膜腐蚀掉，而Si几乎不被腐蚀。
b.硅片最外层的Si几乎是以 H 键为终端结构，表面呈疏水性。  
c. 在酸性溶液中，硅表面呈负电位，颗粒表面为正电位，由于两者之间的吸引力，粒子容易附着在晶片表面。
d. 去除金属杂质的原理：
① 用HF清洗去除表面的自然氧化膜，因此附着在自然氧化膜上的金属再一次溶解到清洗液中，同时DHF清洗可抑制自然氧化膜的形成。故可容易去除表面的Al、Fe、Zn、Ni等金属。但随自然氧化膜溶解到清洗液中一部分Cu等贵金属（氧化还原电位比氢高），会附着在硅表面，DHF清洗也能去除附在自然氧化膜上的金属氢氧化物。
② 实验结果：
报道Al3+、Zn2+、Fe2+、Ni2+ 的氧化还原电位E0 分别是 - 1.663V、-0.763V、-0.440V、0.250V比H+ 的氧化还原电位（E0=0.000V）低，呈稳定的离子状态，几乎不会附着在硅表面。
③ 如硅表面外层的Si以 H 键结构，硅表面在化学上是稳定的，即使清洗液中存在Cu等贵金属离子，也很难发生Si的电子交换，因经Cu等贵金属也不会附着在裸硅表面。但是如液中存在Cl— 、Br—等阴离子，它们会附着于Si表面的终端氢键不完全地方，附着的Cl— 、Br—阴离子会帮助Cu离子与Si电子交换，使Cu离子成为金属Cu而附着在晶片表面。
④ 因液中的Cu2+ 离子的氧化还原电位（E0=0.337V）比Si的氧化还原电位（E0=-0.857V）高得多，因此Cu2+ 离子从硅表面的Si得到电子进行 还原，变成金属Cu从晶片表面析出，另一方面被金属Cu附着的Si释放与Cu的附着相平衡的电子，自身被氧化成SiO2。
⑤ 从晶片表面析出的金属Cu形成Cu粒子的核。这个Cu粒子核比Si的负电性大，从Si吸引电子而带负电位，后来Cu离子从带负电位的Cu粒子核 得到电子析出金属Cu，Cu粒子状这样生长起来。Cu下面的Si一面供给与Cu的附着相平衡的电子，一面生成SiO2。
⑥ 在硅片表面形成的SiO2，在DHF清洗后被腐蚀成小坑，其腐蚀小坑数量与去除Cu粒子前的Cu粒子量相当，腐蚀小坑直径为0.01 ~ 0.1 μm，与Cu粒子大小也相当，由此可知这是由结晶引起的粒子，常称为金属致粒子（MIP）。
   3.SC-2清洗  
1 清洗液中的金属附着现象在碱性清洗液中易发生，在酸性溶液中不易发生，并具有较强的去除晶片表面金属的能力，但经SC-1洗后虽能去除Cu等金属，而晶片表面形成的自然氧化膜的附着（特别是Al）问题还未解决。
2 硅片表面经SC-2液洗后，表面Si大部分以 O 键为终端结构，形成一层自然氧化膜，呈亲水性。  
3 由于晶片表面的SiO2和Si不能被腐蚀，因此不能达到去除粒子的效果。
实验表明：
据报道将经过SC-2液，洗后的硅片分别放到添加Cu的DHF清洗或HF+H2O2清洗液中清洗、硅片表面的Cu浓度用DHF液洗为1014 原子/cm2，用HF+H2O2洗后为1010 原子/cm2。即说明用HF+H2O2液清洗去除金属的能力比较强，为此近几年大量报导清洗技术中，常使用HF+H2O2来代替DHF清洗。

三.离心喷淋式化学清洗抛光硅片

系统内可按不同工艺编制贮存各种清洗工艺程序，常用工艺是：
FSI“A”工艺： SPM+APM+DHF+HPM
FSI“B”工艺： SPM+DHF+APM+HPM
FSI“C”工艺： DHF+APM+HPM$s
RCA工艺： APM+HPM
SPM .Only工艺： SPM
Piranha HF工艺： SPM+HF
上述工艺程序中：
SPM=H2SO4+H2O2 4：1 去有机杂质沾污
DHF=HF+D1.H2O (1-2%) 去原生氧化物，金属沾污
APM=NH4OH+ H2O2+D1.H2O 1：1：5或 0.5：1：5
去有机杂质，金属离子，颗粒沾污
PM=HCL+ H2O2+D1.H2O 1：1：6
去金属离子Al、Fe、Ni、Na等
再结合使用双面檫洗技术可进一步降低硅表面的颗粒沾污

四. 新的清洗技术

A.新清洗液的开发使用
1）.APM清洗
a. 为抑制SC-1时表面Ra变大，应降低NH4OH组成比，例： NH4OH：H2O2：H2O = 0.05：1：1
    当Ra = 0.2nm的硅片清洗后其值不变，在APM洗后的D1W漂洗应在低温下进行。
b. 可使用兆声波清洗去除超微粒子，同时可降低清洗液温度，减少金属附着。
c. 在SC-1液中添加界面活性剂、可使清洗液的表面张力从6.3dyn/cm下降到19 dyn/cm。
选用低表面张力的清洗液，可使颗粒去除率稳定，维持较高的去除效率。
使用SC-1液洗，其Ra变大，约是清洗前的2倍。用低表面张力的清洗液，其Ra变化不大（基本不变）。
  在SC-1液中加入HF，控制其PH值，可控制清洗液中金属络合离子的状态，抑制金属的再附着，也可抑制Ra的增大和COP的发生。
  在SC-1加入螯合剂，可使洗液中的金属不断形成螯合物，有利抑制金属的表面的附着。
2）.去除有机物： O3 + H2O
3）.SC-1液的改进： SC-1 + 界面活性剂芯片,
SC-1 + HF
SC-1 + 螯合剂
4）.DHF的改进：
DHF + 氧化剂（例HF+H2O2）
DHF + 阴离子界面活性剂
DHF + 络合剂
DHF + 螯合剂
5）酸系统溶液：  
HNO3 + H2O2
HNO3 + HF + H2O2、
KHF + HCL
6）.其它： 电介超纯去离子水
1）.如硅片表面附着有机物，就不能完全去除表面的自然氧化层和金属杂质，因此清洗时首先应去除有机物。
2）.据报道在用添加2-10 ppm O3 的超净水清洗，对去除有机物很有效，可在室温进行清洗，不必进行废液处理，比SC-1清洗有很多优点。
HF ＋ H2O2清洗
1. 据报道用HF 0.5 % + H2O2 10 %，在室温下清洗，可防止DHF清洗中的Cu等贵金属的附着。
2. 由于H2O2氧化作用，可在硅表面形成自然氧化膜，同时又因HF的作用将自然氧化层腐蚀掉，附着在氧化膜上的金属可溶解到清洗液中，并随去离子水的冲洗而被排除。在APM清洗时附着在晶片表面的金属氢氧化物也可被去除。晶片表面的自然氧化膜不会再生长。
3. Al、Fe、Ni等金属同DHF清洗一样，不会附着在晶片表面。
4. 对n+、P+ 型硅表面的腐蚀速度比n、p 型硅表面大得多，可导致表面粗糙，因而不适合使用于n+、P+ 型的硅片清洗。
5. 添加强氧化剂H2O2（E0=1.776V），比Cu2+ 离子优先从Si中夺取电子，因此硅表面由于H2O2 被氧化，Cu以Cu2+ 离子状态存在于清洗液中。即使硅表面附着金属Cu，也会从氧化剂H2O2 夺取电子呈离子化。硅表面被氧化，形成一层自然氧化膜。因此Cu2+ 离子和Si电子交换很难发生，并越来越不易附着。
DHF ＋ 界面活性剂的清洗
据报道在HF 0.5%的DHF液中加入界面活性剂，其清洗效果与HF + H2O2清洗有相同效果。
E. DHF＋阴离子界面活性剂清洗
据报道在DHF液，硅表面为负电位，粒子表面为正电位，当加入阴离子界面活性剂，可使得硅表面和粒子表面的电位为同符号，即粒子表面电位由正变为负，与硅片表面正电位同符号，使硅片表面和粒子表面之间产生电的排斥力，因此可防止粒子的再附着。
F. 以HF / O3 为基础的硅片化学清洗技术
此清洗工艺是以德国ASTEC公司的AD-(ASTEC-Drying) 专利而闻名于世。其HF/O3 清洗、干燥均在一个工艺槽内完成,。而传统工艺则须经多道工艺以达到去除金属污染、冲洗和干燥的目的。在HF / O3清洗、干燥工艺后形成的硅片H表面 (H-terminal) 在其以后的工艺流程中可按要求在臭氧气相中被重新氧化.

五. 总结

1.用RCA法清洗对去除粒子有效，但对去除金属杂质Al、Fe效果很小。
2. DHF清洗不能充分去除Cu，HPM清洗容易残留Al。
3. 有机物，粒子、金属杂质在一道工序中被全部去除的清洗方法，目前还不能实现。
4. 为了去除粒子，应使用改进的SC-1液即APM液，为去除金属杂质，应使用不附着Cu的改进的DHF液。
5. 为达到更好的效果，应将上述新清洗方法适当组合，使清洗效果最佳。

“太阳能公司将污染留给中国”----华盛顿邮报

2008年3月9日的《华盛顿邮报》报道了中国太阳能多晶硅公司----洛阳中硅的污染情况：
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http://www.washingtonpost.com/wp-dyn/content/article/2008/03/08/AR2008030802595.html
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# E3 x1 k& e- D; G' d原文附有照片。
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Solar Energy Firms Leave Waste Behind in China
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By Ariana Eunjung Cha' o2 E+ ~2 M7 R
Washington Post Foreign Service
/ s" U6 C' D8 [; kSunday, March 9, 2008; A01

GAOLONG, China -- The first time Li Gengxuan saw the dump trucks from the nearby factory pull into his village, he couldn't believe what happened. Stopping between the cornfields and the primary school playground, the workers dumped buckets of bubbling white liquid onto the ground. Then they turned around and drove right back through the gates of their compound without a word.

This ritual has been going on almost every day for nine months, Li and other villagers said.

In China, a country buckling with the breakneck pace of its industrial growth, such stories of environmental pollution are not uncommon. But the Luoyang Zhonggui High-Technology Co., here in the central plains of Henan Province near the Yellow River, stands out for one reason: It's a green energy company, producing polysilicon destined for solar energy panels sold around the world. But the byproduct of polysilicon production -- silicon tetrachloride -- is a highly toxic substance that poses environmental hazards.

"The land where you dump or bury it will be infertile. No grass or trees will grow in the place. . . . It is like dynamite -- it is poisonous, it is polluting. Human beings can never touch it," said Ren Bingyan, a professor at the School of Material Sciences at Hebei Industrial University.

The situation in Li's village points to the environmental trade-offs the world is making as it races to head off a dwindling supply of fossil fuels.

Forests are being cleared to grow biofuels like palm oil, but scientists argue that the disappearance of such huge swaths of forests is contributing to climate change. Hydropower dams are being constructed to replace coal-fired power plants, but they are submerging whole ecosystems under water.

Likewise in China, the push to get into the solar energy market is having unexpected consequences.

With the prices of oil and coal soaring, policymakers around the world are looking at massive solar farms to heat water and generate electricity. For the past four years, however, the world has been suffering from a shortage of polysilicon -- the key component of sunlight-capturing wafers -- driving up prices of solar energy technology and creating a barrier to its adoption.

With the price of polysilicon soaring from $20 per kilogram to $300 per kilogram in the past five years, Chinese companies are eager to fill the gap.

In China, polysilicon plants are the new dot-coms. Flush with venture capital and with generous grants and low-interest loans from a central government touting its efforts to seek clean energy alternatives, more than 20 Chinese companies are starting polysilicon manufacturing plants. The combined capacity of these new factories is estimated at 80,000 to 100,000 tons -- more than double the 40,000 tons produced in the entire world today.

But Chinese companies' methods for dealing with waste haven't been perfected.

Because of the environmental hazard, polysilicon companies in the developed world recycle the compound, putting it back into the production process. But the high investment costs and time, not to mention the enormous energy consumption required for heating the substance to more than 1800 degrees Fahrenheit for the recycling, have discouraged many factories in China from doing the same. Like Luoyang Zhonggui, other solar plants in China have not installed technology to prevent pollutants from getting into the environment or have not brought those systems fully online, industry sources say.

"The recycling technology is of course being thought about, but currently it's still not mature," said Shi Jun, a former photovoltaic technology researcher at the Chinese Academy of Sciences.

Shi, chief executive of Pro-EnerTech, a start-up polysilicon research firm in Shanghai, said that there's such a severe shortage of polysilicon that the government is willing to overlook this issue for now.

"If this happened in the United States, you'd probably be arrested," he said.

An independent, nationally accredited laboratory analyzed a sample of dirt from the dump site near the Luoyang Zhonggui plant at the request of The Washington Post. The tests show high concentrations of chlorine and hydrochloric acid, which can result from the breakdown of silicon tetrachloride and do not exist naturally in soil. "Crops cannot grow on this, and it is not suitable for people to live nearby," said Li Xiaoping, deputy director of the Shanghai Academy of Environmental Sciences.

Wang Hailong, secretary of the board of directors for Luoyang Zhonggui, said it is "impossible" to think that the company would dump large amounts of waste into a residential area. "Some of the villagers did not tell the truth," he said.

However, Wang said the company does release a "minimal amount of waste" in compliance with all environmental regulations. "We release it in a certain place in a certain way. Before it is released, it has gone through strict treatment procedures."

Yi Xusheng, the head of monitoring for the Henan Province Environmental Protection Agency, said the factory had passed a review before it opened, but that "it's possible that there are some pollutants in the production process" that inspectors were not aware of. Yi said the agency would investigate.

In 2005, when residents of Li's village, Shiniu, heard that a new solar energy company would be building a factory nearby, they celebrated.

The impoverished farming community of roughly 2,300, near the eastern end of the Silk Road, had been left behind during China's recent boom. In a country where the average wage in some areas has climbed to $200 a month, many of the village's residents make just $200 a year. They had high hopes their new neighbor would jump-start the local economy and help transform the area into an industrial hub.

The Luoyang Zhonggui factory grew out of an effort by a national research institute to improve on a 50-year-old polysilicon refining technology pioneered by Germany's Siemens. Concerned about intellectual property issues, Siemens has held off on selling its technology to the Chinese. So the Chinese have tried to create their own.

Last year, the Luoyang Zhonggui factory was estimated to have produced less than 300 tons of polysilicon, but it aims to increase that tenfold this year -- making it China's largest operating plant. It is a key supplier to Suntech Power Holdings, a solar panel company whose founder Shi Zhengrong recently topped the list of the richest people in China.

Made from the Earth's most abundant substance -- sand -- polysilicon is tricky to manufacture. It requires huge amounts of energy, and even a small misstep in the production can introduce impurities and ruin an entire batch. The other main challenge is dealing with the waste. For each ton of polysilicon produced, the process generates at least four tons of silicon tetrachloride liquid waste.

When exposed to humid air, silicon tetrachloride transforms into acids and poisonous hydrogen chloride gas, which can make people who breathe the air dizzy and can make their chests contract.

While it typically takes companies two years to get a polysilicon factory up and running properly, many Chinese companies are trying to do it in half that time or less, said Richard Winegarner, president of Sage Concepts, a California-based consulting firm.

As a result, Ren of Hebei Industrial University said, some Chinese plants are stockpiling the hazardous substances in the hopes that they can figure out a way to dispose of it later: "I know these factories began to store silicon tetrachloride in drums two years ago."

Pro-EnerTech's Shi says other companies -- including Luoyang Zhonggui -- are just dumping wherever they can.

"Theoretically, companies should collect it all, process it to get rid of the poisonous stuff, then release it or recycle. Zhonggui currently doesn't have the technology. Now they are just releasing it directly into the air," said Shi, who recently visited the factory.

Shi estimates that Chinese companies are saving millions of dollars by not installing pollution recovery.

He said that if environmental protection technology is used, the cost to produce one ton is approximately $84,500. But Chinese companies are making it at $21,000 to $56,000a ton.

In sharp contrast to the gleaming white buildings in Zhonggui's new gated complex in Gaolong, the situation in the villages surrounding it is bleak.

About nine months ago, residents of Li's village, which begins about 50 yards from the plant, noticed that their crops were wilting under a dusting of white powder. Sometimes, there was a hazy cloud up to three feet high near the dumping site; one person tending crops there fainted, several villagers said. Small rocks began to accumulate in kettles used for boiling faucet water.

Each night, villagers said, the factory's chimneys released a loud whoosh of acrid air that stung their eyes and made it hard to breath. "It's poison air. Sometimes it gets so bad you can't sit outside. You have to close all the doors and windows," said Qiao Shi Peng, 28, a truck driver who said he worries about his 1-year-old son's health.

The villagers said most obvious evidence of the pollution is the dumping, up to 10 times a day, of the liquid waste into what was formerly a grassy field. Eventually, the whole area turned white, like snow.

The worst part, said Li, 53, who lives with his son and granddaughter in the village, is that "they go outside the gates of their own compound to dump waste."

"We didn't know how bad it was until the August harvest, until things started dying," he said.

Early this year, one of the villagers put some of the contaminated soil in a plastic bag and went to the local environmental bureau. They never got back to him.

Zhang Zhenguo, 45, a farmer and small businessman, said he has a theory as to why: "They didn't test it because the government supports the plant."

Researchers Wu Meng and Crissie Ding contributed to this report.