CN116666500B - 锗光电探测器及通过热失配应力提高其长波响应的方法 - Google Patents
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Abstract
本发明提供锗光电探测器及通过热失配应力提高其长波响应的方法，属于锗光电探测器技术领域，在单晶硅衬底上高温外延生长单晶锗层，然后沉积硅帽层，所述硅帽层厚度为0.5‑5微米，自然冷却到室温下，去除硅帽层后，再制成探测器。从高温下降到室温时锗因热膨胀系数大收缩较多而产生水平拉伸应力，其中，覆盖硅帽层的锗因衬底和帽层的双重拉伸而具有更大应力；室温下去掉硅帽层，原硅帽层贡献的应力被部分保留：即与锗层降温这一自退火过程产生的不可恢复的塑性形变相联系的这部分应力被保留，不随硅帽层去除而释放；而与可恢复的弹性形变相联系的这部分应力将随硅帽层去除而释放。因此曾经沉积硅帽层的锗层具有更大的拉伸引力。
Description
技术领域
本发明涉及锗光电探测器技术领域，具体涉及一种锗光电探测器及通过热失配应力提高其长波响应的方法。
背景技术
硅光芯片技术是在硅衬底上以CMOS工艺集成光电子器件的技术，结合了集成电路技术的超大规模、超高精度制造的特性和光子技术超高速率、超低功耗的优势。硅光技术在光通信、数据中心、光互连、激光雷达、医疗健康、光计算等领域具有广阔的应用前景，通过过去20年学术界、工业界的广泛重视和深入研发，已经在电信和数据中心等领域大规模应用。如图1所示，硅的带隙1.12eV决定了硅探测器响应的截止波长为1.1微米左右，在硅衬底上外延生长带隙更低的锗材料充当吸收材料，可以将探测截止波长增加到1.6微米左右，覆盖1310纳米、1550纳米等最重要的红外光通信波长，这一技术突破是硅光芯片发展的最关键环节之一。
然而，如图1所示，锗对红外光波的吸收系数在1550纳米以后急剧下降，使得锗光电探测器无法满足更长红外波段的探测需求，而这些需求对于正在兴起的光传感（如激光雷达、生物医学检测、夜视等）和扩增的光通信波段非常重要，导致锗光电探测器在这些领域的应用受到限制。如图2所示（图中Eg(L)、Eg(Г)分别为L、Г波矢方向的禁带宽度），研究发现，锗层中的拉伸应力显著改变能带结构和光电性质，双轴向的拉伸应力将会减少锗的带隙并将锗转变为直接带隙材料，相应地，锗光电探测器的长波响应能力和截止波长都将增加，有机会应用在前述扩增波段光通信、激光雷达、生物医学检测和夜视等原本受限的领域。
硅衬底上外延生长的锗层，在外延的高温下为完全弛豫状态；锗的热膨胀系数大于硅，决定了降温时锗的收缩程度也大于硅，锗层中由此产生双轴向的拉伸引力。利用这一热膨胀系数差异导致的热失配应力结合掺杂诱导的应力，能够实现硅基外延锗的直接带隙并观测到硅基锗激光。硅光芯片采用硅基锗外延的集成光电探测器，因此锗光电探测器中已经具有拉伸引力。然而，同采用其他方法产生拉伸应力的锗光电探测器相比，如采用氮化硅、金属等应力源层在金属/半导体/金属（MSM）锗光电探测器中诱发应力（参见专利CN103985788B、CN106653940B），或锡、铅等杂质诱发应力（参见专利CN112534590A、CN110729373B）等，锗外延光电探测器依靠热失配产生的拉伸应力较低，并且硅光芯片常用的绝缘体上硅SOI衬底（由于埋氧化层BOX的阻隔）还会进一步降低热失配应力，其提升锗光电探测器长波响应和增加截止波长的效果就非常有限。
专利CN103985788B、CN106653940B中利用氮化硅、金属等诱导MSM锗光电探测器拉升应力，MSM类型的探测器虽然具有较高的带宽，但暗电流巨大，金属对光的吸收较高，导致信噪比有限，应用场景受到限制；专利CN112534590A、CN110729373B在锗层中掺入锡、铅等杂质，对长波响应提升效果显著，但是生长高质量的锗锡、锗铅合金非常困难，其外延厚度受到限制、容易产生缺陷和高暗电流，器件性能受限，并且掺杂的实施需要额外的掺杂源，还对外延设备腔体造成污染。因此，需要开发新的拉伸应变锗制造技术，克服以上方法的缺点，或作为以上方法的协同或补充，以取得更好的长波响应提升效果。
发明内容
本发明的目的在于提供一种锗光电探测器及通过热失配应力提高其长波响应的方法，解决现有拉伸应变锗制造技术中存在的锗光电探测器应用场景受限制、容易产生缺陷和高暗电流，器件性能受限等技术问题。
本发明公开了一种锗光电探测器及通过热失配应力提高其长波响应的方法，包括以下步骤：
在单晶硅衬底上高温外延生长单晶锗层，然后高温沉积硅帽层，所述硅帽层厚度为0.5-5微米，自然冷却到室温下，去除硅帽层后，再制成锗光电探测器。
工作原理：高温下外延锗和沉积多晶硅，无论锗上是否生长硅帽层，各层材料在高温下均为应力弛豫状态；从高温下降到室温时锗因热膨胀系数大收缩较多而产生水平拉伸应力。由于锗与硅之间存在4.2%的晶格失配，锗外延层不是完美的单晶结构，会产生位错等多种缺陷。包含各种缺陷的锗外延层，在温度变化过程中会同时产生可以恢复的弹性形变（Elastic deformation）和不可恢复的塑性形变（Plastic deformation）。室温下去掉硅帽层，原硅帽层贡献的应力会被部分保留：即与锗层降温这一自退火过程产生的不可恢复的塑性形变相联系的这部分应力被保留，不随硅帽层去除而释放；而与可以恢复的弹性形变相联系的这部分应力将随硅帽层去除而释放。因此，与不覆盖硅帽层的情况相比，其上曾经覆盖硅帽层的锗层具有更大的拉伸引力。采用高温下沉积厚硅帽层并在室温下将其去除这一工艺过程，可以在锗探测器中获得更大的拉升应力和长波响应。
进一步的，所述高温外延生长单晶锗层的具体方法为采用先低温、再高温的两步进行锗外延，所述低温温度为300-400℃，所述高温温度为600-700℃。两步锗外延的方式能减少缺陷密度。
进一步的，所述单晶锗层为纯锗。
进一步的，所述单晶锗层为包括硅、锡、铅、碳一种或多种元素的锗合金。
进一步的，去除所述硅帽层后形成非掺杂锗外延层、N+掺杂锗外延层和N++掺杂锗外延层，所述N++掺杂锗外延层嵌入所述N+掺杂锗外延层上部。
进一步的，所述N+掺杂锗外延层厚度0.05~0.2微米，表面掺杂浓度为1x1018~5x1019cm-3；所述N++掺杂锗外延层，厚度0.02-0.2微米，表面掺杂浓度为1x1019~5x1020cm-3。
进一步的，所述N+掺杂锗外延层通过外延时的原位掺杂方式获得，或通过生长未掺杂的锗外延后通过离子注入获得。
进一步的，所述N++掺杂锗外延层通过锗外延之后的离子注入形成。
进一步的，所述硅帽层为沉积的多晶硅、非晶硅、外延的单晶硅或这些材料组合而成的叠层结构。
进一步的，所述硅帽层通过湿法腐蚀去除。
进一步的，所述湿法腐蚀具体步骤为采用四甲基氢氧化铵TMAH溶液，通过控制时间刻蚀去除硅片表面和背面的硅帽层（TMAH室温下对氧化硅和锗的刻蚀速率极低，可以认为只去除多晶硅和硅片背面的少量单晶硅，不会刻蚀氧化硅和锗）。
进一步的，所述单晶硅衬底为SOI衬底或体硅衬底。
进一步的，所述单晶硅衬底为SOI衬底时，所述单晶硅衬底自下而上包含了硅衬底、埋氧化硅层和顶硅层三个部分。
进一步的，所述顶硅层自下而上又包含了 P层、掺杂浓度渐次增高的P+层和P++层。
进一步的，所述氧化硅通过两次沉积，分别沉积第一氧化硅层和第二氧化硅层，所述第一氧化硅层用于定义锗外延窗口，所述第二氧化硅层用于定义接触孔。
进一步的，所述单晶锗层包括非掺杂外延锗层和掺杂外延锗层。
进一步的，所述单晶锗层厚度与所述第一氧化硅层厚度差距不超过0.1微米。确保锗外延后表面基本平坦，有利于同CMOS工艺兼容实现晶体管集成和多层技术布线。
进一步的，所述探测器上设置有钝化保护层。可以提高锗光电探测器的可靠性和使用寿命。
进一步的，所述探测器上设置有减反膜。减少光发射和取得更好的应用效果。
本发明的有益效果包括：
采用覆盖厚硅帽层的热失配应力增强技术，在锗材料中获得水平方向拉伸应变增强的效果，提升了红外长波方向下的光电响应和增加了响应截止波长；避免了现有掺杂方法产生应力带来的缺陷问题和高昂的设备工艺成本，也没有MSM探测器中金属诱导应变的负面效应，即较高的暗电流、光损耗及信噪比受限等问题；同时适用垂直入射圆饼型探测器和水平光耦合的波导型PIN、PN探测器，也可以用于雪崩光电二极管APD；不仅适合SOI衬底，也适合体硅衬底，具有高度的灵活性和优良的经济性；材料和工艺同硅CMOS工艺兼容，结构简单，成本低廉。
附图说明
为了更清楚地说明本发明实施例的技术方案，下面将对实施例中所需要使用的附图作简单地介绍，应当理解，以下附图仅表示出了本发明的部分实施例，因此不应看作是对范围的限定，对于本领域普通技术人员来讲，在不付出创造性劳动的前提下，还可以根据这些附图获得其它相关的附图。
图1为常见半导体材料的光吸收系数随波长变化的曲线；
图2为拉伸应变增加使锗的带隙降低并转变为直接带隙半导体的示意图；
图3为本发明锗光电探测器制备过程中加入硅帽层后的结构示意图；
图4为本发明锗光电探测器制备完成后的结构示意图。
图中箭头代表应变的方向。
图中：1-SOI衬底，2-硅衬底，3-埋氧化层，4-顶硅层，5-第一氧化硅层，6-单晶锗层，7-硅帽层，8-非掺杂锗外延层，9-N+掺杂锗外延层，10-N++掺杂锗外延层，11-第二氧化硅层。
具体实施方式
为使本发明实施方式的目的、技术方案和优点更加清楚，下面将结合本发明实施方式中的附图，对本发明实施方式中的技术方案进行清楚、完整地描述，显然，所描述的实施方式是本发明一部分实施方式，而不是全部的实施方式。
实施例1
一种锗光电探测器及通过热失配应力提高其长波响应的方法，如图3-图4所示，其制备步骤包括：
选取8吋P(100) SOI衬底1，硅衬底2厚度725微米，电阻率约10欧姆∙厘米，埋氧化层3厚度3微米，顶硅层4厚度0.22微米；
以光刻胶掩蔽其他区域，对锗探测器区域的SOI顶硅层4进行中等剂量的P+型离子注入，后续用于横向传输载流子汇集光电流；
以光刻胶掩蔽其他区域，对锗探测器区域SOI顶硅层4进行环状的离子注入形成重掺杂的P++型离子注入，并高温快速热退火激活，后续用于同金属电极形成欧姆接触；
沉积0.6微米第一氧化硅层5，并通过光刻和干法刻蚀第一氧化硅层5，定义选择性硅基锗外延的窗口；
选择性硅基锗外延0.6微米，采用先低温（约350℃）、再高温（约650℃）两步锗外延的方式减少缺陷密度；
在约650℃下，采用低压化学气相沉积（LPCVD）多晶硅0.5-5微米，此处考虑工艺时间及控制等因素，优选1微米的多晶硅厚度；
自然冷却到室温下，采用四甲基氢氧化铵TMAH溶液，通过控制时间刻蚀去除硅片表面和背面的多晶硅1微米，并增加约30%的过刻蚀（TMAH室温下对氧化硅和锗的刻蚀速率极低，可以认为只去除多晶硅和硅片背面的少量单晶硅，不会刻蚀氧化硅和锗）；
分别光刻锗N+、N++区域并进行离子注入，然后快速热退火激活，形成非掺杂锗外延层8， N+掺杂锗外延层9， N++掺杂锗外延层10；
沉积0.5微米第二氧化硅层11作为金属化前介质PMD（Pre-Metal Dielectric）
光刻接触孔：此时探测器负极上方是0.5微米厚的第二氧化硅层11，正极区域上方则是第一和第二氧化硅层厚度叠加即0.6+0.5即1.1微米厚度，如此大的氧化层厚度落差情况下，需要避免一次光刻和刻蚀情形下正极接触孔打开时负极接触孔的严重过刻蚀，因为过刻蚀会造成高掺杂的多晶硅被大量刻蚀而降低欧姆接触质量。应当进行两次光刻和刻蚀，分别打开正极和负极上方的氧化层。本实施例采用先光刻和刻蚀正极接触孔、后光刻和刻蚀负极接触孔的顺序，以尽量减少敏感、脆弱的锗材料层暴露。
采用常规的两步退火法，在接触孔内形成低电阻的金属硅化物NiSi：1）溅射10纳米金属镍，接触孔中镍同正极接触区的P++硅、负极接触区的N++锗接触，其余镍附着在第二氧化硅层上面或接触孔的氧化硅侧壁上面；2）280℃/300秒/氮气气氛退火，与硅接触的镍反应生成高阻、富镍的硅化物NixSi相（x大于1，以Ni2Si为主），与氧化硅接触的镍不反应而保持金属镍状态；3）在加热的浓硫酸中腐蚀180秒，去除尚未反应生成硅化物的金属镍；4）400℃/30秒/氮气气氛退火，将富镍的硅化物转变为稳定、低阻的NiSi相硅化物。
溅射0.6微米铝，光刻金属布线图形，以干法刻蚀形成接触孔填充和互连线，并退火合金完成金属化步骤。
本发明在硅基锗外延层上覆盖较厚的硅帽层，利用锗的热膨胀系数更大、冷却到室温时收缩更多而获得水平拉伸应力，覆盖较厚硅帽层的锗层应力大于不覆盖或覆盖较薄硅帽层的锗层应力，从而提高红外光长波方向的响应和增加截止波长。本发明的技术方案和实施例，虽然只是针对垂直入射型的硅基锗PIN探测器，但是其技术原理和方法同样适用于横向耦合入光的波导型硅基锗PIN、PN型探测器，也适用于垂直入射或波导耦合横向入光的硅基锗雪崩光电二极管（Avalanche Photodiode, APD）。除了覆盖较厚硅帽层提升锗层应力和采用室温下的TMAH溶液刻蚀去除硅帽层之外，其余结构和工艺在硅基锗探测器结构设计和制造中都是常规性的，此处不再赘述。
以上即为本实施例列举的实施方式，但本实施例不局限于上述可选的实施方式，本领域技术人员可根据上述方式相互任意组合得到其他多种实施方式，任何人在本实施例的启示下都可得出其他各种形式的实施方式。上述具体实施方式不应理解成对本实施例的保护范围的限制，本实施例的保护范围应当以权利要求书中界定的为准，并且说明书可以用于解释权利要求书。
Claims (10)
1.一种通过热失配应力提高锗光电探测器长波响应的方法，其特征在于：包括以下步骤：
在单晶硅衬底上高温外延生长单晶锗层(6)，然后高温沉积硅帽层(7)，所述硅帽层(7)厚度为0.5-5微米，自然冷却到室温下，去除硅帽层(7)后，再制成锗光电探测器。
2.根据权利要求1所述的一种通过热失配应力提高锗光电探测器长波响应的方法，其特征在于：所述高温外延生长单晶锗层(6)的具体方法为采用先低温、再高温的两步进行锗外延，所述低温温度为300-400℃，所述高温温度为600-700℃。
3.根据权利要求1所述的一种通过热失配应力提高锗光电探测器长波响应的方法，其特征在于：所述单晶锗层(6)为纯锗；或所述单晶锗层(6)为包括硅、锡、铅、碳一种或多种元素的锗合金。
4.根据权利要求1所述的一种通过热失配应力提高锗光电探测器长波响应的方法，其特征在于：去除所述硅帽层(7)后制备非掺杂锗外延层（8）、N+掺杂锗外延层（9）和N++掺杂锗外延层（10），所述N++掺杂锗外延层（10）嵌入所述N+掺杂锗外延层（9）上部。
5. 根据权利要求4所述的一种通过热失配应力提高锗光电探测器长波响应的方法，其特征在于：所述N+掺杂锗外延层（9）厚度0.05~0.2微米，表面掺杂浓度为1x1018~5x1019 cm-3；所述N++掺杂锗外延层（10）厚度0.02-0.2微米，表面掺杂浓度为1x1019~5x1020 cm-3。
6.根据权利要求4所述的一种通过热失配应力提高锗光电探测器长波响应的方法，其特征在于：所述N+掺杂锗外延层（9）通过外延时的原位掺杂方式获得，或通过生长未掺杂的锗外延后采用离子注入获得；所述N++掺杂锗外延层（10）通过离子注入获得。
7.根据权利要求1所述的一种通过热失配应力提高锗光电探测器长波响应的方法，其特征在于：所述硅帽层（7）为多晶硅、非晶硅或外延的单晶硅或通过这几种材料组合而成的叠层材料。
8.根据权利要求1所述的一种通过热失配应力提高锗光电探测器长波响应的方法，其特征在于：所述硅帽层（7）通过湿法腐蚀去除。
9.根据权利要求8所述的一种通过热失配应力提高锗光电探测器长波响应的方法，其特征在于：所述湿法腐蚀具体步骤为采用四甲基氢氧化铵TMAH溶液，通过控制时间刻蚀去除硅片表面和背面的硅帽层（7）。
10.一种锗光电探测器，其特征在于：通过权利要求1-9中任一项所述的一种锗光电探测器及通过热失配应力提高其长波响应的方法制得。
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